reactivation of the levant passive margin during the late tertiary and formation of the jaffa basin offshore central israel re examination of the stratigraphic record in the continental margin of israel indicates that the levant passive margin was reactivated in the late tertiary subsidence and presumably sedimentation rates increased after prolonged gradual decay the shelf slope facies transition zone was revived faulting and magmatism resumed and the judea hills began to rise two parallel fault systems with large vertical displacements were formed or reactivated between the levant basin and the continent in the levant basin a new 4 km thick section accumulated and at a middle level between the two faults a local basin was formed and filled with a 2500 m thick section that basin termed here the jaffa basin provides good age control it was initiated in the mid late oligocene was mainly active in the miocene and was completely buried by sediments in the plio pleistocene we suggest that at the early stage of the arabia africa breakup in conjunction with the suez rifting the jaffa basin was formed between two segments of a left lateral fault that allowed arabia to slip northward relative to the mediterranean lithosphere when this fault failed to transform the motion both the suez rift and the jaffa basin were abandoned and the plate motion jumped inland to the dead sea transform

geology geochronology and tectonic setting of the jorullo volcano region michoacan mexico abstract the jorullo monogenetic volcano erupted 250 years ago at the southern border of the trans mexican volcanic belt tmvb an area that records a long history of magmatic and tectonic activity the oldest rocks that crop out in the area are early oligocene 32 7 0 2 ma dioritic to granitic 60 72 wt sio2 plutons intruded by younger to contemporaneous 30 3 0 1 and 33 3 0 7 ma aplitic 76 78 wt sio2 dikes and mineralized veins volcanic rocks related to these intrusives form a thick sequence exposed in cliffs from the base of which an altered andesite 58 wt sio2 was collected the nearby epithermal cu mineralization of the inguaran mining area to the southeast is also related to this tertiary volcanic episode the oldest expression of the tmvb in this area is represented by pliocene 3 2 2 4 ma basaltic andesite to andesite 53 63 wt sio2 lavas with pyroxene olivine forming extensive mesas eroded mounds and thick elongate flows that were unconformably deposited on top of the oligocene rocks their emplacement was followed by erosion approximating a rate of 100 m ma over the past 3 ma this led to the formation of an inverted topography reflecting the higher resistance to erosion of these lavas that were originally deposited in valleys and today constitute prominent mesas during the last 1 ma at least twenty six monogenetic scoria cones formed in the study area erupting about 10 2 km3 of lava and pyroclastics dominantly composed of olivine and pyroxene bearing andesites 61 vol dre lesser basaltic andesites 30 vol dre and a minority of olivine rich basalts 10 vol dre these eruptive products include the k rich basaltic trachyandesites of cerro la pilita a scoria and lava cone located close to jorullo the 27 ka san hilario and esmeralda lavas which are located 13 km southeast of jorullo are the southernmost eruptive products of the tmvb and also the youngest volcanoes after jorullo in this area mapping of main faults and scoria cone alignments on regional and local scales reveals that the ascent of dikes feeding quaternary eruptions in the jorullo area was controlled by west northwest and west southwest basement structures northeast oriented subduction related horizontal compressional stress probably associated with crustal fractures and a major northwest trending seismically active fault zone that passes near paricutin and jorullo

oxygen and hydrogen isotope stratigraphy of the rustenburg layered suite bushveld complex constraints on crustal contamination new do values for plagioclase pyroxene and olivine and limited whole rock dd values are presented for samples from the rustenburg layered suite of the bushveld complex south africa in combination with existing data these provide a much more complete composite o isotope stratigraphy for the intrusion throughout the layered suite mineral do values indicate that the magmas from which they crystallized had do values that were about 7 1 that is 1 4 higher than expected for mantle derived magmas suggesting extensive crustal contamination more limited h isotope data suggest that the oh present within whole rocks regardless of the degree of alteration is of magmatic origin and not an alteration phenomenon there appears to be no systematic change in do value with stratigraphic height and this requires the contamination to have taken place in a staging chamber before emplacement of the magma s into the present chamber large amounts 30 40 of contamination by the lower to middle crust are needed to explain these do values which is in general agreement with previous estimates based on srand nd isotope data alternatively smaller amounts of contamination 20 by sedimentary rocks or their partial melts represented by the country rock can explain the data but it is not apparent how such material could have been present at the depth of the staging chamber in the lower to middle crust

cretaceous tertiary structures and kinematics of the serbomacedonian metamorphic rocks and their relation to the exhumation of the hellenic hinterland macedonia greece the kinematic pattern and associated metamorphism of the predominant ductile deformation and the subsequent deformational stages of the serbomacedonian metamorphic rocks and granitoids are presented in terms of peri tethyan tectonics a systematic record of structural and metamorphic data gives evidence of a main top to ene to ese ductile flow of cretaceous age 120 90 ma associated with a crustal stretching and unroofing a subordinate wsw to wnw antithetic sense of movement of the tectonic top is observed in places the associated metamorphic conditions are estimated at 4 5 7 5 kbar and 510 580 c during eocene to miocene times these fabrics were successively deformed by low angle extensional de ductile shear zones with top to ne and sw sense of movement and brittle shear zones of similar kinematic pattern suggesting a transition from ductile to brittle deformation de deformation was accompanied during its later stages by nw se directed shortening we also discuss the relation of this cretaceous tertiary deformation of the serbomacedonian metamorphic rocks with the eocene to miocene ductile top to southwestward crustal shear of the adjacent rhodope crystalline rocks we regard the serbomacedonian and the rhodope metamorphic rocks to represent related metamorphic provinces the most recent exhumation and cooling history of which is bracketed between the eocene and neogene

filling the bushveld complex magma chamber lateral expansion roof and floor interaction magmatic unconformities and the formation of giant chromitite pge and ti v magnetitite deposits his mind was like a soup dish wide and shallow irving stone on william jennings bryan a compilation of the sr isotopic stratigraphy of the bushveld complex shows that the evolution of the magma chamber occurred in two major stages during the lower open system integration stage lower critical and lower main zone there were numerous influxes of magma of contrasting isotopic composition with concomitant mixing crystallisation and deposition of cumulates larger influxes correspond to the boundaries of the zones and sub zones and are marked by sustained isotopic shifts major changes in mineral assemblages and development of unconformities during the upper closed system differentiation stage upper main zone and upper zone there were no major magma additions other than that which initiated the upper zone and the thick magma layers evolved by fractional crystallisation the lower and lower critical zones are restricted to a belt that runs from steelpoort and burgersfort in the northeast to rustenburg and northam in the west and an outlier of the lower and lower critical zone up to the lg4 chromitite layer in the far western extension north of zeerust it is only in these areas that thick harzburgite and pyroxenite layers are developed and where chromitites of the lower critical zone occur these chromitites include the economically important c 1 m thick lg6 and mg1 layers exposed around both the eastern and western lobes of the bushveld complex the upper critical zone has a greater lateral extent than the lower critical zone and overlies but also onlaps the floor rocks to the south of the steelpoort area the source of the magmas also appears to have been towards the south as the mg chromitite layers degrade and thin northward whereas the lg layers are very well represented in the north and degrade southward sr and os isotope data indicate that the major chromitite layers including the lg6 mg1 and ug2 originated in a similar way extremely abrupt and stratigraphically restricted increases in the sr isotope ratio imply that there was massive contamination of intruding melt which hit the roof of the chamber and incorporated floating granophyric liquid which forced the precipitation of chromite kruger 1999 kinnaird et al 2002 therefore each chromitite layer represents the point at which the magma chamber expanded and eroded and deformed its floor nevertheless this was achieved by in situ contamination by roof rock melt of the intruding critical zone liquids that had an orthopyroxenitic to noritic lineage the main zone is present in the eastern and western lobes of the bushveld complex where it overlies the critical zone and onlaps the floor rocks to the south and the north where it is also the basal zone in the northern lobe the new magma first intruded the northern lobe north of the thabazimbi murchison lineament interacted with the floor rocks incorporated sulphur and precipitated the platreef along the floor rock contact before flowing south into the main chamber this exceptionally large influx of new magma then eroded an unconformity on the critical zone cumulate pile and initiated the main zone in the main chamber by precipitating the merensky reef on the unconformity the upper zone magma flowed into the chamber from the southern bethal lobe as well as the tml this gigantic influx eroded the main zone rocks and caused very large scale unconformable relationships clearly evident as the gap areas in the western bushveld complex the base of this influx which is also coincident with the pyroxenite marker and a troctolitic layer in the northern lobe is the petrological and stratigraphic base of the upper zone sr isotope data show that all the pge rich ores including chromitites are related to influxes of magma and are thus related to the expansion and filling of the magma chamber dominantly by lateral expansion with associated transgressive disconformities onto the floor rocks coincident with major zone changes these positions in the stratigraphy are marked by abrupt changes in lithology and erosional features over which succeeding lithologies are draped the outcrop patterns and the concordance of geochemical isotopic and mineralogical stratigraphy indicate that during crystallisation the bushveld complex was a wide and shallow lobate sill like sheet and the rock strata and mineral deposits are quasi continuous over the whole intrusion

