stratigraphy composition and form of the deccan basalts western ghats india in the western ghats between latitudes 18 20 n and 19 15 n 7000 km2 of deccan basalt have been mapped with the primary objective of establishing a flow stratigraphy as a guide to the volcanic history of the flood basalts using over 70 measured vertical sections major and trace element analyses of nearly 1200 samples and rare earth and87sr 86sr determinations for over 60 samples we divide the basalt into three subgroups and ten formations in this paper we describe the seven principal formations in the area and the most prominent individual flows

geology and geochemistry of the sangamner mafic dike swarm western deccan volcanic province india implications for regional stratigraphy abstract numerous large ne sw to e w trending mafic dikes outcrop around sangamner in the western deccan volcanic province this area is part of a broader region postulated to be a shieldlike feature and a major eruption center a combination of field geochemical and isotopic sr and nd characteristics is used here to understand the relationship of this dike swarm with the associated lava flows and their position in the established deccan stratigraphy many dikes are compositionally similar to the khandala and poladpur formations belonging to the lonavala and wai subgroups respectively while one dike is similar to the ambenali formation one dike has a composition distinct from all other dikes in this area as well as from most stratigraphic units although there are many similarities in composition with the bushe formation as well as the boyhare member of the khandala formation while several dikes are geochemically similar to specific flows members within certain formations their isotopic composi

metamorphic consequences of thrust emplacement fall mountain new hampshire metamorphic pressure temperature p t paths from the upper and lower plates of the fall mountain nappe southwest new hampshire reveal different thermal histories in the two structural levels upper plate rocks experienced early low p high t contact metamorphism to peak p t conditions of 700 750 c 3 4 5 kbar these p t conditions were followed by loading to approximately 5 6 kbar and then by nearly isobaric cooling to approximately 500 c 4 5 kbar upper plate rocks then experienced minor heating with unloading and finally cooling the lower plate rocks also experienced early low p high t contact metamorphism but only to p t conditions of 480 510 c 2 3 5 kbar nappe emplacement resulted in nearly isothermal loading to 500 c 5 6 kbar which was followed by either heating or cooling depending on the proximity of the sample to the upper plate the early pressures recorded in both the upper and lower plate rocks suggest a depth of approximately 10 km for origination of the nappe the post nappe pressures 5 6 kbar imply a total tectonic thickness of approximately 20 km or a doubling of the overburden thickness diffusion calculations suggest that the emplacement of the nappe took less than approximately 10 m y and u pb chronology on zircon suggests that it occurred between 400 and 410 ma 4o ar 39 ar cooling ages on micas indicate that the terrane was cooled to approximately 300 c by 340 ma

geologic thermobarometry of retrograded metamorphic rocks an indication of the uplift trajectory of a portion of the northern scandinavian caledonides kyanite grade pelitic schists from the efjord area 68 15 n 16 45 e northern norway show abundant textural evidence of retrograde reequilibration during late orogenic uplift and cooling of this part of the scandinavian caledonides graphical analysis of mineral compositions in seven samples that show a range of retrograde textures indicates that they reached final chemical equilibrium under nonuniform pressure temperature conditions three element partitioning geothermometers and geobarometers yield consistent pt estimates in individual samples which together define a trend in pt space the sample showing the fewest signs of retrogression yields the highest pressure and temperature estimates whereas samples exhibiting abundant reequilibration textures give lower pt estimates we suggest that the trend indicated by these samples reflects a portion of the pt trajectory assumed by the efjord area during uplift thus the judicious use of element partitioning geothermometers and geobarometers on samples showing signs of retrograde metamorphism may yield important new information concerning the late tectonic evolution of metamorphic terrains

elemental and nd sr pb isotope geochemistry of flows and dikes from the tapi rift deccan flood basalt province india abstract the deccan traps are a large rift associated continental flood basalt province in india parts of which have been studied extensively in terms of geochemistry palaeomagnetism and stratigraphy however the basalts of the tapi rift in the central part of the province have been little studied thus far two ene wsw trending tectonic inliers of the deccan basalts in this region forming ridges rising from younger alluvium are made up of basalt flows profusely intruded by basaltic dikes both of these ridges lie along a single lineament although they are not physically continuous the flows are aphyric plagioclase phyric and giant plagioclase basalts and the dikes are aphyric or plagioclase phyric we consider the two inliers to have been originally continuous from the presence of bouldery remnants of a major dioritic gabbro dike along both samples of this dike from both ridges have previously yielded typical deccan ages of 65 6 0 5 ma and 65 6 0 6 ma by the 40 ar 39 ar incremental heating technique initial 87 sr 86 sr ratios and end t values and present day pb isotopic ratios of most dikes indicate that they are isotopically similar to lavas of the mahabaleshwar and panhala formations of the western ghats about 450 km to the south their mantle normalized trace element patterns have small pb and ba peaks one dike has a strong bushe formation affinity and a nd sr isotopic composition more extreme than that of any other deccan rock yet sampled with end t 20 2 and 87 sr 86 sr t 0 72315 its mantle normalized element pattern shows large pb th and u peaks and large nb ta troughs its elemental and isotopic chemistry reflects substantial continental contamination the flows cut by the mahabaleshwar type dikes are isotopically similar to the poladpur formation lavas of the western ghats their mantle normalized element patterns show modest peaks at rb ba and pb and rather low nb and ta relative to la indicating that they have been contaminated to intermediate degrees the mantle normalized element patterns of all the flows and dikes show enrichment in the light rare earth elements with small or no eu anomalies the entire flow dike sequence is similar to the wai subgroup of the western ghats in terms of its elemental and isotopic chemistry and stratigraphic relationships wai subgroup like lavas i e some of the younger magma types originally identified from the southern part of the western ghats are previously known from the central northern and northeastern deccan and many have been thought to be far travelled flows erupted in the southwestern deccan although at least the dikes and probably the giant plagioclase basalt flows of our study area are locally generated and emplaced our new data extend the known outcrop area of these widespread magma types substantially and these magma types indeed appear to have a nearly province wide distribution

