stable organic carbon isotope stratigraphy across oceanic anoxic event 2 of demerara rise western tropical atlantic ocean drilling program odp leg 207 recovered expanded sections of organic carbon rich laminated shales on demerara rise western tropical atlantic high resolution organic carbon isotope and total organic carbon toc records are presented which span the cenomanian turonian boundary interval ctbi including the oceanic anoxic event oae 2 from four sites oriented along a nw striking depth transect these records represent the first high resolution carbon isotope records across oae 2 from the south american margin of the tropical atlantic due to the scarcity of age significant fossils the main purpose of this study was to develop a detailed carbon isotope stratigraphy in order to correlate the ctbi across the depth transect and to tie this to biostratigraphically well defined sections in the western interior basin pueblo usa boreal shelf seas eastbourne england and western tethys oued mellegue tunisia all four sections studied document a 6 increase of 13corg values at the base of the ctbi which is followed by an interval of elevated 13corg values and a subsequent decrease our results supply an important stratigraphic base for subsequent paleoceanographic studies on late cenomanian to early turonian sediments from demerara rise and elsewhere

paragenetic and mineral chemical relationships in orthoamphibole bearing gneisses from enderby land east antarctica a record of proterozoic uplift abstract polymetamorphic orthoamphibole bearing gneisses from the vicinity of shear zones in casey bay enderby land antarctica record both the overprinting of archaean granulite lithologies by proterozoic metamorphism and the subsequent evolution of the latter episode during localized deformation mineral chemistry and zoning relationships in orthoamphibole garnet kyanite quartz and later orthoamphibole garnet cordierite quartz assemblages are used together with interpretation of reaction and corona textures to constrain the proterozoic pressure temperature path experienced by the rocks consideration of reaction topologies p t x fe mg a1 relationships in orthoamphibole bearing assemblages and standard geothermobarometry indicate that the gneisses underwent a near isothermal decompression p t history steep positive dp dt from 8 kbar and 700 c to 5 5kbar and 650 c this uplift path is correlated with the general effects of rayner complex metamorphism and deformation which occurred after 1100 ma in a major erogenic belt south of casey bay

an unusual high mg garnet spinel orthopyroxenite from southern india evidence for ultrahigh temperature metamorphism at high pressure conditions we report here a garnet spinel orthopyroxenite in close association with an ultrahigh temperature uht granulite from the central part of the madurai granulite block in southern india the garnet spinel orthopyroxenite is almost entirely composed of orthopyroxene spinel and rare garnet in a granular texture spinels in the rock are characterized by high mg x mg 0 69 0 71 with low cr and fe 3 consistent with compositions reported from spinels occurring within xenoliths in kimberlites and high pressure temperature p t alpine complexes the orthopyroxenes have high al content al 2 o 3 up to 4 85 wt typical of equilibration under high p t conditions the p t estimates derived for the garnet spinel orthopyroxenite indicate temperatures of around 1000 c and pressures exceeding 17 kbar the data indicate that uht metamorphism in this locality traversed from above 17 kbar to 11 kbar prior to the final stage of isothermal decompression our study reports the highest pressures obtained by far for extreme crustal metamorphism in southern india and elsewhere in gondwana the multi stage decompression observed in the uht rocks associated with the high p t garnet spinel orthopyroxenite could be correlated to extension of the crust and possibly of the lithospheric mantle and or its delamination with the asthenospheric mantle as the ultimate heat source during the final stage of amalgamation of the gondwana supercontinent

ultrahigh temperature metamorphism and decompression history of sapphirine granulites from rajapalaiyam southern india implications for the formation of hot orogens during gondwana assembly sapphirine bearing mg al granulites from rajapalaiyam in the southern part of the madurai block provide critical evidence for late neoproterozoic cambrian ultrahigh temperature uht metamorphism in southern india poikiloblastic garnet in quartzo feldspathic and pelitic granulites contain inclusions of fine grained subidioblastic to xenoblastic sapphirine associated with quartz suggesting that the rocks underwent t 1000 c peak metamorphism quartz inclusions in spinel within garnet are also regarded as clear evidence for a uht condition inclusions of orthopyroxene within porphyroblastic garnet in the sapphirine bearing rocks show the highest al 2 o 3 content of up to 10 3 wt suggesting t 1050 1070 c and p 8 5 9 5 kbar temperatures estimated from ternary feldspar and other geothermometers t 950 1000 c further support extreme thermal metamorphism in this region xenoblastic spinel inclusions in sapphirine coexisting with quartz suggest that the spinel quartz assemblage pre dates the sapphirine quartz assemblage probably implying a cooling from t 1050 c or an anticlockwise p t path the fmas reaction sapphirine quartz garnet orthopyroxene sillimanite indicates a cooling from the sapphirine quartz stability field after the peak metamorphism corona textures of orthopyroxene cordierite sapphirine orthopyroxene sapphirine cordierite and cordierite spinel around garnet suggest subsequent near isothermal decompression followed by decompressional cooling toward t 650 750 c and p 4 5 5 5 kbar the sapphirine quartz association and related textures described in this study have an important bearing on the uht metamorphism and exhumation history of the madurai block as well as on the tectonic evolution of the continental deep crust in southern india our study provides a typical example for extreme metamorphism associated with collisional tectonics during the late neoproterozoic cambrian assembly of the gondwana supercontinent

gedrite garnet sillimanite bearing granulites from amessmessa area south in ouzzal hoggar algeria some granulites from the amessmessa area south in ouzzal unit hoggar contain the peak assemblage gedrite garnet sillimanite quartz that was used to estimate the p t conditions of metamorphism the rocks developed symplectites and corona textures by the breakdown of the primary paragenesis to orthopyroxene cordierite and spinel the successive parageneses formed in separate microdomains according to a clockwise p t path geothermometry geobarometry and phase diagram calculations indicate that the textures formed by decompression and cooling from 7 9 kbar and 850 900 ct o 3 5 4 5 kbar and 700 800 c this p t evolution is consistent with low to medium a h2o between 0 4 and 0 7 and is similar to the metamorphic conditions deduced in al mg granulites from the north of in ouzzal

nutrients as the dominant control on the spread of anoxia and euxinia across the cenomanian turonian oceanic anoxic event oae2 model data comparison 1 the cenomanian turonian oceanic anoxic event oae2 is characterized by large perturbations in the oxygen and sulfur cycles of the ocean potentially resulting from changes in oxygen supply via oxygen solubility and ocean circulation and in marine productivity we assess the relative impact of these mechanisms comparing model experiments with a new compilation of observations for seafloor dysoxia anoxia and photic zone euxinia the model employed is an intermediate complexity earth system model which accounts for the main ocean dynamics and biogeochemistry of the cretaceous climate the impact of higher temperature and marine productivity is evaluated in the model as a result of higher atmospheric carbon dioxide and oceanic nutrient concentrations the model shows that temperature is not alone able to reproduce the observed patterns of oceanic redox changes associated with oae2 observations are reproduced in the model mainly via enhanced marine productivity due to higher nutrient content responsible for 85 of the change higher phosphate content could have been sustained by increased chemical weathering and phosphorus regeneration from anoxic sediments which in turn induced an enhanced nitrogen nutrient content of the ocean via nitrogen fixation the model also shows that the presence of seafloor anoxia as suggested by black shale deposition in the proto north atlantic ocean before the event might be the result of the silled shape and lack of deep water formation of this basin at the late cretaceous overall our model data comparison shows that oae2 anoxia was quasi global spreading from 5 of the ocean volume before the event to at least 50 during oae2

ultrahigh temperature metamorphism followed by two stage decompression of garnet orthopyroxene sillimanite granulites from ganguvarpatti madurai block southern india the mg al granulites from ganguvarpatti consist of orthopyroxene sillimanite garnet quartz as peak assemblage with a few porphyroblasts of cordierite and sapphirine these assemblages were strongly overprinted by late symplectites and coronas orthopyroxene inclusions in garnet and porphyroblast cores have the highest xmg 0 80 and al2o3 content 10 7 wt the estimated near peak metamorphic conditions 1 000 50 c and 11 kbar using garnet orthopyroxene geothermobarometry are consistent with those determined using a petrogenetic grid the proposed multi stage evolution process implies an initial decompression deduced from multi phase symplectites followed by cooling during biotite formation further late decompression is explained from the orthopyroxene rims on biotite this proposed p t path thus suggests a unique and complex evolution history for the uht granulites of southern india present results are comparable with similar adjacent terranes in the gondwana supercontinent but the lack of structural and geochronological data makes a link with any major metamorphic event uncertain