trapped liquid paleo porosity and formation time scale of a chromitite ortho pyroxenite cumulate section bushveld south africa to evaluate compaction and interstitial melt expulsion during cumulate formation a 20 m cumulate section including the ug2 and ug3 chromitites from a 264 m drill core through the upper critical zone of the bushveld complex south africa has been studied the cumulates in the studied section are as follows 3 m plagioclase pyroxenite to pyroxenite pegmatoid footwall pyroxenite at the lower contact to ug2 0 7 m ug2 chromitite 6 8 m pyroxenite 0 24 m ug3 chromitite 2 0 m plagioclase rich pyroxenite changing locally to norite the two 5 cm leader stringers ug3a and ug3b and 7 m total of olivine pyroxenites grading into plagioclase pyroxenites all pyroxenites are dominated by orthopyroxene opx and the cumulate sequence is topped by mottled anorthosite grading into norite stratigraphic concentrations of major and trace elements of 52 bulk rock samples were determined bulk rock mg numbers are 0 79 0 81 throughout the silicate cumulate units and 0 40 0 46 in the chromitite layers the stratigraphic distribution of six incompatible trace elements k rb ba cs zr and th has been used to determine the amount of trapped liquid ftl or paleo porosity in the cumulate rocks final porosities volume fractions based on averages from the six trace elements are 0 06 0 33 in the pyroxenites in chromitite layers trapped melt fractions of 0 12 0 36 are calculated from incompatible trace element concentrations but bulk sio2 concentrations and x ray tomography yield 0 04 0 17 higher porosities hence the bulk silicate fraction in the chromitites may not necessarily correspond to the trapped liquid fraction as poikilitic opx was crystallizing while the silicate melt still equilibrated using a previously derived experiment based model for compaction time scales gravitationally driven chemical compaction in the ug2 ug3 pyroxenite section is calculated to occur within 1 10 years this time frame corresponds to the times necessary to cool a 20 m layer by 10 50 c the temperature interval argued to encompass the liquidus and almost complete solidification compaction within a decade can in fact easily develop the paleo porosities indirectly observed today and is probably stopped by crystallization of the interstitial liquid contrary to previous assertions melt expulsion from the cumulate pile does not hinder compaction calculated permeabilities would allow for the migration of an order of magnitude higher amount of melt than has to be expelled from the 20 m pile of cumulate the pegmatoid zones in the chromitite footwalls enriched in incompatible trace elements are consistent with a collection of interstitial melts expelling from the underlying compacting pyroxenites their entrapment below the chromitite layers suggests that these act as permeability barriers this is in part due to their finer grain size compared with the pyroxenites but is mainly due to the crystallization of large poikilitic opx during compaction

origin of the ug2 chromitite layer bushveld complex chromitite layers are common in large mafic layered intrusions a widely accepted hypothesis holds that the chromitites formed as a consequence of injection and mixing of a chemically relatively primitive magma into a chamber occupied by more evolved magma this forces supersaturation of the mixture in chromite which upon crystallization accumulates on the magma chamber floor to form a nearly monomineralic layer to evaluate this and other genetic hypotheses to explain the chromitite layers of the bushveld complex we have conducted a detailed study of the silicate rich layers immediately above and below the ug2 chromitite and another chromitite layer lower in the stratigraphic section at the top of the lower critical zone the ug2 chromitite is well known because it is enriched in the platinum group elements and extends for nearly the entire 400 km strike length of the eastern and western limbs of the bushveld complex where we have studied the sequence in the central sector of the eastern bushveld the ug2 chromitite is embedded in a massive 25 m thick plagioclase pyroxenite consisting of 60 70 vol granular cumulus orthopyroxene with interstitial plagioclase clinopyroxene and accessory phases throughout the entire pyroxenite layer orthopyroxene exhibits no stratigraphic variations in major or minor elements mg number1 4 79 3 81 1 however the 6 m of pyroxenite below the chromitite footwall pyroxenite is petrographically distinct from the 17 m of hanging wall pyroxenite among the differences are 1 phlogopite k feldspar and quartz are ubiquitous and locally abundant in the footwall pyroxenite but generally absent in the hanging wall pyroxenite and 2 plagioclase in the footwall pyroxenite is distinctly more sodic and potassic than that in the hanging wall pyroxenite an45 60 vs an70 75 the lower critical zone chromitite is also hosted by orthopyroxenite but in this case the rocks above and below the chromitite are texturally and compositionally identical for the ug2 we interpret the interstitial assemblage of the footwall pyroxenite to represent either interstitial melt that formed in situ by fractional crystallization or chemically evolved melt that infiltrated from below in either case the melt was trapped in the footwall pyroxenite because the overlying ug2 chromitite was less permeable if this interpretation is correct the footwall and hanging wall pyroxenites were essentially identical when they initially formed however all the models of chromitite formation that call on mixing of magmas of different compositions or on other processes that result in changes in the chemical or physical conditions attendant on the magma predict that the rocks immediately above and below the chromitite layers should be different this leads us to propose that the bushveld chromitites formed by injection of new batches of magma with a composition similar to the resident magma but carrying a suspended load of chromite crystals the model is supported by the common observation of phenocrysts including those of chromite in lavas and hypabyssal rocks and by chromite abundances in lavas and peridotite sills associated with the bushveld complex indicating that geologically reasonable amounts of magma can account for even the massive 70 cm thick ug2 chromitite the model requires some crystallization to have occurred in a deeper chamber for which there is ample geochemical evidence

time dependence of mantle plumes some simple theory dynamical calculations indicate that plate tectonics are likely to have a major effect on the initiation of plumes from the basal boundary layer in the mantle and their subsequent evolution plume start because the boundary layer is hotter than the overlying mantle and hence unstable to perturbations the growth rate of perturbations is too weakly dependent on wavelength to control the geographic distribution of plumes instead the distribution of plumes is strongly influenced by the shape of large perturbations associated with slabs and surface plates plumes evolve because downward return flow is needed to replace the material tapped from the boundary layer this flow downwarps the top of the boundary layer several hundred kilometers from the plume and thus restricts further flow of hot material toward an active plume the geometry of this downwarp is strongly influenced by the initial relief of the top of the boundary layer and hence by plate processes the actual history of plates and slabs is thus needed in numerical models for comparison with real data on plume fluxes

paleogene continental margin truncation in southwestern mexico geochronological evidence the reasons for and mechanisms of continental margin truncation in sw mexico where mesozoic cenozoic plutons are situated directly on the pacific coast are not yet well understood large scale dextral and or sinistral displacements of the continental margin terranes now forming parts of baja california or the chortis block have been proposed the well defined along coast nw se decreasing granitoid intrusion age trend 1 2 cm yr in the 100 ma 40 ma time interval between puerto vallarta and zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the chortis block from those latitudes toward the se changes in the dip and velocity of the nnw sse trending cretaceous tertiary subduction zone resulted in a landward migration of the magmatic arc taking into account certain stratigraphic affinities of chortis and the oaxaca and mixteca terranes together with the known displacement rates along the north america caribbean plate boundary the northwesternmost paleoposition of the chortis block with respect to sw mexico was near zihuatanejo in contrast between zihuatanejo and the isthmus of tehuantepec the cessation of the tertiary magmatism decreased more rapidly 7 7 cm yr although the trend is not so obvious starting in the late eocene chortis moved about 1100 km to the se along a transform boundary associated with the opening of the cayman trough based on our geochronological data and structural relationships between mylonite zones and plutons in the acapulco tehuantepec area we propose an approximately 650 km se movement of chortis from about 40 25 ma with a velocity of 6 5 4 3 cm yr since this is considerably slower than the decreasing age trend obtained by us using the geochronological data we consider batholith formation in this segment to predate and postdate the offshore passage of the north america farallon caribbean triple junction geological observations and paleomagnetic data do not give strong support for large scale right lateral displacements of crustal blocks like the baja california given the isotopic data presented the continental margin truncation in sw mexico seems to be the consequence of an interaction of mechanisms of these we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment on the other hand the lateral removal of material associated with the displacement of chortis is more important in the southeastern segment

settling and compaction of chromite cumulates employing a centrifuging piston cylinder and application to layered mafic intrusions the time scales and mechanics of gravitationally driven crystal settling and compaction is investigated through high temperature 1 280 1 500 c centrifuge assisted experiments on a chromite basalt melt system at 100 1 500g 0 5 gpa subsequently the feasibility of this process for the formation of dense chromite cumulate layers in large layered mafic intrusions lmis is assessed centrifugation leads to a single cumulate layer formed at the gravitational bottom of the capsule the experimentally observed mechanical settling velocity of a suspension of 24 vol chromite is calculated to be about half 0 53 of the stokes settling velocity with a sedimentation exponent n of 2 35 3 gravitational settling leads to an orthocumulate layer with a porosity of 0 52 all porosities as fraction formation times for such a layer from a magma with initial chromite contents of 0 1 1 vol are 140 3 5 days equal to a growth rate of 0 007 0 3 m day for grain sizes of 1 2 mm more compacted chromite layers form with increasing centrifugation time and acceleration through chemical compaction an increase of grain contact areas and grain sizes together with a decrease in porosity is best explained by pressure dissolution at grain contacts reprecipitation and grain growth into the intergranular space and a concomitant expulsion of intergranular melt the relation between the porosity in the cumulate pile and effective pressure integrated over time h a t is best fit with a logarithmic function in fact confirming that a pressure dissolution reprecipitation process is the dominant mechanism of compaction the experimentally derived equation allows calculating compaction times 70 80 chromite at the bottom of a 1 m thick chromite layer are reached after 9 250 years whereas equivalent compaction times are 0 2 0 9 years for olivine both for 2 mm grain size the experiments allow to determine the bulk viscosities of chromite and olivine cumulates to be of magnitude 109 pa s much lower than previously reported as long as melt escape from the compacting cumulate remains homogeneous fluidization does not play any role however channelized melt flow may lead to suspension and upward movement of cumulate crystals in lmis chromitite layers are typically part of a sequence with layers of mafic minerals compaction occurs under the additional weight of the overlying layers and can be achieved in a few years to decades