geochemistry of the palitana flood basalt sequence and the eastern saurashtra dykes deccan traps clues to petrogenesis dyke flow relationships and regional lava stratigraphy recent studies of large mafic dyke swarms in the deccan traps flood basalt province india indicate that some of the correlative lava flows reached several hundred kilometers in length here we present field petrographic mineral chemical and whole rock geochemical including sr nd isotopic data on the palitana lava sequence and nearby dykes in the saurashtra region of the northwestern deccan traps these rocks are moderately evolved many with low ti nb characteristics we infer that most dykes are notably and systematically less contaminated by ancient continental crust than the palitana flows but four dykes are equally or significantly more contaminated with some of the most extreme sr nd isotopic compositions seen in the entire deccan traps initial end is as low as 18 0 a bhimashankar type and a poladpur type dyke are present several hundred kilometers from the type section of these magma types in the western ghats escarpment we find no geochemical correlations between the palitana sequence and three subsurface sequences in ne saurashtra containing abundant picritic rocks surface lavas previously studied from saurashtra or the western ghats sequence intriguingly the eastern saurashtra dykes cannot have been feeders to any of these lava sequences feeder dykes of these sequences may be located in southwestern or central saurashtra or in the dhule nandurbar dediapada areas across the gulf of cambay 200 300 km east of palitana our results indicate polycentric flood basalt eruptions not only on the scale of the deccan traps province but also within the saurashtra region itself

structure and emplacement of the nandurbar dhule mafic dyke swarm deccan traps and the tectonomagmatic evolution of flood basalts flood basalts such as the deccan traps of india represent huge typically fissure fed volcanic provinces we discuss the structural attributes and emplacement mechanics of a large linear tholeiitic dyke swarm exposed in the nandurbar dhule area of the deccan province the swarm contains 210 dykes of dolerite and basalt 1 km in length exposed over an area of 14 500 km2 the dykes intrude an exclusively basaltic lava pile largely composed of highly weathered and zeolitized compound pahoehoe flows the dykes range in length from 1 km to 79 km and in thickness from 3 to 62 m almost all dykes are vertical with the others nearly so they show a strong preferred orientation with a mean strike of n88 because they are not emplaced along faults or fractures they indicate the regional minimum horizontal compressive stress 3 to have been aligned n s during swarm emplacement the dykes have a negative power law length distribution but an irregular thickness distribution the latter is uncommon among the other dyke swarms described worldwide dyke length is not correlated with dyke width using the aspect ratios length thickness of several dykes we calculate magmatic overpressures required for dyke emplacement and depths to source magma chambers that are consistent with results of previous petrological and gravity modelling the anomalously high source depths calculated for a few dykes may be an artifact of underestimated aspect ratios due to incomplete along strike exposure however thermal erosion is a mechanism that can also explain this whereas several of the nandurbar dhule dykes may be vertically injected dykes from shallow magma chambers others particularly the long ones must have been formed by lateral injection from such chambers the larger dykes could well have fed substantial 1 000 km3 and quickly emplaced a few years flood basalt lava flows this work highlights some interesting and significant similarities and contrasts between the nandurbar dhule dyke swarm and regional tholeiitic dyke swarms in iceland sudan and elsewhere

geochemical and isotopic signatures of surficial sediments from the western continental shelf of india inferring provenance weathering and the nature of organic matter abstract bottom sediments from thirteen transects sampled mainly at depth contours of 30 100 and 200 m along the western continental margin of india falling in two offshore sectors from dwaraka to goa and cape comorin to goa were analyzed for calcium carbonate major elements in silicate fraction trace and rare earth elements in the bulk fraction and carbon and nitrogen isotopes in organic matter to understand the provenance of sediments weathering patterns in the source areas and nature of organic matter major elements such as si ti mg and discrimination plots involving alkalies and silica have shown a distinct north south provinciality with the source signatures pointing at deccan basalt for sediments in the north from dwaraka to goa and gneisses and granulites of south india in the south shale normalized ree patterns and ree content suggest additional local sources mainly in the southwest coast of india degree of alteration determined using chemical index of alteration cia plagioclase index of alteration pia and a cn k plots have suggested that the sediments are moderately weathered despite the intense orographic rainfall in the source area variability in degree of chemical weathering is seen with the change in provenance in general the sediments north of goa with a provenance of deccan basalts show a higher chemical weathering index corg n molar ratios are mostly between 10 and 12 minimum and maximum being 6 9 and 14 respectively which are closer to values for marine organic matter this is corroborated by marine 13c values 18 2 to 21 6 while they broadly fall within the marine values heavier values are seen in northern transects off porbandar and dwaraka where the sediments have a provenance different from those in the south this could be due to a higher contribution from c4 type vegetation in the north consistent with heavier 13c values in terrestrial plants in source regions heavier 15n values are observed for the northern transects north of ratnagiri which is supported by the intense denitrification in the northern arabian sea

a model for garnet and plagioclase growth in pelitic schists implications for thermobarometry and p t path determinations numerical models of the progressive evolution of pelitic schists in the ncmnkfmash system with the assemblage garnet biotite chlorite staurolite plagioclase muscovite quartz h2o are presented with the goal of predicting compositional changes in garnet and plagioclase along different p t paths the numerical models support several conclusions that should prove useful for interpreting the p t paths of natural parageneses i garnet may grow along p t vectors ranging from heating with decompression to cooling with compression p t paths deduced from garnet zoning that are inconsistent with these growth vectors are self contradictory ii there is a systematic relation between garnet and plagioclase composition and growth such that for most p t paths garnet growth requires plagioclase consumption furthermore mass balance in a closed system requires that as plagioclase is consumed the remaining plagioclase becomes increasingly albitic inclusions of plagioclase in the core of garnet should be more anorthitic than those near the rim and zoned matrix plagioclase should have rims that are more albitic than the cores complex plagioclase textures may arise from the local variability of growth and precipitation kinetics iii a decrease of fe fe mg in a garnet zoning profile is a reliable indicator of increasing temperature for the assemblage modelled however there is no single reliable p monitor and inferences about p can only be made by considering plagioclase and garnet together iv consumption of garnet during the production of staurolite removes material from the outer shell of a garnet and may make recovery of peak metamorphic compositions and p t conditions impossible low peak temperatures typically recorded by staurolite bearing assemblages may reflect this phenomenon v diffusional homogenization of garnet affects the computed p t path and results in a clockwise rotation of the computed p t vector relative to the true p t path

origin of the deccan trap flows at mahabaleshwar inferred from nd and sr isotopic and chemical evidence abstract the deccan flows at mahabaleshwar are divisible into a lower and an upper group based on nd and sr isotopic ratios which define two correlated trends this distinction is supported by incompatible element ratios and bulk compositions the data reflect contamination in a dynamic system of magmas from an lil depleted e juv 8 mantle by two different negative e juv endmembers one undoubtedly continental crust the other either continental crust or enriched mantle the depleted mantle source anomalously high in 87 sr 86 sr may have been in the subcontinental lithosphere or a region of rising indian ocean morb mantle