fluid assisted interaction of peraluminous metapelites with trondhjemitic magma within the petronella shear zone limpopo complex south africa abstract the principal problem concerning the evolution of precambrian granulite complexes located between granite greenstone cratons is their interaction with the underthrusted greenstone belts during exhumation from the lower crust besides evidence for pervasive and localized fluid fluxes arising from devolatilization of the greenschist and amphibolite facies rocks in the course of their burial underneath the granulites this interaction might also be expressed in formation of diverse magmas of granitic trondhjemitic and granodioritic composition we present results of a petrological fluid inclusion and thermobarometric study of interaction between fluidized trondhjemitic magma and peraluminous metapelitic granulites associated with the regional high grade petronella shear zone located in the southern marginal zone smz of the limpopo granulite complex south africa the hot t 1000 c trondhjemitic magma which presumably originated from partial melting of a basaltic amphibolite material at the base of the granulite complex or at the top of the underthrusted greenstone blocks intruded granulites at p 8 0 9 5 kbar 24 28 km depth at 2 667 0 9 ga during the exhumation of the smz the magma heated and assimilated orthopyroxene cordierite metapelites and dragged them to a depth of 18 20 km 6 3 6 5 kbar the magma was heterogeneously saturated with mgo feo al 2 o 3 by the dissolved metapelites this process provoked crystallization of several garnet generations from the trondhjemitic melt various mineral assemblages included in the different generations of garnet allowed application of twq method combined with perple x pseudo sections to trace sub isobaric cooling of the magma from t 900 600 c at 5 5 6 5 kbar the isobaric cooling also affected the associated metapelites fluid inclusions trapped in garnet and quartz in the trondhjemite show that the magma transported carbonic fluid with densities corresponding to the late stages of magma cooling 600 650 c and 5 5 6 5 kbar carbonic fluids coexisted with aqueous salt fluids preserved as inclusions with salinity up to 20 58 wt nacl eq these low water activity fluids a h 2 o 0 3 bearing na k and ca salts being exsolved from the magma on cooling and solidification provoked formation of complex na gedrite biotite sillimanite quartz staurolite plagioclase bearing assemblages after cordierite in metapelites at temperatures 630 570 c and pressures 5 5 6 5 kbar these data provide evidence that hot trondhjemitic melts played a critical role in the exhumation of granulites onto the adjacent granite greenstone craton the trondhjemite transferred heat from the lower to the middle crust and transported large volumes of external aqueous carbonic salt fluids that participated in the rehydration of a significant portion of the smz

oscillating redox conditions in the vocontian basin se france during oceanic anoxic event 2 oae 2 abstract the cenomanian turonian boundary 94 ma was marked by an episode of widespread organic carbon burial largely associated with low oxygen concentrations in bottom oceanic waters named oceanic anoxic event 2 oae 2 in the vocontian basin se france the pont d issole section exposes a 22 m interval called niveau thomel the local sedimentary expression of the oae 2 characterized by levels of laminated dark shales with marine derived organic matter enrichments up to 2 5 wt an episodic disruption in total organic carbon toc enrichment is observed in bioturbated limestones within the niveau thomel reflecting bottom water reoxygenation associated to the plenus cold event oscillations in redox conditions during the oae 2 are evidenced by variations in the degree of pyritisation of the sediments reflecting suboxic to anoxic conditions associated with organic rich intervals these variations are additionally highlighted by enrichments in redox sensitive trace elements mo v zn in sediments containing at least 1 5 wt toc which may reflect a threshold between suboxic and anoxic conditions at pont d issole section low mn and fe concentrations in 0 3 wt toc samples mark a large o2 depletion in pore waters phosphorus released from the sediments under deoxygenated conditions could have contributed to sustain the primary productivity in the basin promoting further o2 consumption a large negative excursion in the sulfur isotopic composition of pyrite 34spy with minimum values down to 40 suggests that microbial sulfate reduction was non sulfate limited and was characterized by pyrite precipitation in an open system with regard to the marine sulfate reservoir this feature is probably linked to a higher sulfate availability within the vocontian basin due to important large igneous province outgassing at the onset of the oae 2

high pressure and ultrahigh temperature metamorphism at komateri northern madurai block southern india we report for the first time the evidence for prograde high pressure hp metamorphism preceding a peak ultrahigh temperature uht event in the northernmost part of the madurai block in southern india mg al rich grt ged rocks from komateri in karur district contain poikiloblastic garnet with numerous multi phase inclusions although most of the inclusion assemblages are composed of gedrite quartz and secondary biotite rare staurolite sapphirine and spinel quartz are also present the xmg mg fe mg of staurolite 0 45 0 49 is almost consistent with that reported previously from namakkal district in the palghat cauvery shear zone system xmg 0 51 0 52 north of the madurai block the hp event was followed by peak uht metamorphism at t 880 1040 c and p 9 8 12 5 kbar as indicated by thermobarometric computations in the grt ged rock and associated mafic granulite symplectic intergrowth of spinel xmg 0 50 0 59 zno 1 7 wt and quartz a diagnostic indicator of uht metamorphism probably formed by decompression at uht conditions the rocks subsequently underwent retrograde metamorphism at t 720 760 c and p 4 2 5 1 kbar the p t conditions and clockwise exhumation trajectory of the komateri rocks comparable to similar features recorded from the palghat cauvery shear zone system suggest that the madurai block and the palghat cauvery shear zone system underwent similar hp and uht metamorphic history probably related to the continent continent collision during the final stage of amalgamation of gondwana supercontinent

sapphirine and corundum bearing granulites from karur madurai block southern india we report new occurrences of sapphirine and corundum bearing granulites intercalated within orthogneisses at lachmanapatti and malappatty in the northern part of madurai block sapphirine in these localities occurs either as needle like intergrowth with cordierite and corundum in symplectites and medium to fine grained euhderal to subhedral crystals associated with cordierite and corundum lachmanapatti or in association with plagioclase corundum and gedrite malappatty the sapphirine from lachmanapatti is highly magnesian xmg 0 87 0 94 with higher cr content up to 0 9 pfu as compared with those in other localities in the madurai block the sapphirine corundum association reported in this study has important bearing on the ultrahigh temperature metamorphism and exhumation history of the northern madurai block as well as on the tectonic evolution of the continental deep crust in southern india

ultrahigh temperature metamorphism in the achankovil zone implications for the correlation of crustal blocks in southern india abstract the achankovil zone of southern india a nw se trending lineament of 8 10 km in width and 100 km length is a kinematically debated crustal feature considered to mark the boundary between the madurai granulite block in the north and the trivandrum granulite block in the south both these crustal blocks show evidence for ultrahigh temperature metamorphism during the pan african orogeny although the exhumation styles are markedly different the achankovil zone is characterized by discontinuous strands of cordierite bearing gneiss with an assemblage of cordierite garnet quartz plagioclase spinel ilmenite magnetite orthopyroxene biotite k feldspar sillimanite the lithology preserves several peak and post peak metamorphic assemblages including 1 orthopyroxene garnet 2 perthite and or anti perthite 3 cordierite orthopyroxene corona around garnet and 4 cordierite quartz symplectite after garnet we estimate the peak metamorphic conditions of these rocks using orthopyroxene bearing geothermobarometers and feldspar solvus which yield 8 5 9 5 kbar and 940 1040 c the highest p t conditions so far recorded from the achankovil zone the retrograde conditions were obtained from cordierite bearing geothermobarometers at 3 5 4 5 kbar and 720 60 c from orthopyroxene chemistry we record a multistage exhumation history for these rocks which is closely comparable with those reported in recent studies from the madurai granulite block but different from those documented from the trivandrum granulite block an evaluation of the petrologic and geochronologic data together with the nature of exhumation paths leads us to propose that the achankovil zone is probably the southern flank of the madurai granulite block and not a unit of the trivandrum granulite block as presently believed post tectonic alkali granites that form an array of suturing plutons along the margin of the madurai granulite block and within the achankovil zone but are absent in the trivandrum granulite block suggest that the boundary between the madurai granulite block and the trivandrum granulite block might lie along the tenmalai shear zone at the southern extremity of the achankovil zone

sulfur isotopes track the global extent and dynamics of euxinia during cretaceous oceanic anoxic event 2 the mesozoic era is characterized by numerous oceanic anoxic events oaes that are diagnostically expressed by widespread marine organic carbon burial and coeval carbon isotope excursions here we present coupled high resolution carbon and sulfur isotope data from four european oae 2 sections spanning the cenomanian turonian boundary that show roughly parallel positive excursions significantly however the interval of peak magnitude for carbon isotopes precedes that of sulfur isotopes with an estimated offset of a few hundred thousand years based on geochemical box modeling of organic carbon and pyrite burial the sulfur isotope excursion can be generated by transiently increasing the marine burial rate of pyrite precipitated under euxinic i e anoxic and sulfidic water column conditions to replicate the observed isotopic offset the model requires that enhanced levels of organic carbon and pyrite burial continued a few hundred thousand years after peak organic carbon burial but that their isotope records responded differently due to dramatically different residence times for dissolved inorganic carbon and sulfate in seawater the significant inference is that euxinia persisted post oae but with its global extent dwindling over this time period the model further suggests that only 5 of the global seafloor area was overlain by euxinic bottom waters during oae 2 although this figure is 30 greater than the small euxinic fraction present today 0 15 the result challenges previous suggestions that one of the best documented oaes was defined by globally pervasive euxinic deep waters our results place important controls instead on local conditions and point to the difficulty in sustaining whole ocean euxinia

strontium isotope stratigraphy in the upper cenomanian shallow water carbonates of the southern apennines short term perturbations of marine 87sr 86sr during the oceanic anoxic event 2 abstract strontium isotope stratigraphy of shallow water carbonates from the southern apennines italy indicates that the late cenomanian early turonian evolution of marine 87sr 86sr is not accurately depicted by the marine reference curve using the low mg biotic calcite of well preserved rudist shells as a study material and carbon isotope stratigraphy for correlation to the standard ammonite biozonal scheme of northern europe new 87sr 86sr values are proposed for four different stratigraphic levels the middle and uppermost part of the guerangeri zone the lower and upper part of the geslinianum zone the southern apennines data suggest that a sharp positive shift at the onset of oceanic anoxic event 2 precedes the well known sr isotope negative shift the positive shift is interpreted in terms of enhanced rates of chemical weathering driven by global warming forced by volcanogenic co2 outgassing a stratified ocean is invoked to reconcile the high gradient and the short lag time of the perturbation with estimates of present day total mass and residence time of strontium in the ocean the sharp switch to the negative shift is related to the collapse of water column density gradient driven by thermal instability during a late cenomanian cool event more active ocean circulation suddenly delivered to surface waters the signal of increased submarine volcanism that had accumulated in deep waters