second look at suspect terranes in southern mexico the boundary between the xolapa and the guerrero mixteca and juarez or oaxaca terranes is a zone of normal faulting indicating north south subhorizontal extension stratigraphic and geochronometric evidence dates tectonic uplift of the xolapa terrane as late cretaceous and tertiary we propose that the xolapa terrane represents a late mesozoic early tertiary magmatic arc built near or on north american continental crust and we discuss as possible tectonic uplift mechanisms 1 extension associated with back arc rifting 2 extension during gravitational spreading of the upper and middle crust and 3 transtension within a strike slip regime established during formation of the caribbean both far and near field deformations indicate distributed transtension therefore a single regional tectonic framework can account for the mesozoic and cenozoic geologic history of these terranes

analogue models of an early cenozoic transpressive regime in southern mexico implications on the evolution of the xolapa complex and the north american caribbean plate boundary

lithospheric thinning by midplate mantle plumes and the thermal history of hot plume material ponded at sublithospheric depths mantle plumes supply hot material to the asthenospheric channel which spreads beneath initially normal oceanic lithosphere at midplate hotspots the fate of this material and the thermal structure of the lithosphere depend on the vigor of secondary convection through the steep thermal gradient above the ponded plume material it has been proposed either that 1 secondary convection is so vigorous that material hotter than the normal mantle adiabat is cooled a short distance downstream of the hotspot such that the geotherm resembles that of younger lithosphere or 2 that the plume material retains its excess temperature for a significant time and the lithosphere thins slowly a parameterized method of modeling transient secondary convection is developed to examine this question calculations indicate that secondary convection is likely to be sluggish unless partial melting greatly lowers viscosity in addition vigorous secondary convection would greatly retard upstream asthenospheric flow of plume material beneath the nose of the hawaiian swell and asthenospheric flow of hot material away from hotspots on slowly moving plates like cape verde this conclusion is compatible with limited seismic evidence that indicates that the lithosphere is not detectably thinned beneath the hawaiian swell

new regional gravity and magnetic maps of the levant a new compilation of regional gravity and magnetic maps of the levant based on detailed local maps and data bases for various parts of the region is presented the most apparent feature of the levant gravity map is the regional high associated with the mediterranean basin high values extend eastward on land to the dead sea rift dsr suggesting a relatively thin basic crust under northern israel and lebanon the magnetic pattern offers additional evidence on the transform nature of the dsr it also suggests that the gulf of suez is associated with an arcuate lineament which extends in a nne direction deep into the mediterranean sea possibly up to the cyprean arc this apparent lineament may indicate an ancient shear zone

southward migration of volcanic activity in the central mexican volcanic belt asymmetric extension within a two layer crustal stretching model abstract magmatic activity in the miocene to present mexican volcanic belt mvb is migrating to the south volcanic activity is associated with extension which is being accommodated along a major 1000 km long e w oriented continental rift in turn the mvb is cross cut by older though active n s to nnw oriented major extensional faults along which the polygenetic volcanoes are aligned a southward migration pattern is also observed for the monogenetic volcanism which however appears to be related to e w and n60 e oriented extensional faults we suggest that the southward migration of volcanic activity can be explained in terms of a two layer crustal stretching model brittle and ductile domains the layers would be separated at an upper crustal level by a zone of simple shear decoupling at the brittle ductile transition zone bdtz the overall movement above the bdtz is southward directed the only direction to which the central mvb can extend and grow our model suggests that the magmas that feed the volcanism become stored at the bdtz evidence supporting this assumption is provided by the al in hornblende thermobarometer and the ol pl sior pseudoternary diagram which indicate average pressures between 2 5 and 4 kbar the magmas feeding the monogenetic volcanism ascend rapidly along active e w and n60 e extensional faults large strain rates i e they do not have enough time to form large magma chambers the magmas feeding the polygenetic volcanism are emplaced along n s to nnw faults lower strain rates these magmas remain stored for longer periods at the bdtz of the n s to nnw faults and therefore form large magmatic chambers shaping vertical overshoots of several kilometers of height the results from geobarometry indicating magma emplacement depth at around 8 km for the polygenetic volcanism and 12 km for the monogenetic volcanism are in good agreement with the rheological constraints of a bdtz at about 10 km of depth we envisage a feedback mechanism regarding magma storage and shallowing of the bdtz i e magma emplacement shallows the bdtz in turn this shallowing controls the new zones for magma emplacement a southward directed process

halogen variations in the paleoproterozoic layered mafic ultramafic intrusions of east kimberley western australia implications for platinum group element mineralization a number of paleoproterozoic layered mafic ultramafic intrusions in the central part of the halls creek orogen of east kimberley western australia have been explored for platinum group elements chromium nickel copper cobalt and gold here we report on the halogen geochemistry of apatite and biotite in a number of these intrusions interstitial apatite is ubiquitous in these intrusions and in most samples tends to be relatively enriched in f and oh end members and relatively poor in cl 20 mole fluorapatite occurs in the more evolved igneous rocks and in samples from the margins of the intrusions that apparently have been contaminated by metamorphic country rock cl f ratios generally increase with bulk rock molar mg mg fe ratios as observed in other intrusions only a few samples show cl enrichment as high as that seen in the stillwater and bushveld complexes beneath the major stratabound platinum group element deposits the most cl rich compositions observed occur in the upper part of the springvale intrusion where it is associated with troctolite and in a single sample from the mcintosh intrusion for the former intrusion it is suggested that volatiles migrating out of the lower part of the mafic stratigraphy stabilized olivine at the expense of pyroxene associated biotite tends to contain little no more than 10 mole cl or f it is concluded that the east kimberley intrusions contained a low to moderate volatile component that during the combined processes of crystallization degassing and fractionation of interstitial halogen bearing minerals produced a late mobile interstitial silicate liquid or volatile rich fluid phase of variable cl f content that in turn caused most of the observed variations within any given intrusion the exceptions include the margins of the intrusions that appear to have been affected by country rocks either during emplacement assimilation or during later metamorphism apatite s generally low cl f ratio the lack of primary amphibole and the high background sulfur concentrations of the east kimberley intrusions suggest that these magmas were relatively volatile poor the development of high grade platinum group element enriched horizons by late stage hydrothermal processes that could have mobilized significant amounts of the platinum group elements and sulfur is considered to be unlikely

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

platinum group element segregation and mineralization in a noritic ring complex formed from proterozoic siliceous high magnesium basalt magmas in the vestfold hills antarctica

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

crustally contaminated komatiite primary source of the chromitites and marginal lower and critical zone magmas in a staging chamber beneath the bushveld complex in the lowermost rocks of the rustenburg layered suite bushveld complex levels of total cr 2 o 3 are up to ten times higher than its putative solubility 0 16 cr 2 o 3 in the magmas that many investigators have accepted as parental to the dunites harzburgites pyroxenites and norites of the lower lower critical and upper critical zones various existing models fail to explain this chromium paradox in so far as they may explain why chromite is precipitated but do not account for its abundance in many cumulates simple mixing calculations and quantitative adiabat 1ph runs support our model in which a heterogeneous liquid crystals system was evolved in a staging chamber beneath the complex by mixing of komatiitic melts with 20 to 30 of crustal melts at depths appropriate to pressures between 10 and 4 5 kbars we contend that crystal settling and the effects of elutriation during magma ascent had generated a high sio 2 cap with modal olivine orthopyroxene and chromite increasing with depth in the staging chamber the magma from the high sio 2 cap of the staging chamber was first injected into the largely sedimentary transvaal supergroup generating what is now termed the marginal zone slurries of crystals plus liquid from the spasmodically rejuvenated main body of the staging chamber were then injected into the shallower bushveld chamber to form the lower and critical zones chromite within the slurries introduced in this way could account for their high gross cr content frequent reversals of normal evolutionary trends evident in the cumulates are attributable to the episodic nature of the influxes from the staging chamber the lithologically diverse nature of the floors upon which the chromitite layers accumulated peridotites pyroxenites norites and anorthosites was therefore more a function of the timing of emplacement of the cr rich slurries generated in the deeper reservoir than of specific evolutionary processes within the bushveld chamber itself