petrogenetic significance of ferro enstatite orthopyroxene in basaltic dikes from the tapi rift deccan flood basalt province india abstract some basaltic dikes from the tapi rift in the deccan flood basalt province india contain abundant clusters of prismatic quench crystals of ferro enstatite orthopyroxene fe 39 42 mg 53 57 ca 2 4 such profuse orthopyroxene clusters are not yet known from any other flood basalt province in the world based on petrography mineral chemistry and whole rock chemistry we conclude that these crystals are formed due to assimilation of shale by deccan basalt magma this interpretation is consistent with the known bedrock geology of the area these quench crystals constitute the first direct petrographic evidence for crustal contamination in the deccan province the liquidus temperature for the dikes is estimated at 1220 10 c with the crystallization temperature of the orthopyroxene at 1080 c or less

nature of sub volcanic magma chambers deccan province india evidence from quantitative textural analysis of plagioclase megacrysts in the giant plagioclase basalts sub volcanic magma chambers might be a widespread component of flood basalt provinces and their presence can be revealed in some cases by plagioclase megacrystic basalts in at least four levels within the deccan flood basalt sequence the generally low abundance of small plagioclase crystals increases to 5 25 with some as large as 30 mm long these giant plagioclase basalt gpb flows were formed by mixing of megacryst rich and megacryst poor magmas the crystal size distributions csd of these megacrysts mostly plot as almost straight lines on a classic csd diagram for a plagioclase growth rate of 10 10 mm s steady state magma chamber models and simple continuous growth suggest residence times of 500 1500 years however the lack of crystals smaller than 2 mm suggests that coarsening may have been involved and crystal shape can help define the environment where this happened plagioclase megacrysts are very tabular and commonly form clusters of sub parallel crystals characteristics that are also found in the plagioclase of anorthosites formed by flotation at the top of shallow magma chambers and crystallization in a high peclet number regime e g skaergaard sept iles a possible history is as follows 1 plagioclase megacrysts crystallize in a convecting magma chamber just below the lava pile 2 currents sweep the crystals to the top of the chamber where they accumulate as a result of their buoyancy the crystals coarsen in response to the continuous supply of hot magma 3 new magma sweeps through the plagioclase mush mingles and mixes then erupts to form the gpbs the residence time recorded by the megacrysts in the gpbs is that of the magma chamber where the megacrysts formed not that of the magmas that make up the megacryst poor part of the gpbs or the other megacryst poor lavas lavas with megacrysts similar to the gpbs are uncommon but widespread galapagos surtsey etc and suggest the presence of sub volcanic magma chambers elsewhere

metamorphic p t paths from pelitic schists and greenstones from the south west tauern window eastern alps petrological data from intercalated pelitic schists and greenstones are used to construct a pressure temperature path followed by the upper schieferhulle ush series during progressive metamorphism and uplift in the south west tauern window italy pseudomorphs of al epidote fe epidote albite oligoclase chlorite after lawsonite and data on amphibole crystal chemistry indicate early metamorphism in the lawsonite albite chlorite subfacies of the blueschist facies at p 7 8 kbar geothermometry and geobarometry yield conditions of final equilibration of the matrix assemblage of 475 25 c 5 6 kbar calculations with plagioclase and phengite inclusions in garnet indicate early garnet growth at pressures of 7 5 kbar garnet zoning patterns are complex and reversals in zoning can be correlated between samples thermodynamic modelling of these zoning profiles implies garnet growth in response to four distinct phases of tectonic activity fluid inclusion data from coexisting immiscible h2o co2 nacl fluids constrain the uplift path to have passed through temperatures of 380 30 c at 1 3 0 2 kbar there is no evidence for metamorphism of ush at pressures greater than 7 5 kbar in this area of the tauern window this is in contrast to pressures of 10 kbar recorded in the lower schieferhulle only 2 3 km across strike a history of differential uplift and thinning of the intervening section during metamorphism is necessary to reconcile the p t data obtained from these adjacent tectonic units

effects of diffusional modification of garnet growth zoning on p t path calculations the effect of intragranular diffusion on chemical zoning in garnet and on p t paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth syn and post growth diffusion of mg fe mn ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635 c compositions from zoned garnet as modified by diffusion hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure temperature p t paths from isobaric and polybaric model histories p t path calculations based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635 c show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path the magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition the effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating only models that reached peak temperatures approximately 15 30 c higher compositions of garnets with radii less than 1 mm that reached actual peak temperatures of 605 c along temperature time histories characteristic of regional metamorphism experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories path distortions were significant at heating cooling rates less than 10 c ma but rapidly diminished for rates faster than this in polybaric simulations diffusion effects were least noticeable when the actual pressure temperature conditions changed in a clockwise sense i e convex to higher p and higher t but apprecciable modification was seen in path models that underwent counterclockwise changes in p and t reequilibration of garnet rim compositions occurred during cooling on all paths and temperature maxima obtained from garnet biotite geothermometry underestimated actual peak conditions by 40 to 70 c calculations suggest that p t path trajectories calculated from garnets of at least 1 mm size and that experienced actual thermal maxima below 585 c are not likely to be distorted by diffusional effects during regional metamorphism however p t path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history the partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling seriously complicating the task of quantitatively estimating diffusion effects on path calculations

uncertainties in differential thermodynamic gibbs method p t paths a combined analytical and numerical evaluation of the uncertainties in p t paths is made for three assemblages that propagates errors in the parameters initial pressure initial temperature initial composition change of composition of monitor parameters endmember entropy and endmember volume propagated errors along an isobaric heating path t 77 c for assemblage 1 grt bt pl qtz ms chl h2o using as monitor parameters the mole fractions of almandine spessartine grossular and anorthite and ignoring uncertainties in thermodynamic properties are approximately 320 bars 1 and 8 3 c 1 if rim compositional uncertainties of 5 in major cations are assumed or 2 5 c 1 and 50 bars 1 if electron microprobe analytical uncertainties are assumed for compositions the largest source of uncertainty is from the errors in the monitor parameters and p t path uncertainties can depend critically on which monitor parameters are used if the mole fraction of annite is used as a monitor parameter in place of the anorthite content of plagioclase then propagated uncertainties are worse than 29 c 1 and 5800 bars 1 p t path uncertainties also depend on assemblage 1 precisions in assemblages 2 grt bt pl qtz ms sil h2o and 3 grt bt qtz kfs sil h2o using as monitor parameters the mole fractions of almandine spessartine grossular and anorthite are calculated to be 267 bars and 43 4 c and 372 bars and 8 2 c respectively estimates of the accuracies in p t paths that include potential errors in endmember entropy of 1 j mol k and in endmember volume of 1 cm3 mol are 324 bars and 8 5 c assemblage 1 341 bars and 48 6 c assemblage 2 and 388 bars and 9 7 c assemblage 3 use of different garnet plagioclase and muscovite activity models can change the length of a p t path by as much as 15 but does not typically change directions in p t space significantly models that incorporate changes of fluid composition shorten p t paths in assemblages 1 and 3 but do not change trajectories significantly assemblage 2 is virtually unaffected by fluid phase models for the mineral assemblages considered here and using appropriate monitor parameters propagated errors are small compared to the total path length suggesting that the differential thermodynamic approach is a precise and accurate method for determining amounts of heating or thickening during metamorphism and hence for interpreting orogenic processes