discovery of sapphirine bearing mg al granulites in the north china craton implications for paleoproterozoic ultrahigh temperature metamorphism abstract we report here for the first time the occurrence of mg al granulites within the khondalite belt of the north china craton and provide robust evidence for extreme crustal metamorphism at ultrahigh temperature uht conditions in this region the uht indicator assemblages in these rocks include equilibrium sapphirine quartz high alumina orthopyroxene sillimanite quartz and high temperature perthites the surrounding khondalites also carry spinel quartz equilibrium assemblage the sapphirines occurring in various microstructural associations are markedly magnesian x mg 0 69 0 77 with a composition varying from 7 9 3 to 2 2 1 end members spinel is a solid solution of mg spinel and hercynite x mg 0 41 0 54 with zno content varying from 0 48 to 1 79 wt garnet shows a compositional range between pyrope and almandine x mg 0 39 0 48 with minor grossular 2 6 3 8 mol and spessartine 0 5 1 8 mol the orthopyroxenes display marked compositional variations in terms of al 2 o 3 content with the core of the medium grained orthopyroxene adjacent to sillimanite in the matrix showing the highest al 2 o 3 content 8 4 wt the history of prograde to peak metamorphism of these rocks is indicated by inclusions of sapphirine spinel quartz and sillimanite within garnet sapphirine formed during the peak stage through the reaction spl qtz sil spr occurrence of sapphirine and quartz inclusions trapped in sillimanite suggests the reaction spr qtz grt opx sil a series of retrograde reactions are observed in these rocks as corona and symplectite textures around early coarse grained minerals such as opx sil qtz crd grt sil qtz crd grt qtz crd opx grt o 2 crd qtz fe 2 o 3 in mag or ilm and finally opx kfs h 2 o bt qtz whereas the stability of sapphirine quartz and orthopyroxene sillimanite quartz in the matrix assemblage suggests t 1000 c and p 10 kbar peak metamorphism conventional geothermobarometric estimates confirm the ultrahigh temperature nature and also trace the retrograde conditions 930 970 c at p 10 kbar al in opx 900 1000 c perthite 930 990 c grt opx and 910 940 c sapphirine spinel electron microprobe dating of monazites in textural association with the uht assemblages yield a sharply defined isochron age of 1917 48 ma which correlates with the 1927 11 ma obtained in a previous study from monazites in the surrounding khondalites carrying spinel quartz assemblage an evaluation of the assemblages and textures using appropriate petrogenetic grids indicate that following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and we interpret these trajectories to be part of an overall anti clockwise p t evolution the study area located within the northern margin of the north china craton defines a collisional orogenic belt where continental collision and deep subduction was possibly followed by extensional collapse of the orogen evidence for the extensional phase is provided by the several mafic magmatic intrusives of paleoproterozoic age occurring in the region which suggest a possible link between the thermal anomaly associated with extreme crustal metamorphism in this region with mantle derived magmas the uht metamorphism in north china craton coincided with the assembly of the paleoproterozoic supercontinent columbia

co2 rich fluid inclusions in staurolite and associated minerals in a high pressure ultrahigh temperature granulite from the gondwana suture in southern india abstract we report the first detailed fluid inclusion study on mg al rocks from the gondwana suture zone along the palghat cauvery shear zone system in southern india these rocks underwent high pressure p 12 kbar and ultrahigh temperature t 990 c metamorphism during the late neoproterozoic associated with the collisional assembly of the gondwana supercontinent fluid inclusions of texturally primary and pseudosecondary categories occur in plagioclase staurolite and kyanite in these rocks inclusions of secondary nature are found in garnet and apatite all the trapped fluids show melting temperatures in the range of 58 6 to 56 6 c suggesting a dominantly pure co 2 composition the secondary inclusions show high homogenization temperatures t h of 12 9 to 30 0 c indicating low density of 0 59 0 83 g cm 3 although most of the primary and pseudosecondary inclusions also contain moderate to low density fluids 0 67 0 95 g cm 3 t h 3 5 to 27 4 c rare high density inclusions 1 00 1 06 g cm 3 t h 26 1 to 14 1 c were discovered in matrix plagioclase and porphyroblastic staurolite the estimated co 2 isochores for the high density primary and pseudosecondary inclusions intersect the clockwise p t trajectory of the rocks at around 8 3 9 0 kbar at 1000 c which broadly coincide with the peak metamorphic conditions of the area since these fluid inclusions show textural evidence for entrapment during the growth of the host high grade minerals we interpret the high density co 2 fluids to be traces of a co 2 rich synmetamorphic fluid that was present during the peak ultrahigh temperature metamorphism despite the preservation of high density co 2 inclusions in staurolite and plagioclase the primary and pseudosecondary inclusions in most of the other minerals show wide density variation which suggests the fluids in these inclusions underwent significant density reversal density decrease during decompression of the rocks from high pressure through the peak ultrahigh temperature stage along a clockwise p t trajectory the finding of pure co 2 inclusions in these mg al granulites conforms with similar observations from many other granulite facies terrains with the carbonic fluids probably derived from mantle reservoirs and transferred through deep seated shear zones that characterize this gondwana suture

petrology and u th pb geochronology of the sapphirine quartz bearing metapelites from rajapalayam madurai block southern india evidence for polyphase neoproterozoic high grade metamorphism abstract strongly migmatitic sapphirine bearing granulites occur as dm to m sized enclaves in garnetiferous leucogranites in the southeastern part of the madurai block mb the largest section of neoproterozoic crust in the southern granulite terrain of india sgt early mineral assemblages biotite sillimanite quartz garnet biotite quartz break down via successive dehydration melting reactions to high t and high p phase assemblages e g garnet melt orthopyroxene sillimanite melt orthopyroxene cordierite melt melt consuming reactions along the retrograde path led to partial resorption of garnet to orthopyroxene sillimanite orthopyroxene sapphirine and or orthopyroxene cordierite as well as orthopyroxene sillimanite assemblages to cordierite biotite quartz symplectites the occurrence of uht metamorphism is indicated by the presence of small inclusions of sapphirine quartz intergrowths in garnet this reaction sequence is interpreted in a kfmash partial petrogenetic grid taking both melt producing and melt consuming reactions into account which yields a clockwise heating cooling decompression p t path culminating at 8 11 kbar and 1000 1100 c in situ u th pb monazite dating has been performed to more precisely constrain the ages given by chemical electron microprobe on monazite in various textural positions monazite inclusions in garnet often yield a bimodal distribution of early 950 850 ma and late neoproterozoic 600 500 ma ages which is in good agreement with the results of u pb and epma zircon dating in other places of the southeastern madurai block by contrast monazite inclusions in other mafic minerals orthopyroxene cordierite biotite or as interstitial phase exclusively yield ages between 550 and 480 ma the early neoproterozoic population is interpreted to reflect the timing of migmatisation and peak uht metamorphism whereas the late neoproterozoic ages are inferred to date cooling and exhumation with granitic magmas to a higher crustal level of the madurai block

ultra high temperature metamorphism and multistage decompressional evolution of sapphirine granulites from the palni hill ranges southern india sapphirine granulites from a new locality in the palni hill ranges southern india occur in a small enclave of migmatitic highly magnesian metapelites mg 85 72 within massive enderbitic orthogneiss they show a variety of multiphase reaction textures that partially overprint a coarse grained high pressure assemblage of bt opx ky grt pl qtz the sequence of reactions as deduced from the corona and symplectite assemblages together with petrogenetic grid considerations records a clockwise p t evolution with four distinct stages 1 equilibration of the initial high p assemblage in deep overthickened crust 12 kbar 800 900 c was followed by a stage of near isobaric heating presumably as a consequence of input of extra heat provided by the voluminous enderbitic intrusives during heating kyanite was converted to sillimanite and biotite was involved in a series of vapour phase absent melting reactions which resulted in the ultra high temperature assemblage opx crd kfs spr sil grt qtz bt coexisting with melt equilibration at c 950 1000 c 11 10 kbar 2 subsequently as a result of decompression of the order of 4 kbar at ultra high temperature a sequence of symplectite assemblages opx sil spr spr crdopx spr crdopx crdopx crd spl crd spl developed at the expense of garnet orthopyroxene and sillimanite this stage of near isothermal decompression implies rapid ascent of the granulites into mid crustal levels possibly due to extensional collapse and erosion of the overthickened crust 3 development of late biotite through back reaction of melt with residual garnet indicates a stage of near isobaric cooling to c 875 c at 7 8 kbar i e relaxation of the rapidly ascended crust to the stable geotherm 4 a second period of near isothermal exhumation up to c 6 5 kbar 850 c is indicated by the partial breakdown of late biotite through volatile phase absent melting reactions available isotope data suggest that the early part of the evolutionary history stages 1 3 is presumably coeval with the early proterozoic metamorphism in the extended granulite terrane of the nilgiri biligirirangan and shevaroy hills to the north while the exhumation of the granulites from mid crustal levels stage 4 occurred only during the pan african thermotectonic event which led to the accretion of the kerala khondalite belt to the south