stress transmission across an active plate boundary an example from southern mexico abstract paleostress tensors calculated from inversion of fault slip data at 60 stations between zihuatanejo and tehuantepec in southern mexico were separated into four distinct groups and assigned to time intervals between 150 ma and recent the change in orientation of the maximum horizontal stress closely traces the change in direction of convergence between the north american and the farallon cocos plates for the same time interval depending on the angle between the convergence direction and the plate margin both thrust fault and strike slip stress regimes were active based on this comparison we infer that the stress distribution in the continental crust of southern mexico has been controlled by interplate stress transmission latest cretaceous early tertiary mylonites at the northern edge of the xolapa complex portray left lateral oblique crustal extension and oceanward migration of the upper plate the orientation of extension at mid crustal level grossly correspond to the orientation of the minimum compressive stress at upper crustal level during the last 20 ma a tensional stress regime as a response to surface uplift has been dominant in southern mexico

the halogen geochemistry of the bushveld complex republic of south africa implications for chalcophile element distribution in the lower and critical zones solidification of the interstitial liquid the stratigraphic distribution halogen bearing minerals especially apatite are minor but ubiof s cu and the pge in the critical zone cannot readily be quitous phases throughout the bushveld complex interstitial apatite explained either by precipitation of sulfide as a cotectic phase or as is near end member chlorapatite below the merensky reef lower a function of trapped liquid abundance evidence from potholes and and critical zones and has increasingly fluorian compositions with the pge rich driekop pipe of the bushveld complex imply that increasing structural height above the reef main and upper zones migrating cl rich fluids mobilized the base and precious metal cl f variations in biotite are more limited owing to crystal chemical sulfides we suggest that the distribution of sulfide minerals and controls on halogen substitution but are also consistent with a the chalcophile elements in the lower and critical zones reflects a decrease in the cl f ratio with structural height in the complex general process of vapor refining and chromatographic separation of a detailed section of the upper lower zone to the critical zone is these elements during the evolution and migration of a metalliferous characterized by an upward decrease in sulfide mode from 0 01 cl rich fluid phase 0 1 to trace 0 001 cu tends to correlate with other incompatible elements in most samples whereas the platinum group elements pge can behave independently particularly in the critical zone the decrease in the cl f ratio of apatite in the

birth and decay of an intracontinental magmatic swell early cretaceous tectonics of southern israel the early cretaceous history of southern israel is an example of the formation growth and decay of an in tracontinental swell associated with magmatism in this paper we document and quantify this process and analyze its geody namic mechanism the magmatism and the uplifting contin ued for about 10 myr and produced a 150 to 200 km wide dome from whose crest nearly 1000 m of sediments were re moved shortly after the magmatism ended a period of cooling and subsidence began and sediments started to cover the swell s margins however this subsidence was much smaller than the initial uplift and did not bury the swell s crest which remained elevated for an additional 10 myr the fact that most of the uplift remained indicates that the major cause for uplifting was the permanent buoyancy effect of in trusions transient effects such as heating at the base of the lithosphere and dynamic support of the asthenosphere may have also occurred but their contribution to the local uplifting was minor the distinction between temporal and permanent effects is possible because of the long term history provided by the stratigraphic record a full life cycle of a fossil swell in currently active swells this distinction is much more com plicated and from this point of view this case provides unique information the tectonostratigraphic analysis we present il lustrates how geological observations can be used to calculate tectonic uplift and to estimate accordingly the volume of magmatism that produced it taking into account the sea level rise during uplift and considering the topographic relief that remained after the erosion the calculated tectonic uplift amounts to about 500 m or 40 higher than the uplift inferred only from the amount of erosion thermal mechanical mod eling indicates that this uplift can be explained by intrusions with a volume equivalent to a 4 to 5 km thick sill consistent with other geological and geophysical observations

late cretaceous shortening and early tertiary shearing in the central sierra madre del sur southern mexico insights into the evolution of the caribbean north american plate interaction 1 a wide region in the central part of the sierra madre del sur sms southern mexico records two deformational phases between the late cretaceous and early tertiary the first is a progressive approximately e w shortening phase that spans from the coniacian to earliest paleocene and involves deformation of cretaceous marine sedimentary units the second phase corresponds to paleocene to early eocene deformation that also affects continental sediments and is characterized by gentle folding and counterclockwise rotation of previous shortening structures associated with strike slip faulting here we present geologic geochronologic and structural data of two key areas of the sierra madre del sur the guerrero morelos platform gmp and the huajuapan tamazulapan area in western oaxaca to describe the geometry kinematics and timing of the late cretaceous and early tertiary deformation two regional magmatic episodes constrain the deformational history 1 the first between the maastrichtian and the paleocene 68 57 ma documents the end of late cretaceous shortening in the gmp and 2 the second between the late eocene and early oligocene 37 29 ma has a more regional distribution the time and space analysis of deformation and magmatism in southern mexico led us to exclude flat subduction or collision of the guerrero terrane to the west as the cause for late cretaceous shortening in the gmp considering the similarity in the time and style of deformation with that of the northern chortis block we favor an interpretation in which the tectonic evolution of the central and eastern sms is the result of progressive interaction of the caribbean plate with the southernmost edge of north america since the late cretaceous

postnappe stacking extension and exhumation of high pressure low temperature rocks in the island of crete greece the island of crete consists largely of nappes of contrasting lithologies and metamorphism that were stacked southward during an oligocene early miocene n s compression with the lower nappes undergoing a high pressure low temperature hp lt metamorphism this was followed by a n s miocene crustal extension that caused thinning of the nappes and uplift of the hp lt metamorphic rocks ductile conditions took place in the lower nappes associated with a retrograde greenschist metamorphism while semiductile to brittle conditions took place in the upper nappes a major normal detachment fault separates the lower and the upper nappes the quartz texture analyses and the symmetry of structures indicate bulk coaxial deformation while the results of strain analysis suggest both constrictional and flattening strains a younger ne sw compression affected both the thinned nappe pile and the late miocene pliocene sedimentary basins finally a pliocene ne sw extension led to further uplift and exhumation of the hp lt metamorphic rocks this cyclic tectonometamorphic process of alternate compression and extension took place during the migration of the hellenic orogenic belt toward the most external units including successive tectonic events

uplift and subduction erosion in southwestern mexico since the oligocene pluton geobarometry constraints details of the late oligocene to middle miocene uplift and tectonic erosion episodes of the southwestern continental margin of mexico can be inferred using altot geobarometry of igneous hornblendes geochronology and field relations on the basis of such analyses carried out between acapulco and huatulco we find the following 1 calc alkaline batholiths exposed along the coast from acapulco to huatulco mostly in the 35 25 ma age range were emplaced at depths between 13 and 20 km 2 the contact relationships between these plutons and their host rocks and the exposure of volcanic counterparts 70 km from the coastline indicate a landward decrease in the amount of uplift 3 a comparison of the time differences between intrusion and cooling ages of batholiths along the coast suggest that cooling rates were in general higher between acapulco and huatulco than those along the margin between puerto vallarta and manzanillo 700 km northwest of acapulco 4 the uplift of this coastal belt occurred during the late stages of magmatism and after its cessation triggering intensive subaerial erosion of supracrustal rocks and the exposure of midcrustal rocks such as granitic batholiths and amphibolite facies metamorphic assemblages of the xolapa complex these findings in conjunction with the geometry of the present continental margin as well as the offshore tectonic and stratigraphic features support previous interpretations of very active late oligocene to middle miocene subduction erosion after the onset of strike slip tectonics related to the detachment and subsequent eastward displacement of the chortis block subduction erosion involved both trench sediments and crystalline continental framework rocks different rates of continental framework erosion are assessed on the basis of the bathymetric fluctuations of the upper slope trench sediments and the age of the accretionary prism subsidence of the offshore continental basement suggests intense episodes of basal erosion of lower continental crust whereas the construction of the present day accretionary prism and the uplift of the upper slope indicate a decline in the frontal and basal erosion of the continental framework comparing the calculated depths of pluton crystallization with the present depth of the continental crust subducted slab boundary interpreted using previously published seismic refraction and gravity models we conclude that onshore basal erosion played a subordinate role during miocene episodes of subduction erosion major removal of lower crustal sections was probably restricted to offshore regions plate reconstructions of the cocos plate and its predecessors with respect to north america indicate that the uplift and probably the offshore subduction erosion in this region coincided with the initial stages of the subhorizontal trajectory of the guadalupe plate beneath southwestern mexico

highly siderophile and strongly chalcophile elements in magmatic ore deposits an ore deposit by definition must be economically viable that is to say it must contain sufficient material at high enough grade to make it possible to mine and process it at a profit bates and jackson 1987 this requires the elements to be collected and concentrated by some phase and for them to be deposited close to the surface of the earth at the oxygen fugacities found in the crust native fe is not normally stable and thus the highly siderophile elements defined as ru rh pd re os ir pt and au cannot behave as siderophile elements except in rare cases such as on disko island klock et al 1986 where the magma is sufficiently reduced for native fe to be present however if mafic magmas become saturated in a base metal sulfide liquid the highly siderophile elements behave as highly chalcophile elements table 1 thus these elements are generally found in association with base metal sulfide minerals which crystallized from a magmatic sulfide liquid namely pyrrhotite pentlandite chalcopyrite cubanite pyrite an exception to this is au although au is strongly chalcophile and is produced as a by product from many platinum group element pge deposits table 2 most primary au deposits consist of native au groves et al 1998 these will not be discussed in this chapter there are many pge deposits i e accumulations of pge minerals and base metal sulfides containing pge bates and jackson 1987 around the world but most of these do not constitute pge ore deposits because they are either too small or their grade is too low or other political or infrastructure factors prevent the economic exploitation of the deposit bates and jackson 1987 for the purpose of this work we have defined pge ore deposits as those which have significant production 2 of the annual world