volcanological and tectonic control of stratigraphy and structure in the western deccan traps many of the world s flood basalt provinces form elevated plateaux at the margins of continents although in most cases their present large elevation is not the result of mountain building processes several explanations have recently been put forward to explain such occurrences of epeirogeny the deccan trap basalt province forms one such elevated plateau and results are presented here showing how the epeirogenic uplift in this region combined with crustal subsidence probably associated with the rifting of the indian continental margin has affected the structure of the basalt sequence trace element analytical data are used for samples from numerous vertical sections through the deccan traps lava series along and around the western ghats ridge in india the results reinforce the previously defined stratigraphy of the mahabaleshwar area and extend it over a region covering some 36 000 km2 reaching as far south as belgaum and the trap basement contact these results show that the lava pile is not flat lying but forms a very low amplitude anticlinal fold structure plunging southwards by up to 0 3 over most of the area although in the south there is evidence of a reversal of this plunge the fold is interpreted as being the result of two tilting processes 1 westward tilting near the coast due to the foundering of the passive continental margin and 2 epeirogenic uplift along the whole west coast of india producing the observed topography and the peninsula wide drainage patterns and also the easterly component of dip variations in the magnitude of the latter effect along the western continental margin may also be important in generating the plunge of the fold although the possibility of some component of depositional dip may also be important this latter possibility can be modelled using a simple computer program the results of this modelling show that a migrating linear volcanic edifice fits the observations best

the spi lake formation of the central hearne domain western churchill province canada an axial intracratonic continental tholeiite trough above the cogenetic kaminak dyke swarm the spi group is a package of plagioclase porphyritic amygdaloidal basalt flows and sandstone spi lake formation conformably overlain by coarse conglomerate and sandstone old boot formation these unconformably overlie the kaminak lake segment of the central hearne supracrustal belt western churchill province the spi group developed in a small isolated basin 8 km2 and is older than the paleoproterozoic hurwitz group our data demonstrate that the basalts of the spi lake formation are genetically related to the ca 2450 ma kaminak dyke swarm a speculation previously promoted by some workers based on field characteristics alone they are continental tholeiites with large ion lithophile and light rare earth element enriched multielement profiles with prominent high field strength element troughs the 143nd 144nd compositions overlap the contemporaneous chondritic bulk earth data all of these are features we infer to have resulted from mixing of melts from dominantly depleted mid ocean ridge basal

strontium isotope zoning in garnet implications for metamorphic matrix equilibration geochronology and phase equilibrium modelling in principle garnet growth rates may be calculated from 87 rb 86 sr and 87 sr 86 sr measurements in garnet subsamples and the surrounding rock matrix because of low rb sr garnet should passively record the matrix decay of 87 rb to 87 sr as a progressive increase in 87 sr 86 sr from core to rim this concept was tested by collecting rb sr data for five garnet grains from four major orogenic belts eastern vermont c 380 ma western new hampshire c 320 ma southern chile c 75 ma and northwestern italy c 35 ma both normal sr isotope zoning increasing 87 sr 86 sr from core to rim and inverse sr zoning decreasing 87 sr 86 sr from core to rim were observed garnet and matrix iso tope data commonly yielded grossly inaccurate model ages incomplete rb and sr equilibration among matrix minerals is invoked to explain the deviations between theoretical v measured zoning patterns and the age disparities initially the reactive matrix is dominated by rapidly equilibrating rb rich mica which imparts high 87 sr 86 sr values in garnet cores progressive participation of slower equilibrating sr rich plagioclase buffers or even reduces 87 sr 86 sr possibly leading to flat or decreas ing 87 sr 86 sr from garnet cores to rims unusually high 87 sr 86 sr in garnet in combination with bulk matrix compositions causes erroneously young apparent ages so metamorphic ages growth rates and associated heating and loading rates are likely suspect although rb sr may be the most suscep tible because of the profound disparities between mica and feldspar zircon reactivity might influence the lu hf system by up to a few per cent the sm nd system seems generally immune to these effects pseudosection analysis and conventional garnet geochronology which presume complete matrix equilibration during metamorphism may require modification to account for differences between whole rock v reactive matrix compositions

petrologic constraints on imbrication metamorphism and uplift in the sw tauern window eastern alps the pressure temperature time deformation pttd history of two tectonic units in the southwest tauern window austria italy is evaluated from p t paths of metamorphism and uplift geochronologic data and observations on fabric development petrologic data indicate burial of the lower schieferhulle lsh to depths of 35 km and the adjacent upper schieferhulle ush to only 24 28 km the units are now separated by only 2 km of section metamorphism was postkinematic in the lsh and synkinematic in the ush correlation of p t path reversals calculated from zoned garnets in the two units however indicates that juxtaposition of the lsh and ush occurred prior to metamorphism temperature time and depth time diagrams for both units show increasing cooling rates and decreasing uplift rates as the rocks approached the surface both the petrologic data and the depth time profiles for the lsh and ush point to a history of differential uplift in the early stages of decompression textural features indicate that the lsh behaved as an essentially rigid body during uplift whereas the ush experienced extensive ductile shearing the pttd data for the lsh and ush are incorporated into a model for continental collision in the eastern alps initial tectonic uplift of the lsh was accompanied by ductile thinning of the ush and overlying austroalpine units final uplift and erosion affected all of the units en masse