age and sedimentary provenance of the southern granulites south india u th pb shrimp secondary ion mass spectrometry abstract southern india lies at a junction in the gondwana forming orogenic belts between the east african orogen and the kuunga orogen it contains voluminous high grade metasedimentary gneisses that make up an important component of the record of collision and amalgamation of gondwana here we present u pb secondary ion mass spectrometry sims isotopic data from detrital zircon cores from throughout southern india that demonstrate dominant neoarchaean to palaeoproterozoic age components that are incompatible with the known ages of potential southern and central indian source regions the original sediments to the trivandrum block gneisses were deposited between 1900 and 515 ma whereas a sample from the achancovil unit and possible also a sample from the madurai block were deposited in neoproterozoic times we speculate that these rocks broadly correlate with southern and western malagasy metasedimentary rocks including the itremo and molo groups and formed an extensive basin or basins that lay on the west side present orientation of the neoproterozoic continent azania in addition metamorphic zircon from four samples yielded an age of 513 6 ma that is interpreted as dating high grade metamorphism throughout much of the southern granulite terrane

mineral chemistry and metasomatic growth of aluminous enclaves in gedrite cordierite gneiss from southwestern new hampshire usa

sapphirine quartz corona around magnesian xmg 0 58 staurolite from the palghat cauvery suture zone southern india evidence for high pressure and ultrahigh temperature metamorphism within the gondwana suture abstract we report the first finding of equilibrium sapphirine quartz assemblage from the palghat cauvery suture zone pcsz in southern india providing unequivocal evidence for extreme crustal metamorphism at ultrahigh temperature uht conditions associated with the collisional assembly of the gondwana supercontinent in the late neoproterozoic cambrian the sapphirine and quartz occur as coronas around mg rich x mg 0 58 staurolite within poikiloblastic garnet in an mg al rich rock suggesting the progress of the prograde dehydration reaction staurolite garnet sapphirine quartz h 2 o although the occurrence of mg rich staurolite was previously noted from some localities in the pcsz no sapphirine quartz direct association has yet been reported the available experimental studies on mg rich staurolite indicate that the mineral is stable at p 15 kbar and suggest that the complex texture that we report here might correspond to a peak high pressure regime prior to the uht event the symplectic sapphirine quartz around staurolite probably implies decompression from p 15 kbar toward the stability of sapphirine quartz at temperatures of c 1000 c along a clockwise p t path the prograde high pressure metamorphism and following uht event correlate with the subduction collision tectonics associated with the final stage of amalgamation of gondwana supercontinent

spinel sapphirine quartz bearing composite inclusion within garnet from an ultrahigh temperature pelitic granulite implications for metamorphic history and p t path abstract we report here a multiphase mineral inclusion composed of quartz plagioclase k feldspar sapphirine spinel orthopyroxene and biotite in porphyroblastic garnet within a pelitic granulite from rajapalaiyam in the madurai granulite block southern india in this unique textural association hitherto unreported in previous studies sapphirine shows four occurrences 1 as anhedral mineral between spinel and quartz spr 1 2 subhedral to euhedral needles mantled by quartz spr 2 3 subhedral to anhedral mineral in orthopyroxene and 4 isolated inclusion with quartz spr 4 spr 1 spr 2 and spr 4 show direct grain contact with quartz providing evidence for ultrahigh temperature uht metamorphism at temperatures exceeding 1000 c associated orthopyroxene shows high mg fe mg ratio 0 75 and al 2 o 3 content up to 9 6 wt also suggesting t 1050 c and p 10 kbar during peak metamorphism coarse spinel spl 1 with irregular grain morphology and adjacent quartz grains are separated by thin films of spr 1 and k feldspar suggesting that spl 1 and quartz were in equilibrium before the stability of spr 1 quartz this texture implies that the p t conditions of the rock shifted from the stability field of spinel quartz to sapphirine quartz petrogenetic grid considerations based on available data from the fmas system favour exhumation along a counterclockwise p t trajectory the irregular shape of the inclusion and chemistry of the inclusion minerals are markedly different from the matrix phases suggesting the possibility that the inclusion minerals could have equilibrated from cordierite bearing silicate melt pockets during the garnet growth at extreme uht conditions

mo total organic carbon covariation in modern anoxic marine environments implications for analysis of paleoredox and paleohydrographic conditions 1 sedimentary molybdenum mo s has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic rich marine facies deposited under oxygen depleted conditions a detailed analysis of mo s total organic carbon toc covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that 1 mo s while useful in distinguishing oxic from anoxic facies is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and 2 patterns of mo s toc covariation can provide information about paleohydrographic conditions especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal these inferences are based on data from four anoxic silled basins the black sea framvaren fjord cariaco basin and saanich inlet and one upwelling zone the namibian shelf representing a spectrum of aqueous chemical conditions related to water mass restriction in the silled basin environments increasing restriction is correlated with a systematic decrease in mo s toc ratios from 45 5 for saanich inlet to 4 5 1 for the black sea this variation reflects control of mo s by mo aq which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal the basin reservoir effect the temporal dynamics of this process are revealed by high resolution chemostratigraphic data from framvaren fjord and cariaco basin sediment cores which exhibit long term trends toward lower mo s toc ratios following development of water column stratification and deepwater anoxia mo burial fluxes peak in weakly sulfidic environments such as saanich inlet owing to a combination of greater mo aq availability and enhanced mo transport to the sediment water interface via fe mn redox cycling and are lower in strongly sulfidic environments such as the black sea and framvaren fjord these observations demonstrate that at timescales associated with deepwater renewal in anoxic silled basins decreased sedimentary mo concentrations and burial fluxes are associated with lower benthic redox potentials i e more sulfidic conditions these conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction e g silled basins and are not valid for low oxygen facies in open marine settings such as continent margin upwelling systems

electron microprobe dating of monazites from high grade gneisses and pegmatites of the kerala khondalite belt southern india monazites of five samples one leptynitic garnet biotite gneiss one khondalite one augen gneiss and two pegmatites from the central and northern part ponmudi unit of the kerala khondalite belt kkb in southern india were analyzed with the electron microprobe dating technique monazites in the augen gneiss and the pegmatites yield grain sizes between 200 800 m th abundances are rather low 10 wt and the distribution of th u and pb within single grains is fairly homogeneous contrasting to this monazites in the leptynitic gneiss and the khondalite are small 150 m they often display very complex th u pb patterns and contain high th concentrations up to 20 wt the statistical treatment of individual ages from the investigated samples revealed three populations of lower proterozoic 1 9 ga upper proterozoic 580 ma and ordovician age 470 ma as well as mid proterozoic ages between 0 8 1 7 ga which are not regarded to be of geological significance lower proterozoic ages are preserved in the cores of monazites from leptynitic gneisses and khondalites they fairly agree with sm nd model ages for similar rocks of the kkb and give a minimum age for first monazite growth or complete homogenization the prominent pan african population with mean values between 540 and 580 ma is present in the leptynitic gneiss the khondalite and the augen gneiss and in line with published isotope ages for the kkb the ordovician population finally marks the emplacement of granitic pegmatites subsequent to the pan african high grade metamorphic event there is an obvious discrepancy between khondalites and leptynitic gneisses on the one hand and augen gneisses on the other concerning the presence of lower proterozoic ages while these are abundant in the former often rimmed by upper proterozoic ages they are completely absent in the latter it appears unlikely that lower proterozoic ages were completely reset during a pan african event exclusively in the augen gneisses while they were preserved in leptynitic gneisses and khondalites it is further concluded that the augen gneisses are of magmatic origin and were derived from porphyritic granites thus the upper proterozoic age of 605 37 ma calculated for the investigated sample approximates the time of magma emplacement which slightly precedes the peak stage of pan african high grade metamorphism in the kkb and of monazite crystallization from the granitic melt a characteristic feature of the investigated monazites is the resetting of lower proterozoic and pan african ages to significantly younger values due to partial pb loss monazites not shielded by other minerals e g garnet suffered selective mobilization of pb along fractures or at their rims while th and u concentrations remained almost unchanged the results presented in this study indicate that this was mainly due to fluid rock interaction it is concluded that thermal diffusion of pb even at the suggested temperatures of 900 c only had minor influence on the th u pb composition in monazite and that the closure temperature for this system must be significantly higher than previously assumed 750 c

first report of sapphirine bearing rocks from the palghat cauvery shear zone system southern india abstract silica deficient mg al granulites from paramati within the palghat cauvery shear system contain sapphirine in association with corundum spinel and sillimanite gedrite which occurs commonly in this locality coexists with cordierite corundum and sillimanite mineral assemblages and reaction textures indicate peak metamorphism at ultrahigh temperature conditions this new locality provides evidence for extreme crustal metamorphism along the archean proterozoic collision boundary in southern india