melt generation and fluid flow in the thermal aureole of the bushveld complex granite sheets emplaced into the migmatite zone of the eastern contact aureole of the bushveld complex resulted from fluid enhanced incongruent biotite melting of the underlying silverton formation shales during prograde metamorphism ba concentrations are extreme in both the sheets 1000 ppm and the hornfels 800 ppm into which they have been emplaced we conclude that a ba rich hydrothermal fluid induced melting in the aureole and that fluid transport of ba2 and to a lesser extent sr2 and eu2 persisted in the melt zones under subsolidus conditions sr isotope systematics from high ba localities define an errorchron of 2161 106 ma with an initial 87sr 86sr ratio of 0 705 0 001 metasedimentary rocks unaffected by fluid infiltration were homogenized at the same time but with an increased initial ratio suggesting that whereas isotope homogenization was achieved between outcrops permeated by fluids there is no evidence of regional homogenization oxygen isotope compositions of psammitic metasediments in the aureole are uncorrelated with distance from the contact suggesting the infiltrating fluid equilibrated isotopically with the metasediments their elevated 18o values 11 3 12 1 are consistent with a fluid source from devolatilization of the sedimentary lithologies textural analysis at both outcrop and thin section scale of arkose and psammites in the aureole lakenvalei and magaliesberg formations shows that the extent of melting was highly heterogeneous even on the grain scale and resulted from heterogeneously distributed infiltration of aqueous fluid on dilatant cracks and grain boundaries cathodoluminescence imaging of quartz shows a marked difference in the amount of fine structure with that in the melted rocks having uniform luminescence in contrast to that in rocks containing little or no melt which preserve textures inherited from the regionally metamorphosed protolith simple finite difference thermal modelling of the aureole suggests that the width of the melt zone 500 m is inconsistent with conductive heat transfer and hence that the thermal structure has been modified by fluid advection

the juchatengo complex an upper level ophiolite assemblage of late paleozoic age in oaxaca southern mexico the juchatengo complex jc suite is located between the proterozoic oaxacan complex to the north and the xolapa complex to the south and was amalgamated by late paleozoic magmatism it consists of mafic and sedimentary rocks that have oceanic affinities with internal pseudostratigraphic structural and metamorphic characteristics which resemble a typical upper level ophiolite assemblage new u pb zircon and previous hornblende k ar analyses yield ages of ca 291 313 ma u pb for plagiogranites and ca 282 277 ma for tonalites intruding the entire sequence including pelagic sediments at the top with a maximum deposition age of ca 278 ma and noteworthy local provenance these data constrain the age of the jc to the late pennsylvanian early permian period hf isotopic analyses obtained from zircons in the jc plagiogranite and tonalite show that they come from a similar primitive mantle source 176hf 177hf 0 282539 0 283091 hf t 3 2 to 15 0 hf t values from near 0 to 2 8 in the tonalites indicate a contribution from the continental crust trace elements and ree patterns in whole rock and zircons point to a primitive mantle source for differentiated mafic plagiogranite dykes and tonalitic plutons geochronological and geochemical data address the generation of new oceanic crust above the subduction zone probably in a backarc setting in this tectonic scenario the jc ophiolite originated due to the convergence of the paleo pacific plate below the already integrated oaxacan and acatlan complexes in western pangea the dextral displacement places the deformation in a transtensional regime during the late paleozoic age

re os isotope and pd ru variations in chromitites from the critical zone bushveld complex south africa abstract rhenium osmium isotope measurements of selected chromitites from the critical zone of the bushveld igneous complex reveal that a significant component of radiogenic os was present in the magma when it formed 2 06 ga ago measured 187 os 188 os ratios increase with stratigraphic height and individual layers have similar values across the complex initial 187 os 188 os ratios for chromitites from the western bushveld decrease from 0 140 at the lg to 0 129 at the mg2 chromitite increasing to 0 153 at the ug2 chromitite the inflection at mg2 mg3 is the boundary between the lower and upper critical zone consistent with previous mineralogical and geochemical studies indicating a shift in magma composition at this point chromite separates have os values of 12 35 at 2 06 ga and are too radiogenic for a purely mantle derivation assimilation of 5 mafic granulitic lower crust in a tholeiitic parent magma can explain the radiogenic values pd ru ratios for the intergranular and chromite separate components of each layer differ by orders of magnitude but relative shifts from layer to layer are similar combined with the re os data the results suggest that pge were concentrated into chromitites by magmatic or early deuteric processes that were specific to each layer post layering upward mobilization of pge by hydrothermal processes probably did not play a significant role in pge enrichment

the transformation of southern israel from a swell to a basin stratigraphic and geodynamic implications for intracontinental tectonics abstract the history of southern israel during the 100 myr period between the cretaceous and the eocene is an example of epeirogenic movements which were controlled by magmatism and heating initially this region was uplifted and became a local swell then it subsided more than its surroundings and became a local basin this study analyzes the subsidence history of the region during its cooling for two purposes first to understand the relations between the tectonic pattern of subsidence and the resulting stratal geometry and second to constrain the volume of magmatism that intruded the lithosphere and perturbed the isostatic equilibrium our results show that lateral changes in the intensity of the tectonically driven subsidence are not always expressed by simple corresponding changes in the thickness of sedimentary units the most prominent example of this was found in the cenomanian section which thickens northward in absolute contrast with the tectonic pattern of subsidence during its deposition the reason for this discrepancy lies in lateral differences in the compactibility of the substrate upon which these sediments were deposited the two most important factors controlling the compactibility of the substrate are the thickness of previously existing sediments and their history of uplift since sediments that are reburied after an uplifting phase do not compact significantly until they reach their previous maximal depth we emphasize the need to reveal the amount of exhumation from the amount of erosion hidden in unconformities our second aim was achieved only in part because we cannot distinguish the subsidence caused by midlithospheric intrusions from secondary factors one possible scenario that satisfies the observations is a combination of midlithospheric intrusions equivalent to a 15 km thick sill plus thermal thinning of the lithosphere of the same order

sequence stratigraphic analysis of the aptian albian lower greensand in southern england abstract facies analysis and its use in sequence stratigraphy is described using the lower greensand of southern england as an example and the resulting interpretations are outlined this work forms an independent test of sequence stratigraphic techniques and the exxon cycle chart sequence stratigraphy is a very powerful tool in the analysis of problematic successions although some modification is made here in emphasizing the use of the transgressive surface changing palaeoenvironmental conditions and the effect that palaeoceanography has on the recognition of sequence boundaries are also discussed broad agreement is found between these results and the cycle chart although changing tidal conditions alter the criteria available for analysis of the aptian albian lower greensand succession comparison between this study and sequence stratigraphic analyses conducted in the more condensed type sections of england and france shows that further sequence boundaries are recognizable

latest cretaceous to miocene deformation events in the eastern sierra madre del sur mexico inferred from the geometry and age of major structures the cretaceous cenozoic major lithologic units and structures of the sierra madre del sur are well known the laramide orogeny is generally considered as the cause of the contractile structures but the details about the migration kinematics and intensity of deformation are poorly known furthermore the deformation events responsible for the post laramide strike slip and normal faults have not been identified in this paper we document the migration of the deformation events that occurred in southern mexico from maastrichtian to miocene time we identify different groups of structures representing three successive deformation events based on the geometry age and kinematics of tectonic structures deformation migrated from west to east the first event corresponding to the laramide orogeny occurred during late cretaceous time in the guerrero morelos platform and ended in the middle eocene in the east within the veracruz basin the oaxacan fault system which bounds the acatlan oaxacan block to the east records laramide shortening from six structural sections we interpret the juxtaposition of the oaxacan complex against the mylonite belt of the sierra de juarez with subsequent uplift of the eastern border of the oaxacan complex and finally the gravitational overriding of the sedimentary cover in a radial centripetal arrangement the second event produced strike slip faulting during ne sw horizontal shortening from eocene to oligocene time the third event produced normal and strike slip faults indicating ne sw horizontal extension during oligocene miocene time major structures produced during these three deformation events are roughly distributed in an arcuate pattern bounding the block formed by the acatlan and oaxacan complexes based on this pattern and the relatively less deformed mesozoic rocks within the acatlan oaxacan block we interpret that most of the deformation resulted from the impingement of this block on thinner crustal domains adjoining the block