geology and geochemistry of pachmarhi dykes and sills satpura gondwana basin central india problems of dyke sill flow correlations in the deccan traps many tholeiitic dyke sill intrusions of the late cretaceous deccan traps continental flood basalt province are exposed in the satpura gondwana basin around pachmarhi central india we present field petrographic major and trace element and sr nd pb isotope data on these intrusions and identify individual dykes and sills that chemically closely match several stratigraphically defined formations in the southwestern deccan western ghats some of these formations have also been identified more recently in the northern and northeastern deccan however the pachmarhi intrusions are significantly more evolved lower mg numbers and higher tio2 contents than many deccan basalts with isotopic signatures generally different from those of the chemically similar lava formations indicating that most are not feeders to previously characterized flows they appear to be products of mixing between deccan basalt magmas and partial melts of precambrian indian amphibolites as proposed previously for several deccan basalt lavas of the lower western ghats stratigraphy broad chemical and isotopic similarities of several pachmarhi intrusions to the northern and northeastern deccan lavas indicate petrogenetic relationships distances these lava flows would have had to cover if they originated in the pachmarhi area range from 150 to 350 km the pachmarhi data enlarge the hitherto known chemical and isotopic range of the deccan flood basalt magmas this study highlights the problems and ambiguities in dyke sill flow correlations even with extensive geochemical fingerprinting

geochemical stratigraphy of deccan flood basalts of the bijasan ghat section satpura range india abstract the 240 m thick bijasan ghat section exposes deccan basalt flows and dykes that have major and trace element compositions suggestive of variable amounts of contamination by continental material most flows and a dyke are chemically similar to lavas of the poladpur formation in the southwestern deccan one flow and one dyke are similar to lavas of the bushe formation in the southwest and one dyke is similar to lavas of the mahabaleshwar formation in the southwest the type sections of these formations are located 350 km to the south of bijasan ghat but many poladpur type lavas a few mahabaleshwar like lavas and dykes and a few broadly bushe like lavas and dykes have recently been identified in the satpura region lavas resembling the thick ambenali fm of the southwest are absent from bijasan ghat and nearby areas our data further extend the known outcrop area of poladpur like flows estimated at 3 105 km2 one of the most widespread deccan magma types seven giant plagioclase basalt flows encountered in seven different sections in this region cannot unambiguously be correlated at present either physically or chemically and therefore are of little use in deciphering the complicated stratigraphic and structural make up of the regional lava pile

the timing of crustal extension and the eruption of continental flood basalts the association between mantle plumes continental flood basalt eruption and the extension and rifting of the crust is well recognized1 3 but there has always been controversy as to whether the plume or the rifting was the initiating factor4 5 was rifting triggered by doming above the thermal plume2 3 6 or was rifting the driving force preceding the associated volcanicity7 8 here i show that recent flow by flow mapping of two continental flood basalt provinces the columbia river basalt group and the deccan basalt group allows one to infer the relative timing of volcanicity and crustal extension in both provinces eruption of the main tholeiitic phase preceded significant extension and crustal thinning this suggests that the primary event in the formation of these provinces was the upwelling of a mantle plume and that the subsequent crustal extension rifting and decompression was in part a consequence of the initiation of that plume

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

coupled c s fe geochemistry in a rapidly accumulating marine sedimentary system diagenetic and depositional implications in the present study we have investigated the c s fe systematics in a sediment core md161 13 from the krishna godavari k g basin bay of bengal the core covers the late holocene period with high overall sedimentation rate of 573 cm kyr 1 pore fluid chemical analyses indicate that the depth of the present sulfate methane transition zone smtz is at 6 mbsf the ta ca mg so42 ratios suggest that both organoclastic degradation and anaerobic oxidation of methane aom drive sulfate reduction at the study site the positive correlation between total organic carbon content toc and chromium reducible sulfur crs content indicates marked influence of organoclastic sulfate reduction on sulfidization coupled occurrence of 34s enriched iron sulfide pyrite with 12c enriched authigenic carbonate zones is the possible records of paleo sulfate methane transition zones where aom driven focused sulfate reduction was likely fueled by sustained high methane flux from underlying gas rich zone aluminum normalized poorly reactive iron fepr al and la yb ratios suggest increasing contribution from deccan basalts relative to that of archean proterozoic granitic complex in sediment flux of krishna godavari basin during the last 4 kyr

madagascar basalts tracking oceanic and continental sources extensive upper cretaceous volcanism in southern madagascar was fed in part by mantle sources resembling those expressed today in the indian ocean at marion and prince edward islands and on the central southwest indian ridge in addition very low end t to 17 4 high 87sr 86sr t to 0 72126 tholeiites in southwestern madagascar were variably but highly contaminated by ancient continental material broadly like that affecting the bushe and poladpur formations of the later deccan traps in india alkalic dikes in southwestern madagascar have a rough analogue in the mahabaleshwar formation of the deccan in that they document the influence of a low 206pb 204pb negative end relatively low 87sr 86sr reservoir a very similar reservoir is manifested at present in mid ocean ridge basalts on the central southwest indian ridge near 40 e the original location of this end member appears likely to have been in the madagascan lithospheric mantle a portion of which may have been removed in the middle cretaceous by the action of the marion hotspot or the rifting of indo madagascar an origin within the hotspot itself also may be possible however recent products of the hotspot appear to lack completely the necessary low 206pb 204pb low end signatures

plume rift interaction in the deccan volcanic province abstract it is widely accepted that the grand volcanic episode of the deccan in india was a consequence of the passage of the northerly drifting indian subcontinent over the reunion starting plume in the late cretaceous this plume also produced the three rift cambay triple junction the three arms being the west coast graben belt the narmada tapi rift zone and the cambay rift deccan related alkalic magmatism both preceded and followed the main tholeiitic phase of 65 ma ago by about 3 my the uniformly tholeiitic thick basalt sequence of the western ghats was derived from direct melting of the plume head but the tholeiitic alkalic magmatism of the three rift belts was due to melting of the hydrous lithospheric mantle due to variable amounts of rifting any lithospheric loading mechanism for generating post tholeiite alkalic lavas does not seem feasible for the deccan nor can it explain the pre tholeiite alkaline magmatism in the deccan similarly conductive heating of the lithosphere by the plume cannot have generated the alkalic melts some amount of actual rifting did occur direct melt supply from the plume is not necessary for magmatism in the rifts though it seems to have occurred in some cases our documentation of indigenous magmatism for the three rift belts calls into question any attempts at inferring the size of the plume head from synchroneity in geochronologic data alone and we negate the possibility or necessity of large distance transport of magmas from the plume through the lithosphere to feed the rifts recent evidence indicates that alkalic magmatism north of the main deccan outcrop preceded the main tholeiite phase by 3 my the cambay graben was active 3 5 my before the main tholeiite phase as the alkalic complexes at the northern end of this graben are 68 5 ma in age so far the oldest well proven deccan related rocks they are the first expression of the reunion plume and the borehole picrites from the triple junction area until now conventionally regarded as the oldest can no longer be considered so accurate radiometric dating of these picrites is therefore much needed