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

first evidence of ultrahigh temperature decompression from the granulite province of southern india sapphirine bearing orthopyroxene sillimanite garnet granulites of the palni hill ranges record evidence of ultrahigh temperature metamorphism during which primary mineral assemblages were partially overprinted by a succession of reaction coronas and symplectites that together suggest decompression model system petrogenetic grids allow constraints to be placed on the evolution of p t conditions as recorded by reaction relationships of successively developed mineral assemblages thermobarometry on garnet bearing assemblages allows minimum constraints to be placed on p t conditions for the primary mineral assemblages primary mineral assemblages suggest p t conditions of c 12 kbar and c 900 1000 c secondary mineral assemblages in reaction textures show a two stage decompression to probable p t conditions of c 5 kbar and c 850 c the inferred p t path is clockwise

silica undersaturated sapphirine spinel and kornerupine granulite facies rocks ne strangways range central australia small pods of silica undersaturated al rich and mg rich granulite facies rocks containing sapphirine pleonastic spinel kornerupine cordierite orthopyroxene corundum sillimanite and gedrite are scattered throughout the ne strangways range central australia these are divided into four distinct rock types namely orthopyroxene rich aluminous granofels and metapelitic gneisses containing sapphirine spinel or kornerupine two granulite facies metamorphic events are recognized of which only the first m1 is considered in this paper peak metamorphic mineral parageneses indicate that the m1 thermal maximum occurred at approximately 900 950 c and 8 9 kbar all samples are characterized by profuse and diverse coronitic and symplectic reaction textures these are interpreted as evidence for the sequential crossing of the following reactions in the system fmas cordierite spinel corundum sapphirine sillimanite cordierite spinel orthopyroxene sapphirine sillimanite sapphirine spinel sillimanite orthopyroxene corundum sapphirine sillimanite cordierite orthopyroxene corundum phase stability relationships in fmas and mash indicate an anticlockwise p t path terminated by isobaric cooling such a path is exemplified by early low p mineral parageneses containing spinel corundum and gedrite and the occurrence of both prograde and retrograde corundum reaction textures preserve evidence for an increase in ah2o and ab2o3 with progressive isobaric cooling this hydrous retrogression resulted from crystallization of intimately associated m1 partial melt segregations there is no evidence for voluminous magmatic accretion giving rise to the high m1 thermal gradient the m1 p t path may be the result of either lithospheric thinning after both crustal thickening and burial of the supracrustal terrane or concomitant crustal thickening and mantle lithosphere thinning

ocean stagnation and end permian anoxia ocean stagnation has been invoked to explain the widespread occurrence of organic carbon rich laminated sediments interpreted to have been deposited under anoxic bottom waters at the time of the end permian mass extinction however to a first approximation stagnation would severely reduce the upwelling supply of nutrients to the photic zone reducing productivity moreover it is not obvious that ocean stagnation can be achieved numerical experiments performed with a three dimensional global ocean model linked to a biogeochemical model of phosphate and oxygen cycling indicate that a low equator to pole temperature gradient could have produced weak oceanic circulation and widespread anoxia in the late permian ocean we find that polar warming and tropical cooling of sea surface temperatures cause anoxia throughout the deep ocean as a result of both lower dissolved oxygen in bottom source waters and increased nutrient utilization buildup of quantities of h2s and co2 in the late permian ocean sufficient to directly cause a mass extinction however would have required large increases in the oceanic nutrient inventory

paleoproterozoic uht metamorphism with isobaric cooling ibc followed by decompression heating in the khondalite belt north china craton new evidence from two sapphirine formation processes

a new particulate mn fe p shuttle at the redoxcline of anoxic basins abstract pelagic redoxclines of anoxic basins and deeps form the suboxic transition between oxygenated surface and anoxic or even sulfidic bottom waters intense element cycling favoured by elevated microbial activity causes steep gradients of physico chemical parameters nutrients and redox sensitive trace metals this study presents a conceptual model for authigenic particle formation at pelagic redoxclines which is based on the tight coupling of mn fe and p cycles besides the well known occurrence of mn oxides textural sem edx and geochemical icp oes icp ms analyses of particles from the redoxclines of the black sea and the baltic sea gotland basin landsort deep evidence the existence of earlier postulated fe oxyhydroxo phosphates and emphasize mixed phases consisting of mn oxides and fe oxyhydroxo phosphates as a new solid species most of the analyzed particles are star shaped of about 5 m in size and occur as single particles or aggregates without any morphological differences between mn oxides fe oxyhydroxo phosphates and mixed phases throughout the redoxcline these minerals show a general succession with maximum abundance of mn oxides above the redoxcline followed by mixed phases and almost pure fe phosphates within and below the redoxcline respectively molar fe p ratios of single particles argue against the formation of known pure fe phosphates like vivianite or strengite at the lower end of the redox transition zone but are consistent with recent experimental findings for colloidal p bearing hydrous ferric oxides moreover morphological similarities suggest the formation of irregular fe oxyhydroxo coatings due to oxidation of upward diffusing fe 2 by oxygen and stepwise replacement of mn iv by fe iii on sinking mno x particles followed by immediate adsorption or even co precipitation of phosphate batch type experiments using biogenic mno x particles demonstrate the efficient potential of fe 2 oxidation by sinking mno x particles when entering sulfidic waters mno x particles are progressively reduced leading to an increasing relative abundance of fe and p rich particles in deeper parts of the water column these particles are also reductively dissolved thereby releasing fe 2 and phosphate to the water column this mn fe p shuttle likely affects phosphate transport throughout the water column and thus impacts primary production at least over longer time scales furthermore the particulate mn fe p shuttle must have played an important role for the cycling of p and certain trace metals in ancient ocean basins e g during certain periods of cretaceous black shale formation and should be considered in future mass balances and modeling approaches dealing with oxic anoxic interfaces of aquatic ecosystems

tropical warming and intermittent cooling during the cenomanian turonian oceanic anoxic event 2 sea surface temperature records from the equatorial atlantic oceanic anoxic event 2 oae 2 occurring during the cenomanian turonian c t transition is evident from a globally recognized positive stable carbon isotopic excursion and is thought to represent one of the most extreme carbon cycle perturbations of the last 100 myr however the impact of this major perturbation on and interaction with global climate remains unclear here we report new high resolution records of sea surface temperature sst based on tex86 and 18o of excellently preserved planktic foraminifera and stable organic carbon isotopes across the c t transition from black shales located offshore suriname french guiana demerara rise ocean drilling program leg 207 site 1260 and offshore senegal cape verde basin deep sea drilling project leg 41 site 367 at site 1260 where both sst proxy records can be determined a good match between conservative sst estimates from tex86 and 18o is observed we find that late cenomanian ssts in the equatorial atlantic ocean 33 c were substantially warmer than today 27 29 c and that the onset of oae 2 coincided with a rapid shift to an even warmer 35 36 c regime within the early stages of the oae a marked 4 c cooling to temperatures lower than pre oae conditions is observed however well before the termination of oae 2 the warm regime was reestablished and persisted into the turonian our findings corroborate the view that the c t transition represents the onset of the interval of peak cretaceous warmth more importantly they are consistent with the hypotheses that mid cretaceous warmth can be attributed to high levels of atmospheric carbon dioxide co2 and that major oaes were capable of triggering global cooling through the negative feedback effect of organic carbon burial led co2 sequestration evidently however the factors that gave rise to the observed shift to a warmer climate regime at the onset of oae 2 were sufficiently powerful that they were only briefly counterbalanced by the high rates of carbon burial attained during even the most extreme interval of organic carbon burial in the last 100 myr

predicting the lithospheric stress field and plate motions by joint modeling of lithosphere and mantle dynamics the way in which basal tractions associated with mantle convection couples with the lithosphere is a fundamental problem in geodynamics a successful lithosphere mantle coupling model for the earth will satisfy observations of plate motions intraplate stresses and the plate boundary zone deformation we solve the depth integrated three dimensional force balance equations in a global finite element model that takes into account effects of both topography and shallow lithosphere structure as well as tractions originating from deeper mantle convection the contribution from topography and lithosphere structure is estimated by calculating gravitational potential energy differences the basal tractions are derived from a fully dynamic flow model with both radial and lateral viscosity variations we simultaneously fit stresses and plate motions in order to delineate a best fit lithosphere mantle coupling model we use both the world stress map and the global strain rate model to constrain the models we find that a strongly coupled model with a stiff lithosphere and 3 4 orders of lateral viscosity variations in the lithosphere are best able to match the observational constraints our predicted deviatoric stresses which are dominated by contribution from mantle tractions range between 20 70 mpa the best fitting coupled models predict strain rates that are consistent with observations that is the intraplate areas are nearly rigid whereas plate boundaries and some other continental deformation zones display high strain rates comparison of mantle tractions and surface velocities indicate that in most areas tractions are driving although in a few regions including western north america tractions are resistive citation ghosh a w e holt and l m wen 2013 predicting the lithospheric stress field and plate motions by joint modeling of lithosphere and mantle dynamics

fluid characteristics of high to ultrahigh temperature metamorphism in southern india a quantitative raman spectroscopic study abstract we report the first quantitative compositional data on fluid inclusions in late neoproterozoic to cambrian ultrahigh temperature uht granulites from major crustal blocks and shear zones in southern india fluid inclusions in various high grade minerals such as garnet orthopyroxene plagioclase cordierite and quartz from these rocks were studied in terms of petrography microthermometry and laser raman spectroscopy measured melting temperatures of the inclusions indicate that the trapped fluid phase is dominantly carbonic raman analyses confirmed a co2 rich composition with only minor dilutants such as n2 5 0 mol h2o 0 50 mol and ch4 0 47 mol the range of most co2 isochores computed from density measurements in fluid inclusions from the uht granulites pass below the peak pressure temperature conditions of metamorphism indicating considerable resetting of fluid densities during decompression in contrast garnetiferous granulites from the salem area along the palghat cauvery shear zone system which underwent high pressure prograde metamorphism contain very high density syn metamorphic pure co2 inclusions such dense carbonic fluid inclusions in garnet and plagioclase coexist with carbonate minerals calcite or dolomite indicating probable derivation from deep seated primary magmatic sources although models on the origin of co2 are debatable the ubiquitous association of carbonic fluids in the uht mineral assemblages is in favour of large scale infiltration of co2 rich fluids during extreme crustal metamorphism associated with collisional orogeny and subsequent extensional collapse at the final stage of amalgamation of the gondwana supercontinent