a plate tectonic model for the mesozoic and early cenozoic history of the caribbean plate abstract we present a model in which the caribbean plate is an intra american feature formed along the caribbean spreading center as opposed to the current model that considers the caribbean plate as a far travelled crustal segment that formed in the pacific region paleomagnetic data which cover an age range from jurassic through paleocene indicate the ophiolite complexes in costa rica and panama formed in an equatorial position the ophiolites did not fundamentally change their position relative to south america since their origin basaltic rocks of the lower part of the ophiolites are of mid ocean ridge type suggesting formation at a spreading center they are interpreted to have formed as part of the proto caribbean crust at a spreading axis in an intra american position during jurassic and early cretaceous times the upper part of the ophiolites is mainly built up by island arc and intraplate basalts the island arc basalts evolved at the central american landbridge which started in the middle cretaceous the intraplate basalts are related to the caribbean plateau basalt which thickened and stiffened the caribbean crust in middle cretaceous to probably campanian times changes in relative plate motions along the middle american convergent plate margin are reflected by orientations and time sequences of paleostress tensors calculated from fault slip data in southern mexico and costa rica kinematic indicators from mylonite zones display sinistral movement along secondary shear zones within the southern mexican crustal complexes and along the northern boundary of the chortis block in the motagua polochic fault system this reflects the eastward drift of the chortis block since the beginning of the cenozoic at this time the chortis block became part of the caribbean plate which moved more than 1000 km towards the east relative to the north and south american plates during the cenozoic

a cretaceous and jurassic geochronology an integrated geomagnetic polarity and geologic time scale for the jurassic and cretaceous periods is presented based on various methods according to the availability of definitive isotopic ages an age calibrated sea floor spreading model is used to interpolate the ages of the kimmeridgian to barremian and the campanian to maestrichtian stages numerical age estimates for the aptian to santonian stage boundaries follow published isotopic age determinations the hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the hettangian to oxfordian stages the new scale results in ages of 208 ma for the base of the jurassic 144 ma for the jurassic cretaceous boundary and 66 5 ma for the top of the cretaceous the integrated biostratigraphic magnetostratigraphic and geochronometric record serves as a working hypothesis for geologic correlation of jurassic and cretaceous strata

accelerated subsidence and sedimentation in the levant basin during the late tertiary and concurrent uplift of the arabian platform tectonic versus counteracting sedimentary loading effects 1 since the middle eocene the northwest arabian platform has been emerging from the water and rising above sea level whereas the adjacent levant basin has been subsiding and accumulating a thick sedimentary section this study investigates these opposing vertical motions and the relative roles of tectonic driven versus isostatic adjustment forces involved such a distinction is strongly dependent on a reliable estimation of the paleobathymetry which in pelagic environments may vary substantially we use two different methods to estimate the paleo water depth in the deep levant basin in the tertiary time isostatic balancing calculations with the adjacent inland region are employed to estimate the paleo water depth in the deep levant basin in the middle eocene just before the commencement of the late tertiary tectonic phase for later periods we use morpho structural elements such as abrasion surfaces and incised canyons to build laterally changing topo bathymetry profiles our results indicate that in the mid eocene water depth in the deep levant basin was 2 3 5 km which gradually decreased to 1 5 km today based on these results our analysis shows that the enhanced subsidence of the levant basin reflects an isostatic response to sedimentary filling of a pre existing deep water basin with no involvement of a downward tectonic force on the contrary we suggest that the regional tectonic force was upward counteracting a sedimentary loading effect further regional implication of this understanding is that the cause for uplifting and exposure of the nw arabian platform in the late tertiary extended far westward beyond the inland region

transgressions regressions and relative sea level changes on the cretaceous shelf of israel and adjacent countries a critical evaluation of cretaceous global sea level correlations cretaceous transgressive regressive cycles over the northern margins of the arabo nubian craton in the region of present day israel and the adjacent countries were reconstructed by tracing the position of the ancient tethyan shorelines many of these cycles suggest inverse relationships to the relative sea level changes deduced from litho and biofacies analyses transgressions are frequently associated with shallowing resulting in local exposure the following rapid relative sea level rise is associated with regressive oscillations caused by prograding sedimentation generally the relative sea level gradually rose throughout the cretaceous as reflected by the onlapping trend of the marine sediments with the exception of one greater relative sea level drop in the middle turonian probably tectonically induced all other relative sea level variations were short stillstands and very minor falls the dominant tectonic control on the relative sea level changes and transgressive regressive cycles during the late coniacian maastrichtian times and on the middle turonian and early late coniacian shallowing events recognised in israel render their previous attribution exclusively to eustasy erroneous the opposing relationship between relative bathymetric changes and transgressive regressive tendencies as well as the poor biostratigraphic control in many of the sea level curves compiled on the basis of these two criteria raise additional doubts as to their presumed significance for global correlations the present study supports jeletzky s 1977 1978 critical analysis and his call for similar critical re evaluation of previous studies throughout the world rather than the emendation of data to improve correlation to a given scheme

structural and compositional constraints on the emplacement of the bushveld complex south africa abstract despite a plethora of petrological studies the emplacement mechanics of the world s largest layered intrusion the 2 05 ga bushveld complex are still poorly understood early models considered the intrusion to comprise separate lopolithic intrusions or even concentric cone sheets but recently overwhelming support for a sill like intrusional form has emerged examination of the contact aureole reveals three groups of emplacement related structures interfinger deformation zones and bridges formed between intruding and dilating magma fingers preserved in both the western and eastern parts of the complex sub bushveld complex intrusions that reflect finger like emplacement into the underlying contact aureole and diapiric domes that characterize the eastern contact aureole formed by diapiric amplification of initial interfinger deformation zones associated with the earliest mafic ultramafic pulse of the bushveld complex most of these structures exhibit a strong nw se preferred orientation while longitudinal terminations and divergence of the sub bushveld intrusions away from their source horizons indicate magma emplacement towards the se this emplacement direction is supported by petrological data lateral lithological facies variations and mineral chemical variations within specific horizons the thickest and most chemically primitive accumulations of the lower zone of the complex are found adjacent to and thin towards the ne and se away from the thabazimbi murchison lineament tml a crustal scale lineament that has undergone polyphase reactivation since at least 2 7 ga the bushveld complex was most likely fed by a feeder dyke that utilised the tml spreading laterally from the dyke axis to form its current sill like geometry the stress field at 2 05 ga was suitably oriented to allow for dilation of the ene trending tml suggesting the kaapvaal craton was subject to a component of nw se extension during this time nw se directed collision of the kaapvaal and zimbabwe cratons is recorded in the limpopo belt which would suggest emplacement under conditions of far field extensional stress within a back arc setting this is corroborated by petrological data that supports a significant component of subducted crust in the bushveld magmas

halogens of bushveld complex south africa δ37cl and cl f evidence for hydration melting of the source region in a back arc setting abstract the bushveld complex is one of only a few layered intrusions anomalously enriched in chlorine this study presents the first chlorine stable isotope analyses of bushveld and associated rocks designed to constrain the source of bushveld cl most samples overlap the range of values for pristine morb and altered seafloor samples 37 cl 4 relative to sea water this contrasts with distal metasedimentary floor rocks 37 cl 0 with little evidence of contamination away from the marginal rocks the cl isotope composition and high cl f ratio is therefore a fundamental characteristic of the lower and critical zone magmas and not derived from infiltration from the country rock these results the high cl f ratio and the boninitic arc like character of bushveld magma are consistent with hydration partial melting of a depleted source in a subduction related setting with cl supplied either by the dehydration of a subducted slab with a heavy cl signature or an overridden wet mantle hotspot

ultramafic rocks in the centre of the vredefort structure south africa geochemical affinity to bushveld rocks abstract the geochemistry and mineral chemistry of ultramafic rocks in the centre of the vredefort structure have been reinterpreted a detailed examination of previously published major element trace and ree analyses as well as mineral analyses strongly suggests a magmatic origin in contrast to previously postulated tectonic emplacement of mantle material a strong geochemical affinity to ultramafic rocks from the bushveld complex is evident the magmatic nature of the ultramafic rocks in the centre of the vredefort structure has implications for the discussion of the metamorphic history of the vredefprt structure which shows evidence of unusually high and as yet not satisfactorily explained geothermal gradients during peak metamorphism

brief reversed polarity interval during the cretaceous normal polarity superchron a striking feature of the geomagnetic time scale is the 34 m y period of normal polarity field bias known as the cretaceous normal polarity superchron new paleomagnetic data from three deep sea drilling project cores sites 317a 402a and 463 indicate that a brief interval of reversed polarity occurred during the superchron this reversed polarity interval may help refine past motions of the pacific plate and aid resolution of the statistical structure of the geomagnetic reversal chronology

are bushveld u type parent magmas boninites or contaminated komatiites three distinct categories of magmas bushveld u type parent magmas boninites and siliceous high magnesium basalts from archaean greenstone belts share the distictive geochemical characteristics of high mgo 9 19 low tio2 less than 1 and high sio2 greater than 52 boninites are generally thought to form by hydrous melting of metasomatized previously depleted upper mantle while siliceous high magnesium basalts shmb in greenstone belts have recently been recognized as the products of combined fractionation and crustal contamination of komatiites both these mechanisms can apparently give rise to similar end products and both mechanisms have been proposed for the petrogenesis of bushveld u type magmas