p t paths from garnet zoning a new technique for deciphering tectonic processes in crystalline terranes a procedure to calculate quantitative rock pressure temperature p t paths based on chemical zoning profiles in garnet combined with mineral chemistry from the other minerals in the assemblage has been applied to samples from diverse tectonic settings with the result that the calculated p t path is shown to be a sensitive monitor of tectonic processes terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the prograde p t path and cooling during decompression in the retrograde path in nappe terranes where hot rocks are emplaced over cool rocks the upper plate shows cooling during decompression whereas the lower plate shows heating during compression in nappe terranes where cool rocks are emplaced over hot the lower plate shows cooling during compression calculation of p t paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes

pressure temperature time path discontinuity in the main central thrust zone central nepal metapelites collected in central nepal reveal a discontinuity in metamorphic pressuretemperature time p t t paths near the base of the main central thrust zone despite an absence of obvious structural breaks garnets in the structurally lowest rocks grew with increasing t and p loading whereas garnets 1 3 km upsection grew with increasing t but decreasing p exhumation monazite grains in structurally lower rocks yield ionmicroprobe th pb ages of 8 9 ma structurally higher monazite grains range from 10 to 22 ma the p t t paths confirm previous interpretations that footwall metamorphism in part resulted from thrust reactivation ca 8 ma but also reflect thermal relaxation following older 20 ma or older thrust movement the main central thrust zone formed during pulses of movement that resulted in progressive transfer of material from the lower to upper plate

post k pb younger 40 ar 39 ar ages of the mandla lavas implications for the duration of the deccan volcanism abstract we present new age determinations derived from 40 ar 39 ar incremental heating experiments for basaltic lava flows from the mandla lobe located on the eastern margin of the main deccan volcanic province some 1000 km from the western ghats escarpment the most reliable estimates of crystallization ages come from 5 plateau ages from plagioclase separates from a stratigraphically controlled succession of 37 lava flows we detect no statistically significant age difference from bottom to top range 63 65 ma and calculate a weighted mean age for the section at 64 21 0 33 ma these lava flows are significantly younger than the majority of the main deccan volcanic activity documented from the western ghats 67 65 ma the new ages are consistent however with geochemical correlation of the mandla lobe lavas with the uppermost succession poladpur ambenali mahabaleshwar formations of the sw deccan and indicate that this post k pb youngest phase of flood basalt activity erupted over much of the province

geochemistry of flood basalts of the toranmal section northern deccan traps india implications for regional deccan stratigraphy unpublished data 1998 andthe southeastern deccan particularly the poladpur andambenali formations mitchell widdowson 1991 bilgrami 1999 for as much as 350 km from their south western type sections in addition lavas isotopically andchemicallyequivalenttothoseoftheambenaliformation fm arepresentinthenortheastern sectoroftheprovinceas far as 900 km from their southwesternmost counter parts mahoney 1988 deshmukh

thermobarometric constraints on the thermal history of the main central thrust zone and tibetan slab eastern nepal himalaya the main central thrust mct south of mt everest in eastern nepal is a 3 to 5km thick shear zone separating chlorite bearing schist in the lower plate from sillimanite bearing migmatitic gneiss in the overlying tibetan slab the metamorphic grade increases through the mct zone toward structurally higher levels previous workers have suggested that either post or synmetamorphic thrust movement has caused this inversion of metamorphic isograds in an effort to quantify the increase in grade and to constrain proposed structural relations between metamorphism and slip on the fault four well calibrated thermobarometers were applied to pelitic samples collected along two cross strike transects through the mct zone and tibetan slab results show an increase in apparent temperature up section in the mct zone from 778 k to 990 k and a decrease in temperature to 850 k in the lower tibetan slab which is consistent with synmetamorphic thrust movement a trend in calculated pressures across this section is less well defined but on average decreases up section with a gradient of 28mpa km resembling a lithostatic gradient pressure temperature paths for zoned garnets from samples within the mct zone modelled using the gibbs method show a significant decrease in temperature and a slight decrease in pressure from core to rim which might be expected for upper plate rocks during synmetamorphic thrust movement samples from the uppermost tibetan slab yield higher temperatures and pressures than those from the lower tibetan slab which may be evidence for later resetting of thermobarometers by intrusion of the large amounts of leucogranite at that structural level

are flood basalt eruptions monogenetic or polygenetic a fundamental classification of volcanoes divides them into monogenetic and polygenetic we discuss whether flood basalt fields the largest volcanic provinces are monogenetic or polygenetic a polygenetic volcano whether a shield volcano or a stratovolcano erupts from the same dominant conduit for millions of years excepting volumetrically small flank eruptions a flood basalt province built from different eruptive fissures dispersed over wide areas can be considered a polygenetic volcano without any dominant vent however in the same characteristic a flood basalt province resembles a monogenetic volcanic field with only the difference that individual eruptions in the latter are much smaller this leads to the question how a flood basalt province can be two very different phenomena at the same time individual flood basalt eruptions have previously been considered monogenetic contrasted by only their high magma output and lava fluidity with typical small volume monogenetic volcanoes field data from hawaiian shield volcanoes iceland and the deccan traps show that whereas many feeder dykes were single magma injections and the eruptions can be considered large monogenetic eruptions multiple dykes are equally abundant they indicate that the same dyke fissure repeatedly transported separate magma batches feeding an eruption which was thus polygenetic by even the restricted definition the same magma conduit this recognition helps in understanding the volcanological stratigraphic and geochemical complexity of flood basalts the need for clear concepts and terminology is however strong we give reasons for replacing monogenetic volcanic fields with diffuse volcanic fields and for dropping the term polygenetic and describing such volcanoes simply and specifically as shield volcanoes stratovolcanoes and flood basalt fields