pacific 187 os 188 os isotope chemistry and u pb geochronology synchroneity of global os isotope change across oae 2 studies of oae 2 sections beyond the atlantic ocean western interior seaway wis and european pelagic shelf are limited here we present initial osmium isotope stratigraphy 187os 188os osiosi from two proto pacific sites that span the cenomanian turonian boundary interval ctbi the yezo group yg section hokkaido japan and the great valley sequence gvs california usa to evaluate the 187os 188os seawater chemistry of the proto pacific additionally we combine new 206pb 238u zircon ca id tims geochronology from five volcanic tuff horizons of the yezo group section to test and facilitate inter basinal integration with the wis using radio isotopically constrained age depth models for both sections and quantitatively constrain the absolute timing and duration of events across the ctbi the yg shows an almost identical osiosi profile to that of the wis and very similar to that of other sites of the proto atlantic and european pelagic oceans turgeon and creaser 2008 and du vivier et al 2014 the characteristics of the osiosi profile are radiogenic and heterogeneous 0 55 0 85 prior to the oae 2 and synchronous with the inferred oae 2 onset the osiosi abruptly become unradiogenic and remain relatively homogeneous 0 20 0 30 before showing a gradual return to more radiogenic view the mathml sourceosi 0 70 throughout the middle to late oae 2 a206a206pb 238u zircon age of an interbedded tuff hk017 in the adjacent horizon to the first unradiogenic osiosi value constrains the age of the osiosi inflection at 94 44 0 14 ma94 44 0 14 ma this age including uncertainty agrees with the interpolated age of the same point in the osiosi profile 94 28 0 25 ma94 28 0 25 ma in the only other dated oae 2 section the wis indicating a coeval shift in seawater chemistry associated with volcanism at the oae 2 onset at the levels of temporal resolution ca 0 1 myr further prior to the onset of oae 2 an enhanced radiogenic inflection in the osiosi profile of the yg is correlative within uncertainty with a similar trend in the wis based on the u pb age depth model the interpolated ages 94 78 0 12 ma94 78 0 12 ma and 94 66 0 25 ma94 66 0 25 ma for this osiosi inflection in the yg and wis respectively indicate that palaeocirculation was sufficient to simultaneously influence transbasinal seawater chemistry in contrast the pre oae 2 osiosi profile for the gvs is disparate to that of the yg and those of the proto atlantic and european pelagic shelf locations we interpret the pre oae 2 heterogeneous osiosi values 0 30 0 95 to record a palaeobasin that was regionally influenced interchangeably by both unradiogenic hydrothermal flux and radiogenic continental flux os we conclude that the osiosi profiles from the proto pacific sections record both trends that are consistent globally oae 2 onset syn and post oae 2 but also show regional differences pre oae 2 between oae 2 sections worldwide as such the osiosi profiles coupled with u pb geochronology facilitate the correlation of oae 2 stratigraphy and demonstrate both regional and global ocean dynamics

three dimensional lithospheric electrical structure of southern granulite terrain india and its tectonic implications 1 the crustal as well as the upper mantle lithospheric electrical structure of the southern granulite terrain sgt is evaluated using the magnetotelluric mt data from two parallel traverses one is an 500 km long n s trending traverse across sgt and another a 200 km long traverse data space occam 3 d inversion was used to invert the mt data the electrical characterization of lithospheric structure in sgt shows basically a highly resistive several thousands of ohm meters upper crustal layer overlying a moderately resistive a few hundred ohm meters lower crustal layer which in turn is underlain by the upper mantle lithosphere whose resistivity shows significant changes along the traverse the highly resistive upper crustal layer is interspersed with four major conductive features with three of them cutting across the crustal column bringing out a well defined crustal block structure in sgt with individual highly resistive blocks showing correspondence to the geologically demarcated salem madurai and trivandrum blocks the 3 d model also brought out a well defined major crustal conductor located in the northern half of the madurai block the electrical characteristics of this south dipping conductor and its close spatial correlation with two of the major structural elements viz karur oddanchatram kodaikanal shear zone and karur kamban painavu trichur shear zone suggest that this conductive feature is closely linked to the subduction collision tectonic processes in the sgt and it is inferred that the archean dharwar craton neoproterozoic sgt terrain boundary lies south of the palghat cauvery shear zone the results also showed that the achankovil shear zone is characterized by a well defined north dipping conductive feature the resistive block adjoining this conductor on the southern side representing the trivandrum block is shown to be downthrown along this north dipping crustal conductor relative to the madurai block suggesting a northward movement of trivandrum block colliding against the madurai block the lithospheric upper mantle electrical structure of the sgt up to a depth of 100 km may be broadly divided into two distinctly different segments viz northern and southern segments the northern lithospheric segment over a major part is characterized by a thick resistive upper mantle while the southern one is characterized by a dominantly conductive medium suggesting a relatively thinned lithosphere in the southern segment

multiple tectonothermal events in the granulite blocks of southern india revealed from epma dating implications on the history of supercontinents abstract we report age data on zircon monazite uraninite and huttonite from a suite of 29 samples covering four major granulite blocks in southern india using an electron microprobe technique the rocks analysed in this study cover all of the major lithounits in these terrains and include garnet bearing and garnet free charnockites garnet biotite gneisses khondalites calc silicate rocks and a suite of orthogneisses biotite gneiss biotite hornblende gneiss two pink metagranites representing the magmatic phase were also analysed zircons from the madras block yield well defined isochrons at 2 5 2 6 ga core to rim analyses of single zircon grains show age zoning with 2 6 2 9 ga igneous cores mantled by 2 4 2 5 ga rims detrital zircons show age up to ca 3 2 ga monazites in this block have cores and rims with 2 5 2 3 ga a suite of 19 samples from the madurai block brings out the multiple tectonothermal events in this terrain zircons from an orthogneiss yield well defined isochrons at 1 7 0 1 ga 0 82 0 05 ga and 0 58 0 04 ga from core inner rim and rim portions respectively zircon grains in other rocks preserve either core or secondary growth ages at 0 8 1 0 ga zircons in a pink metagranite from this block show sharply defined isochrons of 0 68 0 03 ga for the core and 0 57 0 01 ga for the secondary portion a late pan african overprint is observed throughout this block with zircon rims monazite uraninite and huttonite yielding age values in the range of 0 45 0 60 ga zircons from both the trivandrum and nagercoil blocks show a major tectonothermal event at 0 55 ga with faint indications of previous tectonothermal events during 0 8 1 0 and 1 7 2 0 ga monazite data from both the trivandrum and nagercoil blocks are essentially similar to those from the madurai block except for presence of relic monazite in the former our study confirms the notion that the palghat cauvery shear zone marks the major terrain boundary between an archean craton in the north and proterozoic terrains in the south it also strengthens the view of paleoproterozoic accretion and pan african reworking of blocks south of this shear zone the ages of production accretion and reworking in the terrains of southern india yield important information for the histories of columbia 1 8 1 5 ga rodinia 1 1 1 0 ga and gondwana 0 6 0 5 ga supercontinents the southern indian terrains formed part of a worldwide network of orogenic belts that is centered around 1 8 ga and outlines the configuration of columbia accretion of terrains in the madurai block to archean rocks north of the palghat cauvery shear zone at this time is consistent with the ages suggested to be the oldest metamorphic event in the eastern ghats orogen of eastern india and the rayner belt of coastal east antarctica our data confirm earlier evidence that all of southernmost india underwent resetting of isotopic systems during the final accretion of gondwana at the time of the pan african orogeny 0 5 ga the possibility that the trivandrum and nagercoil terrains accreted to the madurai block after 0 7 ga suggests that this pan african zone may also have involved accretion and closure of ocean basins if that happened then this zone may be the long sought suture between east and west gondwana

redox sensitivity of p cycling during marine black shale formation dynamics of sulfidic and anoxic non sulfidic bottom waters abstract a high resolution geochemical record of a 120 cm black shale interval deposited during the coniacian santonian oceanic anoxic event 3 odp leg 207 site 1261 demerara rise has been constructed to provide detailed insight into rapid changes in deep ocean and sediment paleo redox conditions high contents of organic matter sulfur and redox sensitive trace metals cd mo v zn as well as continuous lamination point to deposition under consistently oxygen free and largely sulfidic bottom water conditions however rapid and cyclic changes in deep ocean redox are documented by short term 15 20 ka intervals with decreased total organic carbon toc s and redox sensitive trace metal contents and in particular pronounced phosphorus peaks up to 2 5 wt p associated with elevated fe oxide contents sequential iron and phosphate extractions confirm that p is dominantly bound to iron oxides and incorporated into authigenic apatite preservation of this fe p coupling in an otherwise sulfidic depositional environment as indicated by fe speciation and high amounts of sulfurized organic matter may be unexpected and provides evidence for temporarily non sulfidic bottom waters however there is no evidence for deposition under oxic conditions instead sulfidic conditions were punctuated by periods of anoxic non sulfidic bottom waters during these periods phosphate was effectively scavenged during precipitation of iron oxyhydr oxides in the upper water column and was subsequently deposited and largely preserved at the sea floor after 15 25 ka sulfidic bottom water conditions were re established leading to the initial precipitation of cds zns and pyrite subsequently increasing concentrations of h2s in the water column led to extensive formation of sulfurized organic matter which effectively scavenged particle reactive mo complexes thiomolybdates at site 1261 sulfidic bottom waters lasted for 90 100 ka followed by another period of anoxic non sulfidic conditions lasting for 15 20 ka the observed cyclicity at the lower end of the redox scale may have been triggered by repeated incursions of more oxygenated surface to mid waters from the south atlantic resulting in a lowering of the oxic anoxic chemocline in the water column alternatively sea water sulfate might have been stripped by long lasting high rates of sulfate reduction removing the ultimate source for hs production