cr and sr keys to parental magmas and processes in the bushveld complex south africa abstract large layered intrusions are almost certainly periodically replenished during their protracted cooling and crystallization the exact composition s of the replenishing magma s in the case of the bushveld complex south africa has been debated mainly on the basis of major element composition and likely crystallization sequences the intrusion is dominated by orthopyroxene and plagioclase and so their cr and sr contents and likely partition coefficient values can be used to re investigate the appropriateness of the various proposed parental magmas one magma type with about 12 mgo 1000 ppm cr and 180 ppm sr can explain the genesis of the entire lower and critical zones a number of other magma compositions proposed to produce the critical zone fail to match these trace element constraints by being too poor in cr a fundamentally different magma type was added at the base of the main zone but none of the proposed compositions is consistent with the trace element requirements specifically the cr contents are higher than predicted from pyroxene compositions a further geological constraint is demonstrated from a consideration of the cr budget at this level there is an abrupt decrease from about 0 4 to 0 1 cr 2 o 3 in orthopyroxene across this critical zone main zone transition no realistic proportions of mixing between the residual magma at the top of the critical zone and any proposed added magma composition can have produced a composition that could have crystallized these low cr orthopyroxenes instead it is suggested that the resident magma from the upper critical zone was expelled from the chamber possibly as sills into the country rocks during influx of a dense differentiated magma near the level of the pyroxenite marker in the main zone there is further addition of a ferrobasaltic magma with 6 mgo 111 ppm cr and 350 ppm sr that is consistent with the geochemical requirements

lateral thickening of sedimentary units sediment compaction vs tectonics abstract when a new sedimentary unit is deposited over a region with significant differences in the thickness of the pre existing sedimentary cover the thickness of the new unit may express lateral differences in sediment compaction and not in the intensity of the tectonic driven subsidence this effect is examined here and quantified as a function of the thickness of the pre exiting section it is shown that when the thickness of the pre existing section varies from nearly zero to a few kilometers this effect may easily dominate the tectonic driven subsidence when the pre existing section is thicker than 5 km over the entire region this effect decreases significantly and when the sedimentary cover is more than 10 km thick over the entire basin this effect has no real influence on the trend of lateral thickening another related effect discussed here is the reburial of uplifted regions in light of the assumption that reburied sediments do not compact anymore until they reach their previous maximal depth this emphasizes the need to consider the amount of uplift in backstripping in order to understand the later tectonics

trace element and platinum group element distributions and the genesis of the merensky reef western bushveld complex south africa the merensky reef of the bushveld complex is one of the world s largest resources of platinum group elements pge however mechanisms for its formation remain poorly understood and many contradictory theories have been proposed we present precise compositional data major elements trace elements and platinum group elements pge for 370 samples from four borehole core sections of the merensky reef in one area of the western bushveld complex trace element patterns incompatible elements and rare earth elements exhibit systematic variations including small scale cyclic changes indicative of the presence of cumulus crystals and intercumulus liquid derived from different magmas ratios of highly incompatible elements for the different sections are intermediate to those of the proposed parental magmas critical zone and main zone types that gave rise to the bushveld complex mingling but not complete mixing of different magmas is suggested to have occurred during the formation of the merensky reef the trace element patterns are indicative of transient associations between distinct magma layers the porosity of the cumulates is shown to affect significantly the distribution of sulphides and pge a genetic link is made between the thickness of the merensky pyroxenite the total pge and sulphide content petrological and textural features and the trace element signatures in the sections studied the rare earth elements reveal the important role of plagioclase in the formation of the merensky pyroxenite and the distribution of sulphide

a possible mechanism for epeirogenic uplift continental geology is dominated by vertical movements some of these are the result of crustal shortening and extension associated with large horizontal displacements involving processes which are now understood at least in outline however both uplift and subsidence can occur without associated shortening or extension of the continental crust most investigations of such epeirogenic movements have been concerned with subsidence partly because of the economic importance of the resulting sedimentary basins and partly because of the existence of a simple model for the process1 which can account for the development of several basins in some detail2 4 however uplift has been relatively neglected and cannot be produced by the same mechanism the purpose of this paper is to examine whether epeirogenic uplift can result from the intrusion of large thicknesses of basic magma into the lower part of continental crust this suggestion was considered as a possible mechanism for the regional uplift of the colorado plateau by gilbert5 more than 100 years ago and also by holmes6 though both rejected the idea however a recent discussion of no less than 14 different mechanisms for generating uplift7 did not include this process even though it seems able to produce the observed vertical motions and also to account for some of the features of the evolution of sedimentary basins that cannot be explained by the uniform stretching model3 8 9 although the suggestion is not new the argument used here to support it is and depends on an understanding of the relationship between ocean island volcanoes and mantle circulation

sr and pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the bushveld complex south africa microdrilling and progressive leaching evidence for sub liquidus contamination within a crystal mush progressive leaching of plagioclase for sr iso topes and microdrilling for sr and pb isotopes from grains of plagioclase and orthopyroxene from the critical zone and the lower zone indicates that these minerals are not in isotopic equilibrium leaching suggests critical zone plagioclase either lost rb or had a more radiogenic sri rim relative to the core whereas plagioclase from an upper zone sample is isotopically homogeneous for sri microdrilling analyses of plagioclase from the lower and critical zones consistently have a higher initial 87 sr 86 sr sri and a less radiogenic modeled 238 u 204 pb composition l2 than coexisting ortho pyroxene the range of calculated sri for plagioclase and orthopyroxene is 0 70506 0 70662 34 and 0 70290 0 70654 36 respectively the average difference in sri between mineral pairs was 0 00095 the range of calculated l2 for plagioclase and orthopyroxene is 9 42 10 30 average 9 7 and 9 83 15 75 average 10 1 respectively the range of measured 208 pb 206 pb for plagioclase and orthopyroxene is 34 757 36 439 33 and 36 669 41 845 85 respectively one orthopyroxenite without evidence for more than one population of crystal size distribution nonetheless had sri 0 70654 36 with calculated l2 of 10 32 for larger grains as compared with sri 0 70290 32 and calculated l2 of 9 97 for smaller grain size fractions isotopic results from this study demonstrate that whole rock isotopic data may not provide the appropriate level of detail necessary to address some processes in the bushveld complex however systematic changes have the potential to elucidate the timing of contamination with regard to other processes crystal aging compaction driven recrystallization and mineral exsolution occurring within a slowly cooled crystal liquid vapor mush system

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

geochemical constraints on the petrogenesis of the kamiskotia gabbroic complex and related basalts western abitibi subprovince ontario canada barrie c t gorton m p naldrett a j and hart t r 1991 geochemical constraints on the petrogenesis of the kamiskotia gabbroic complex and related basalts western abitibi subprovince ontario canada precambrian res 50 173 199 the kamiskotia gabbroic complex kgc is a large 170 km 2 late archean synvolcanic intrusion located in the western abitibi subprovince canada it is divided into four stratigraphic units a lower zone lz comprised of adcumulus and mesocumulus peridotites troctolites and gabbro norites mg number 86 73 a middle zone mz comprised of mesocumulus gabbro norites mg number 74 60 mesocumulus and orthocumulus gabbro norites and ferroan gabbro norites of the upper zone uz mg number 64 37 which are overlain and partly along strike with granophyric rocks of intermediate to felsic composition the cumulates are characterized by fiat ree patterns with lan ybr 0 4 2 6 and range from 0 6 2 chondrites for olivine bearing lz adcumulates to 10 25 chondrites for upper uz meso to orthocumulates europium anomalies are strongly positive for the lz and and diminish upsection to the upper uz which has slight negative eu anomalies chilled rocks are found in mixed magma and contact chill outcrops and have a mg number of 54 58 lan ybn 0 8 1 2 and slight negative eu anomalies phase relationships and mass balance calculations using the ree indicate that lz cumulates may be derived from the average chill composition similarly upper uz cumulates may be derived from evolved basalts which overlie the kgc assimilation fractional crystallization modeling indicates that the evolved basalts may be derived from the average chill composition by 70 80 fractionation at low pressures or lower degrees of fractionation accompanied by recharge of the average chill composition into the system the basalt compositions do not reflect significant assimilation of nearby felsic rocks or average archean crust during fractionation primitive kgc liquids are estimated by adding an average olivine bearing adcumulate composition to the kgc chill composition in equal proportions to obtain a liquid with an mg number of 70 the ree content of such a liquid is consistent with a derivation by 34 partial melting of a chemically primitive mantle neodymium isotope ratios for kgc cumulates mineral separates and overlying volcanic rocks plot on a 2710 ma isochron with nd t 2 5 0 8 in close agreement with u pb zircon ages for the kgc and kamiskotia felsic volcanic rocks at 2707 2 ma and 2705 2 ma respectively the initial value is identical to the isotopically depleted morb iike abitibi mantle signature at 2 7 ga and provides supporting evidence for fractionation dominated supracrustal processes with no detectable assimilation of an isotopically enriched crustal component kamiskotia mafic magmatism is similar to more recent rift related systems represented by the skaergaard intrusion east greenland and differentiated tholeiitic volcanic fields in eastern iceland and along the galapagos spreading center