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

continental tholeiitic mafic rocks of the paleoproterozoic hurwitz group central hearne sub domain nunavut insight into the evolution of the hearne sub continental lithosphere the paleoproterozoic hurwitz group of the western churchill province is an erosional remnant of an areally extensive predominantly shallow water intracratonic basin comprised of four major sequences sequence 2 forming the central part of the stratigraphy contains the ameto formation a sequence of pillowed and massive basaltic rocks and associated gabbro sills termed the happotiyik member that are interlayered with subordinate deep water mudstones siltstones and diamictites whole rock geochemical data for the mafic rocks reveals a suite of homogeneous tholeiitic basalts with affinities to both continental and volcanic arc tholeiites compatible trace elements and large ion lithophile elements exhibit scattered behavior whereas all high field strength elements show a systematic increase with zr the rocks are large ion lithophile and light rare earth element enriched and have parallel primitive mantle normalized extended trace element patterns with prominent negative nb ta and ti anomalies end

source and differentiation of deccan trap lavas implications of geochemical and mineral chemical variations the basalt stratigraphy of the deccan trap between mahabaleshwar ghat and belgaum over steps the basement from north to south sr isotope and zr nb ratios and sr rb and ba concentrations correlate portions of the post poladpur stratigraphy over 250 km along the western ghats thereby confirming a southerly component of dip of 0 06 at the southwestern margin the stratigraphy extends upwards from the compositionally uniform ambenali formation cox hawkesworth 1984 into a sequence of grossly heterogeneous flow units which have been allocated to the mahabaleshwar and panhala formations lightfoot hawkesworth 1988 the mahabaleshwar formation is represented only by a sequence of highly fractionated flows termed the kolhapur unit with similar 87sr 86sr0 to the mahabaleshwar 0 7045 but with sr 2 25 succeeding the kolhapur unit are a series of flows with high 87sr 86sr0 0 7045 0 705 zr nb 13 and low sr 230 but trace element concentrations similar to the mahabaleshwar formation these have been allocated to the desur unit of the panhala geochemical variations in flows overlying the ambenali define two distinct trends one is attributed to gabbro fractionation and the other to variations in the compositions of the parental magmas and arguably their source regions there is little evidence for significant crustal contamination in these flows and the degree of fractionation and the composition of the phase extract are shown to vary along strike within the mahabaleshwar formation the high tio2 content of kolhapur unit flows is shown to be the result of shallow level gabbro fractionation rather than the presence of a primitive high ti magma mahabaleshwar formation basalts exhibit a broad negative correlation between the degree of fractionation and sr isotopic composition the endmember with lower 87sr 86sr0 has different zr y from the ambenali basalts and would appear to have been generated by lower degrees of melting of a similar source the other endmember has more radiogenic sr lower zr nb similar zr y but higher mg number the simplest interpretation is that these magmas were more primitive and hence hotter and more able to interact with the lithosphere en route to the surface and that they then mixed to produce the mahabaleshwar array the panhala formation basalts plot on the sr nd array defined by the mahabaleshwar formation and the desur unit basalts plot on an extension of this array this suggests that the source characteristics are also lithospheric the absolute elemental abundances may then be a function of melting and fractionation we are impressed by the apparent switch from crustal lithospheric contributions to mantle lithospheric contributions through the stratigraphy and suggest that this together with the more protracted fractionation of the magma reflects a change in the availability of the lithospheric components accompanying the southerly migration of the volcanic edifice

sr nd and pb isotopic and chemical compositions of central deccan traps lavas and relation to southwestern deccan stratigraphy abstract sr nd and pb isotopic compositions and major and trace element abundances of lavas in five central deccan traps sections are generally very similar to those of lavas in the southwestern deccan western ghats escarpment the combination of strong isotopic and chemical similarities between lavas of the two regions indicates that they shared a closely similar petrogenesis our results indicate that unlike many lavas in the northeastern deccan most of the lavas studied in the central deccan sections can be grouped into different stratigraphic members and chemical types cts belonging to four formations thakurvadi bhimashankar khandala and poladpur present in the type sections of the western ghats bhimashankar and thakurvadi type lavas are found in a section near outram whereas only khandala type lavas make up the mhaishmal ellora and ajanta sections to the east and only poladpur type lavas are present in the easternmost section at lonar this west east sequence is the same as that seen going upward in the western ghats stratigraphy except that bushe type lavas which are located stratigraphically between the khandala and poladpur formations in the western ghats have not been found in this study overall our results indicate that the khandala and poladpur formations extend over west east distances of more than 300 km and some individual members e g the dhak dongar may extend over as much as 500 km the regional dip from igatpuri in the southwest to lonar in the east central deccan is less than 0 5 to the east which is similar to the southward regional dip in the southwestern deccan near horizontal flows in the central and southwestern deccan areas suggest that post deccan tectonic activity has had limited effects on the studied area large volumes of basaltic lavas derived from eruptive centers in the western deccan may have covered both the central and southwestern deccan areas

petrophysical properties of the deccan basalts exposed in the western ghats escarpment around mahabaleshwar and koyna india abstract we present petrophysical properties density p and s wave velocity porosity and poisson s ratio of the deccan basalts from the western ghats escarpment around mahabaleshwar and koyna and characterize the dhawar basement rocks around goa the petrophysical properties of basaltic lava flows show significant variation in the bushe poladpur ambenali and mahabaleshwar formations that are widespread in the deccan volcanic province the bushe formation stands out distinctly from other formations because of its lower density and p and s wave velocity the porosity vesicles and amygdales plays a major role in controlling the variation of the petrophysical properties of the basalts the poisson s ratio of the deccan basalts is largely affected by the vesicular porosity aspect ratio of the amygdales and zeolite content the dharwar basement rocks greywackes and granites are found to be lower in density p and s wave velocity and poisson s ratio than the basaltic rocks the variation of the petrophysical properties with elevation in the western ghats sections roughly follows the geochemical stratigraphy high porosity of the bushe and mahabaleshwar formations constitute a multi layered aquifer system in the deccan volcanic province in the koyna earthquake zone these formations may provide an effective groundwater connectivity between the reservoir and earthquake focal areas the new petrophysical data of the deccan basalts and dharwar basement rocks will help to refine the geophysical models of the southwestern deccan basalt province

a geological map of the southern deccan traps india and its structural implications a geological map of an area of over 125 000 km 2 within the southern deccan traps india is presented which is based upon variations in the basalt geochemistry it represents a synthesis of new data and earlier work the use of large data sets has allowed stratigraphic boundaries to be established with greater accuracy over a much larger area than before the geochemical sections demonstrate that the southward overstepping stratigraphic style encountered along the ghats ridge has an essentially north to south polarity and that there appears to be detectable overstep in an east to west sense however the stratigraphy defines a pronounced coastward dipping monoclinal flexure across the western ghats ridge between 16 n and 18 n the data are consistent with two models of eruption the first considers the deccan eruptive focus to have been fixed throughout the volcanic episode producing an essentially annular flow pattern the second considers the flow pattern to have been generated from a southerly migrating eruptive centre resulting from the northerly drift of india over a fixed hotspot both hypotheses are consistent with recent quantitative models which suggest that rapid and voluminous flood basalt eruptions are the result of continental rifting events associated with plume generated thermal 9highs9 until a causal link between established plume hotspot trails and impact sites can be satisfactorily demonstrated we find it unnecessary to invoke a cataclysmic impact event as an explanation for the deccan eruptions