petrology of gedrite bearing rocks in mid crustal ductile shear zones from the eastern ghats belt india a suite of metapelitic basic and quartzofeldspathic rocks intruded by enderbitic gneiss from the southernmost tip of the eastern ghats belt india and metamorphosed at c 750 800 c 6 kbar were subjected to repeated ductile shear deformation hydration cooling and accompanying alkali metasomatism along narrow shear zones gedrite bearing assemblages developed in the shear zones traversing metapelitic rocks interpretation of the reaction textures in an appropriate p t grid in the system fmash an isothermal isobaric h2o na2o grid in the system nfmash and geothermobarometric data suggest a complex evolutionary history for the gedrite bearing parageneses initially gedrite bearing assemblages were produced due to increase in na2o at nearly constant but high h2o accompanying cooling gedrite was partially destabilized to orthopyroxene albite due to progressively increasing na2o during further cooling and at increased h2o a second generation of gedrite appeared in the rocks

production and preservation of organic matter the significance of iron 1 iron fertilization has been previously recognized as a potential mechanism for enhanced organic matter burial in marine sediments however the singular view of iron as a control on primary production overlooks its role in sedimentary diagenesis a factor that must be evaluated when considering organic matter accumulation this study examines the role of iron as a buffer of pore water sulfide and its implications for marine organic matter burial biogeochemical model experiments indicate that dissolved sulfide buildup in surficial marine sediments is highly sensitive to reactive iron concentration a reduction in reactive iron concentration can initiate dissolved sulfide accumulation the consequences of which include inhibition of bioturbation bioirrigation a decrease in oxygen exposure time and enhanced organic matter burial alternatively an increase in reactive iron concentration can serve to decrease organic matter burial the coupling of iron and phosphorous cycling within marine sediments provides an important positive feedback and therefore this mechanism is designated the sulfide buffer phosphorous trap hypothesis given sufficient organic carbon supply carbonate rich and opal rich sediments should be especially prone to the development of sulfidic conditions because of a deficiency in terrigenous iron widespread chalk and marl deposition during the cretaceous in association with the evolutionary expansion of calcareous plankton may have predisposed many benthic marine environments to the accumulation of toxic hydrogen sulfide and fostered the development of oceanic anoxic events comparison of model results with proxy data from oceanic anoxic event ii middle cretaceous suggests a complex role of iron as a control on both organic matter production and preservation

black shale deposition atmospheric co2 drawdown and cooling during the cenomanian turonian oceanic anoxic event 1 oceanic anoxic event 2 oae2 spanning the cenomanian turonian boundary ctb represents one of the largest perturbations in the global carbon cycle in the last 100 myr the 13ccarb 13corg and 18o chemostratigraphy of a black shale bearing ctb succession in the vocontian basin of france is described and correlated at high resolution to the european ctb reference section at eastbourne england and to successions in germany the equatorial and midlatitude proto north atlantic and the u s western interior seaway wis 13c offset between 13ccarb and 13corg is shown to be a good pco2 proxy that is consistent with pco2 records obtained using biomarker 13c data from atlantic black shales and leaf stomata data from wis sections boreal chalk 18o records show sea surface temperature sst changes that closely follow the 13c pco2 proxy and confirm tex86 results from deep ocean sites rising pco2 and sst during the late cenomanian is attributed to volcanic degassing pco2 and sst maxima occurred at the onset of black shale deposition followed by falling pco2 and cooling due to carbon sequestration by marine organic productivity and preservation and increased silicate weathering a marked pco2 minimum 25 fall occurred with a sst minimum plenus cold event showing 4 c of cooling in 40 kyr renewed increases in pco2 sst and 13c during latest cenomanian black shale deposition suggest that a continuing volcanogenic co2 flux overrode further drawdown effects maximum pco2 and sst followed the end of oae2 associated with a falling nutrient supply during the early turonian eustatic highstand

sulfur isotope records around livello bonarelli northern apennines italy black shale at the cenomanian turonian boundary sulfur isotope ratios for carbonate hosted sulfate show a rapid increase 6 to 9 m y across the livello bonarelli black shale marche umbrian apennines of italy which was deposited at the cenomanian turonian boundary in the middle cretaceous 93 5 ma the increase suggests that rates of pyrite burial increased substantially over the event resulting in a decrease of 9 27 in the oceanic reservoir of so 4 2 the enhanced pyrite burial may have significantly increased the flux of oxygen to the atmosphere 9 27 10 18 mol m y furthermore removal of the large amount of iron associated with this process might have had a profound effect on iron cycle

first report of sapphirine quartz assemblage from southern india implications for ultrahigh temperature metamorphism we report here for the first time the occurrence of sapphirine quartz assemblage in textural equilibrium from quartzo feldspathic and pelitic granulites from southern india the sapphirine bearing rocks occur as layered gneisses associated with pink granite within massive charnockite in rajapalaiyam area in the southern part of madurai block sapphirine occurs in three associations i fine grained subhedral mineral associated with quartz enclosed in garnet ii intergrowth with al rich orthopyroxene up to 9 7 wt al2o3 and iii in symplectitic intergrowth with orthopyroxene al2o3 5 9 6 7 wt and cordierite surrounding garnet the sapphirine in association with quartz is slightly magnesian xmg 0 79 0 80 and low in si content 1 55 1 56 pfu as compared with those associated with orthopyroxene and cordierite xmg 0 77 0 79 si 1 59 1 63 pfu the sapphirine quartz assemblage suggests that the granulites underwent t 1050 c peak metamorphism cores of porphyroblastic orthopyroxene in the sapphirine bearing rocks shows high al2o3 content of up to 9 7 wt suggesting t 1040 1060 c and p 8 kbar fmas reaction of sapphirine quartz garnet sillimanite cordierite indicates a cooling from sapphirine quartz stability field after the peak ultrahigh temperature metamorphism slightly lower temperature estimates from ternary feldspar and sapphirine spinel geothermometers t 950 1000 c also support a post peak isobaric cooling corona textures of orthopyroxene cordierite sapphirine orthopyroxene sapphirine and cordierite spinel around garnet suggest subsequent decompression the sapphirine quartz association and related textures reported in this study have important bearing on the ultrahigh temperature metamorphism and exhumation history of the madurai block as well as on the tectonic evolution of the continental deep crust in southern india

the stability and origin of sodicgedrite in ultrahigh temperature mg al granulites a case study from the gondwana suture in southern india mg al rich rocks from the palghat cauvery shear zone system pcsz within the gondwana suture zone in southern india contain sodicgedrite as one of the prograde to peak phases stable during t 900 990 c ultrahigh temperature metamorphism gedrite in these samples is mg rich mg fe mg x mg 0 69 0 80 and shows wide variation in na2o content 1 4 2 3 wt naa 0 33 0 61 pfu gedrite adjacent to kyanite pseudomorph is in part mantled by garnet and cordierite the gedrite proximal to garnet shows an increase in naa and aliv from the core naa 0 40 0 51 pfu aliv 1 6 1 9 pfu to the rim naa 0 49 0 61 pfu aliv 2 0 2 2 pfu suggesting the progress of the following dehydration reaction ged ky na ged grt crd h2o this reaction suggests that as the reactants broke down during the prograde stage the remaining gedrite became enriched in na to form sodicgedrite which is regarded as a unique feature of high grade rocks with mg al rich and k si poor bulk chemistry we carried out high p t experimental studies on natural sodicgedrite and the results indicate that gedrite and melt are stable phases at 12 kbar and 1 000 c however the product gedrite is na poor with only 0 13 wt na2o naa 0 015 0 034 pfu in contrast the matrix glass contains up to 8 5 wt na2o suggesting that with the progressive melting of the starting material na was partitioned into the melt rather than gedrite the results therefore imply that the occurrence of sodicgedrite in the uht rocks of the pcsz is probably due to the low h2o activity during peak p t conditions that restricted extensive partial melting in these rocks leaving na partitioned into the solid phase gedrite the occurrence of abundant primary co2 rich fluid inclusions in this rock which possibly infiltrated along the collisional suture during the final amalgamation of the gondwana supercontinent strengthens the inference of low water activity