large scale rotations of the chortis block honduras at the southern termination of the laramide flat slab abstract paleogeographic reconstructions place the chortis block adjacent to southern mexico in the late cretaceous and earlier time forming the tectonically disturbed southwestern tip of the north american plate this study assesses the relative motion between chortis and north america during the laramide orogeny later development of the north america caribbean plate boundaries we carried out paleomagnetic analysis of rotations and latitudinal displacement using a total of 90 sites from the atima limestone and the valle de angeles formation redbeds sampled at seven localities in honduras two secondary but ancient magnetizations post date late cretaceous folding at most localities the oldest magnetization is a chrm of high laboratory unblocking temperature in the redbeds 650 c and a moderate temperature in the limestones 500 600 c this component is wnw directed at five localities varying somewhat in declination having a consistent moderate positive inclination mean i 39 7 95 6 8 n 53 sites the age of the chrm is bracketed between 85 and 63 ma based on youngest zircon age and the age of a cross cutting dike an intermediate unblocking temperature component present in both units dec 146 2 inc 28 5 k 13 1 95 6 8 n 37 sites is interpreted as an overprint the chrm indicates an average counterclockwise rotation of 55 4 5 7 with respect to the expected direction the younger overprint was acquired between 50 and 15 ma its mean is also discordant with the expected north america direction implying some neogene rotation of chortis abundant late albian turonian zircons in valle de angeles redbeds explicitly require extending the laramide arc along western mexico into chortis a reconstruction of chortis south of mexico that takes into account 55 degrees of rotation shows a gap between both continental blocks at about 70 ma we propose a model where this gap formed by back arc extension creating the late cretaceous chontal basin at the southern termination of the laramide flat slab

proterozoic c type eclogites hosting unusual tife cr cu mineralization in northeastern brazil abstract mafic to ultramafic rocks occur at bodoco about 640 km west of recife cofolded with early proterozoic gneisses and migmatites close to the tectonic contact with middle to late proterozoic metasedimentary rocks of the salgueiro cachoeirinha group the jadeite content of clinopyroxenes the fe mg partitioning between clinopyroxene and garnet the si activity in phengite and the na content of amphiboles in retrograde symplectites in gabbroic members suggest a progressive metamorphic evolution up to c type eclogites at 13 kbar 525 c followed by a retrograde path of medium p t ratio to the greenschist facies the final event transformed parts of these mafic rocks into banded amphibole gneisses and migmatites the metagabbroic members contain up to 12 rutile metapyroxenitic members bear two types of massive ores a cumulatic cr ti spinel bands and b network ilmenomagnetite the cr ti spinel is unusual in that it has a composition between ulvospinel magnetite and chromite and has a zonation of cr mg enrichment from core to rim which may be explained by peripheral ti loss during high pressure metamorphism several ti occurrences in the floresta area 400 km west of recife including the 20 mt riacho da posse deposit show striking similarities with the bodoco occurrences 1 massive ores with high ti fe ratios related to ultramafic rocks olivine cumulates 2 gabbroic intercalations composed of only garnet and amphibole with the same mineral compositions recorded in bodoco lacking plagioclase but rich in rutile and 3 migmatitic gneissic wall rocks in the vicinity of the contact with younger metasedimentary rocks a similar tectono stratigraphic setting a few km from the contact with the late proterozoic macururo group and high pressure metamorphic host rocks mafic granulites has been described in the literature for the serrote da lage deposit of massive titanomagnetite with a 60 mt interstitial copper mineralization the only other eclogite occurrence recorded in brazil at pouso alegre state of minas gerais has an identical geological setting chemistry of the metagabbroic rocks of bodoco and floresta indicate an oceanic tholeiitic picritic character while the rare earth element patterns of the ultramafic rocks suggest a modified ocean floor peridotite nature positive co ni and very high cu pd ratios in the sulphides also suggest an oceanic origin all these features with exception of the high ti fe ratio support an ophiolitic character of the mafic rocks a detailed geochronological study may help to determine whether the mafic ultramafic rocks represent relics of a true oceanic crust of the same age as the salgueiro cachoeirinha group obducted onto older continental crust on an active late proterozoic continental margin or if the proximity to the contact between basement and metasedimentary rocks is coincidental

early precambrian crustal development changing styles of mafic magmatism the work of sutton watson 1951 was one of the cornerstones in the interpretation of the evolution of early precambrian archaean and early proterozoic gneiss complexes and in the documentation of mafic dyke swarms that intruded recently accreted archaean cratons since then the concept of geological terranes and terrane accretion has been extended back to the archaean and different events can be distinguished by increasingly precise isotopic dating techniques there is growing evidence that with certain clear provisos the principle of uniformitarianism can be applied as equally to the early precambrian as it can through the rest of the geological record in terms of mafic magmatism there are similarities between some ancient and modern suites both intrusive and extrusive but there are also some major differences such as the presence of komatiites in the archaean and the formation of large scale layered intrusions and extensive dyke swarms at the end of the late archaean and in the early proterozoic the mantle has evolved continuously with the progressive extraction of mafic magmas but allowing for this continental tholeiites appear to have changed little with time ancient mid ocean ridge basalts are elusive because of their poor preservation record the geochemistry of mafic volcanics occurring in many greenstone belts commonly suggests a continental contamination component and that these volcanic rocks are most closely akin to those from present day arc systems the relative abundance of early precambrian intrusive mafic suites parallels the crustal growth curve reflecting the global crustal accretion differentiation superevent towards the close of the archaean bimodal tholeiitic noritic mafic magmatism associated with many of these intrusive suites stems from this rapid turnover in the mantle mafic crust continental crust system this in turn gives a possible link between the respective evolutions of archaean to recent komatiitic noritic and boninitic suites

earthquake swarms and slow slip on a sliver fault in the mexican subduction zone the mexican subduction zone is an ideal location for studying subduction processes due to the short trench to coast distances that bring broad portions of the seismogenic and transition zones of the plate interface inland using a recently generated seismicity catalog from a local network in oaxaca we identified 20 swarms of earthquakes m

constrains on the origin and history of the eastern mediterranean basin abstract the basin formed along the northern passive margin of gondwanaland as a relict of the mesozoic neotethys in early mesozoic time though direct information from the basin itself is meagre the geology of its passive margins and the regional framework of the tethys evolution constrain the formation and history of the basin the eastern mediterranean and its passive margins are interpreted to have formed as a result of several faulting and continental breakup phases from early permian to middle jurassic times before pangaea s complete disintegration this led to the detachment of the eratosthenes and tauride blocks from the margins of the levant and herodotus basins respectively the latter merged in the northeast with the cyprus basin that extended along arabia s northern margin the main rifting probably occurred in the triassic leading to seafloor spreading in the herodotus and cyprus basins while along the levant margin important rifting also occurred in the jurassic during the initial rifting stages extension perpendicular to the levant margin was important but subsequently the tauride block drifted mainly northward the existence of the passive margins and slopes of the eastern mediterranean is documented at least from the end of the middle jurassic until then the basin was probably narrow 200 300 km but during the main phase of the africa laurasia separation it grew to twice its present width or more this scenario depends critically on the interpretation that the tauride block was originally located next to northeastern africa on the western side of the eratosthenes block plate convergence and consumption of the northern portion of the basin have taken place since mid cretaceous times but the active convergent boundary on its northern side formed only in the neogene by modification of earlier structures some intra plate deformation also took place especially in the neogene but it hardly changed the original configuration of the remaining portion of the basin

crustal evolution of island arc ultramafic magma galmoenan pyroxenite dunite plutonic complex koryak highland far east russia alaskan type platinum bearing plutons and potassium enriched mafic to ultramafic volcanic rocks are temporally and spatially associated within the late cretaceous paleocene achalvayam valaginskii intra oceanic palaeo arc system allochthonousy present in the kogak highland and kamchatka peninsula far east russia the compositions of the parental magmas to the alaskan type complexes are estimated using the galmoenan platonic complex as an example this complex composed of dunites pyroxenites and minor gabbros is the largest 20 km3 in the system and the best studied owing to associated platinum placer deposits the compositions of the principal mineral phases in the galmoenan intrusive rocks olivine fo 79 92 clinopyroxene 1 3 5 wt al2o3 0 1 0 5 wt tio2 and cr spinel 5 15 wt al2o3 and 0 3 0 7 wt tio2 are typical of liquidus assemblages in primitive island arc magmas in intra oceanic settings and closely resemble the mineral compositions in the achaivayam valaginskii ultramafic volcanic rocks the temporal and spatial association of intrusive and extrusive units and the similarity of their mineral compositions suggest that both suites were formed from similar parental magmas ne composition of the parental magma for the galmoenan platonic rocks is estimated using previously reported data for the achaivayam valaginskii ultramafic volcanic rocks and phenocryst hosted melt inclusions quantitative simulation of crystallization of the parental magma in the galmoenan magma chamber shows that the compositions of the cumulate units are best modelled by fractional cryallization with periodic magma replenishment the model calculations reproduce well the observed mineral assemblages and the trace element abundances in clinopyroxene based upon the estimated composition of the parental magmas and their mantle source we consider that fluxing of a highly refractory mantle wedge similar to the source of boninites by chlorine rich aqueous fluids is primarily responsible for both high degrees of partial melting and the geochemical characteristics of the magmas including their enrichment in platinum group elements