benthic phosphorus regeneration net primary production and ocean anoxia a model of the coupled marine biogeochemical cycles of carbon and phosphorus we examine the relationships between ocean ventilation primary production water column anoxia and benthic regeneration of phosphorus using a mass balance model of the coupled marine biogeochemical cycles of carbon c and phosphorus p the elemental cycles are coupled via the redfield c p ratio of marine phytoplankton and the c p ratio of organic matter preserved in marine sediments the model assumes that on geologic timescales net primary production in the oceans is limited by the upwelling of dissolved phosphorus to the photic zone the model incorporates the dependence on bottom water oxygenation of the regeneration of nutrient phosphorus from particulate matter deposited at the water sediment interface evidence from marine and lacustrine settings modern and ancient demonstrates that sedimentary burial of phosphorus associated with organic matter and ferric oxyhydroxides decreases when bottom water anoxia dysoxia expands steady state simulations show that a reduction in the rate of thermohaline circulation or a decrease of the oxygen content of downwelling water masses intensifies water column anoxia dysoxia and at the same time increases surface water productivity the first effect reflects the declining supply of oxygen to the deeper parts of the ocean the second effect is caused by the enhanced benthic regeneration of phosphorus from organic matter and ferric oxyhydroxides sedimentary burial of organic carbon and authigenic calcium phosphate mineral francolite on the other hand is promoted by reduced ocean ventilation according to the model global scale anoxia dysoxia leads to a more efficient recycling of reactive phosphorus within the ocean system consequently higher rates of primary production and organic carbon burial can be achieved even when the continental supply of reactive phosphorus to the oceans remains unchanged

a model for uranium rhenium and molybdenum diagenesis in marine sediments based on results from coastal locations abstract the purpose of this research is to characterize the mobilization and immobilization processes that control the authigenic accumulation of uranium u rhenium re and molybdenum mo in marine sediments we analyzed these redox sensitive metals rsm in benthic chamber pore water and solid phase samples at a site in buzzards bay massachusetts usa that has high bottom water oxygen concentrations 230 300 mol l and high organic matter oxidation rates 390 mol c cm2 y the oxygen penetration depth varies from 2 to 9 mm below the sediment water interface but pore water sulfide is below detection release of rsm to pore waters during the remineralization of solid phases occurs near the sediment water interface at depths above the zone of authigenic rsm formation this release occurs consistently for mo at both sites but only in the winter for re in buzzards bay and intermittently for u at the buzzards bay site re removal to the solid phase extends to the bottom of the profile while the zone of removal is restricted to 2 9 cm for u and mo authigenic re formation is independent of the anoxic remineralization rate which is consistent with an abiotic removal mechanism the rate of authigenic u formation and its modeled removal rate constant increase with increasing anoxic remineralization rates and is consistent with u reduction being microbially mediated authigenic mo formation is related to the formation of sulfidic microenvironments the depth and extent of mo removal from pore water is closely associated with the balance between iron and sulfate reduction and the consumption of pore water sulfide via iron sulfide formation pore water rsm reach constant asymptotic concentrations in sulfidic sediments but only pore water re is constant at depth in buzzards bay the increases in pore water u at the buzzards bay site are consistent with addition via irrigation and subsequent upward diffusion to the removal zone deep pore water mo concentrations exceed its bottom water concentration due to irrigation induced oxidation and remobilization from the solid phase in sulfidic sediments there is no evidence for higher pore water u or mo concentrations at depth due to the absence of irrigation and or the presence of more stable authigenic rsm phases there are good correlations between benthic fluxes and authigenic accumulation rates for u and mo in sulfidic sediments however results from buzzards bay suggest irrigation ultimately results in the partial loss of u and mo from the solid phase with accumulation rates that are 20 30 of the modeled flux irrigation can augment re possibly u or compromise u mo authigenic accumulation in sediments and is important when determining burial rates in continental margin sediments

anticlockwise evolution of ultrahigh temperature granulites within continental collision zone in southern india abstract we report three new localities of corundum and sapphirine bearing hyper aluminous mg rich and silica poor ultrahigh temperature granulites formed during late neoproterozoic cambrian times within the palghat cauvery shear zone system in southern india from petrologic characteristics mineral chemistry and petrogenetic grid considerations the peak metamorphic conditions of these rocks are inferred to lie around 950 1000 c as suggested by al in orthopyroxene thermometer at pressures above 10 kbar as indicated by the equilibrium orthopyroxene sillimanite gedrite quartz assemblage these rocks preserve several remarkable reaction textures the most prominent among which is the triple corona of spinel sapphirine cordierite on corundum with the whole textural assembly embedded within the matrix of gedrite suggesting the reaction ged crn spl spr crd the formation of sapphirine sillimanite assemblage symplectite associated with relict corundum and porphyroblasitc cordierite is explained by the reaction crd crn spr sil the association of sapphirine cordierite symplectite with gedrite sillimanite assemblage as well as with aluminosilicate boundaries indicates the gedrite consuming reaction ged sil spr crd extensive growth of sapphirine cordierite observed on the rim of gedrite porphyroblasts with spinel occurring as relict inclusions within the sapphirine indicates the reaction ged spl spr crd the pressure temperature p t path defined from the observed mineral assemblages and reaction texture is characterized by anticlockwise trajectory with a prograde segment of initial heating and subsequent deep burial followed by retrograde near isothermal decompression such an anticlockwise trajectory is being reported for the first time from southern india and has important tectonic implications since these rocks were developed at the leading edge of the crustal block that was involved in collisional orogeny and subsequent extension during the final phase of assembly of the gondwana supercontinent we propose that the rocks were subjected to deep subduction and rapid exhumation and the extreme thermal conditions were attained either through input from underplated mantle derived magmas or convective thinning or detachment of the lithospheric thermal boundary layer during or after crustal thickening

thermal evolution of cretaceous tethyan marine waters inferred from oxygen isotope composition of fish tooth enamels 1 the evolution of subtropical 30 35 n upper ocean temperatures through the cretaceous is inferred from the oxygen isotope compositions of 64 fish teeth enamel coming from the western tethyan platform mean 18o values of 22 at the berriasian valanginian boundary decrease with oscillations to 18 5 around the cenomanian turonian boundary and progressively increase to 21 5 by the end of the cretaceous the similarity of this oxygen isotope curve for bioapatites from platform environments with those for foraminifera and bulk carbonates that were deposited in deeper waters and at other paleolatitudes indicates that they record global climatic signals major cooling events at the million year scale can be distinguished 1 at the berriasian valanginian boundary and 2 during the earliest late valanginian a third cooling event is detected during the earliest aptian these events already proposed as icehouse interludes during the lower cretaceous are also recorded at subtropical latitudes a progressive warming is identified from the aptian to the cenomanian turonian interval that corresponds to a thermal optimum and then upper ocean temperatures decreased to the maastrichtian minimum isotopic temperatures range from 15 c to 28 c assuming a 18oseawater of 1 for an ice free world taking more realistic 18oseawater values of 0 for tropical waters during glacial periods within the berriasian valanginian interval and earliest aptian or with above average salinities possibly the maastrichtian temperatures are increased by 4 5 c temperature differences between climatic extremes of the valanginian and cenomanian turonian are estimated to have been 10 c latitudinal thermal gradients for the albian cenomanian turonian and maastrichtian were 0 2 0 3 c latitude and thus weaker than modern oceanic values at about 0 4 c latitude

corundum quartz and mg staurolite bearing granulite from the limpopo belt southern africa implications for a p t path abstract a new occurrence of the rare corundum quartz assemblage and magnesian staurolite has been found in a gedrite garnet rock from the central zone of the neoarchean limpopo belt in zimbabwe poikiloblastic garnet in the sample contains numerous inclusions of corundum quartz sillimanite magnesian staurolite sapphirine orthopyroxene and sapphirine sillimanite assemblages as well as monophase inclusions corundum often containing subhedral to rounded quartz occurs as subhedral to euhedral inclusions in the garnet quartz and corundum occur in direct grain contact with no evidence of a reaction texture the textures and fe mg ratios of staurolite inclusions and the host garnet suggest a prograde dehydration reaction of st grt crn qtz h2o to give the corundum quartz assemblage peak conditions of 890 930 c at 9 10 kbar are obtained from orthopyroxene sapphirine and garnet staurolite assemblages a clockwise p t path is inferred with peak conditions being followed by retrograde conditions of 4 6 kbar and 500 570 c the presence of unusually magnesian staurolite mg fe mg 0 47 0 53 and corundum garnet assemblages provides evidence for early high pressure metamorphism in the central zone possibly close to eclogite facies the prograde high pressure event followed by high to ultrahigh temperature metamorphism and rapid uplifting of the limpopo belt could have occurred as a result of neoarchean collisional orogeny involving the zimbabwe and kaapvaal cratons

trace element abundances in the rock canyon anticline pueblo colorado marine sedimentary section and their relationship to caribbean plateau construction and oxygen anoxic event 2 1 rapid eruption of submarine lava flows during formation of the caribbean plateau correlates closely with ocean anoxic event 2 oae2 which bracketed the cenomanian turonian boundary 93 5 ma these events also correspond with a positive excursion in carbon isotopic composition of seawater hydrothermal activity associated with large scale submarine volcanism may have been responsible for this abrupt change in ocean chemistry we determined the distribution of major minor and trace element abundances in the rock canyon marine sedimentary section pueblo colorado after normalizing element concentrations to zr to remove the variable contribution of terrigenous material to these sediments we detected an interval of concentrated metal abundance anomalies that coincides with the abrupt beginning of the positive 13c isotope excursion the metal abundance anomalies indicate that intermittent hydrothermal activity in the form of both water rock exchange and magmatic degassing introduced large concentrations of trace metals into the cretaceous ocean at the same time that extinctions of benthic species turnover in plankton communities and increases in isotopically light organic carbon burial occurred the stratigraphic position of this interval of trace metal anomalies matches events associated with oae2 and indicates that intermittent hydrothermal activity on a massive scale triggered abrupt changes in carbon burial and deep ocean oxygen contents