a multi isotopic and trace element investigation of the cretaceous tertiary boundary layer at stevns klint denmark inferences for the origin and nature of siderophile and lithophile element geochemical anomalies abstract os sr nd and pb isotope data were collected from a profile across the cretaceous tertiary k t boundary layer at stevns klint denmark nd t 65 ma values from within the boundary layer fish clay are lower by 1 unit than those of the underlying maastrichtian limestone and the overlying danian chalk sequences systematic profile upward changes of pb sr and os isotopic compositions and concentrations in the boundary layer cannot be accounted for by in situ growth of daughter products since the sedimentation of the fish clay while os nd and pb isotopes indicate the admixing of less radiogenic components to the fish clay sr isotopes show elevated radiogenic values in the boundary layer relative to the carbonate sequences beneath and above it the sudden change in lithophile e g sr pb and nd isotope compositions at the base of the fish clay and profile upward trends of 87sr 86sr and 206pb 204pb ratios towards those of the overlying danian chalk are interpreted to reflect recovery from enhanced acid rain induced continental local weathering input to the seawater however a continental crustal source is invalid for the siderophile element os in the light of evidence from chromium isotopes for a cosmic origin of the platinum group elements pges and certain moderately siderophile elements cr ni co v in k t boundary sediments including stevns klint shukolyukov and lugmair science 282 1998 927 929 and supported by the finding of projectile debris bauluz et al earth planet sci lett 182 2000 127 136 and the occurrence of abundant ni rich spinel at many k t sites robin et al nature 363 1993 615 617 kyte nature 396 1998 237 239 we favor to explain the sudden drop of 187os 188os ratios from 0 210 to 0 160 at the k t boundary to derive from global fall out of extraterrestrial matter the present 186os 188os ratio of 0 119836 0 000004 measured in the basal layer of the fish clay is within the uncertainty a chondritic value we therefore exclude the possibility of a major contribution of pges to the sediment from iron meteorites chondrite normalized ru ir n ratios of 0 95 0 14 and os ir n ratios of 0 93 0 14 in the fish clay cannot distinguish between abundance ratios of different types of chondrites and strongly sub chondritic pt ir n ratios of 0 62 0 09 2 suggest differential pge remobilization through the sedimentary column and consequently the alteration of inter element ratios pges and the moderately siderophile elements cr ni v and co form an elemental association with systematically upward decreasing concentrations in the fish clay low co ni ratios of 0 12 in the fish clay relative to values of 0 35 in the over and underlying carbonate sequences support mixing of meteorite derived co ni 0 05 and terrestrial upper mantle crustal co ni 0 3 sources while lithophile element isotope data indicate an increased continental crustal input to the fish clay at the k t transition the uncertainty with respect to possible post depositional alteration of abundance patterns of siderophile and moderately siderophile elements though not affecting the chondritic isotopic composition of os does not allow confirmation of indications from chromium isotopes for a carbonaceous cv type meteorite as the preferred k t impactor type by shukolyukov and lugmair science 282 1998 927 929

highly heterogeneous precambrian basement under the central deccan traps india direct evidence from xenoliths in dykes abstract crustal or mantle xenoliths are not common in evolved tholeiitic flood basalts that cover huge areas of the precambrian shields yet the occasional occurrences provide the most direct and unequivocal evidence on basement composition few xenolith occurrences are known from the deccan traps india and inferences about the deccan basement have necessarily depended on geophysical studies and geochemistry of deccan lavas and intrusions here we report two basalt dykes rajmane and talwade dykes from the central deccan traps that are extremely rich in crustal xenoliths of great lithological variety gneisses quartzites granite mylonite felsic granulite carbonate rock tuff because the dykes are parallel and only 4 km apart and only a few kilometres long the xenoliths provide clear evidence for high small scale lithological heterogeneity and strong tectonic deformation in the precambrian indian crust beneath measured 87sr 86sr ratios in the xenoliths range from 0 70935 carbonate to 0 78479 granite mylonite the rajmane dyke sampled away from any of the xenoliths shows a present day 87sr 86sr ratio of 0 70465 and initial at 66 ma ratio of 0 70445 the dyke is subalkalic and fairly evolved mg no 44 1 and broadly similar in its sr isotopic and elemental composition to some of the lavas of the mahabaleshwar formation the xenoliths are comparable lithologically and geochemically to basement rocks from the archaean dharwar craton forming much of southern india as several lines of evidence suggest the dharwar craton may extend at least 350 400 km north under the deccan lava cover this is significant for precambrian crustal evolution of india besides continental reconstructions

origin of deccan trap lavas evidence from combined trace element and sr nd and pb isotope studies abstract geochemical and isotope results are presented from a new study of the most southern basalts in the deccan trap india three chemical formations are recognised two of which can be correlated with the established stratigraphy in mahabaleshwar and imply a regional southerly dip of 0 06 over a distance of 250 km in detail sr isotope variations within the ambenali and mahabaleshwar formations can be shown to reflect three distinct end members which provide new constraints for petrogenetic models pb isotope data for selected basalts exhibit a wide range with 206 pb 204 pb 16 87 22 45 and a linear correlation on a pb pb diagram the least contaminated ambenali basalts plot within the pb array and interaction with mantle lithosphere involves a shift to less radiogenic pb whereas contamination with crust is characterised by more radiogenic pb unlike the karoo and parana continental flood basalt provinces only four flow units within the southern deccan appear to contain a significant contribution from mantle lithosphere the mahabaleshwar and ambenali formation basalts exhibit a striking negative pb sr isotope trend which is presently regarded as one of the features of interaction with shallow level lithospheric mantle it further suggests that basalts from the walvis ridge kerguelen and ninetyeast ridge all remobilised such shallow level material and that the deccan basalts which were not affected by crustal contamination reflect interaction between asthenospheric material similar to t type morb but related to the reunion hotspot and continental mantle lithosphere of the indian plate

early alkaline magmatism in the deccan traps implications for plume incubation and lithospheric rifting abstract significant lithospheric extension which usually accompanies continental flood volcanism has been generally thought to always follow the main volcanic episode however plume derived uncontaminated early alkaline magmas from the deccan traps india indicate considerable lithospheric extension 3 million yrs my before the main tholeiitic phase indeed such early alkaline magmatism known from many flood basalt provinces of the world seems a necessary and natural consequence of plume head incubation with lithospheric doming and rifting

geology petrochemistry and genesis of the bimodal lavas of osham hill saurashtra northwestern deccan traps abstract the saurashtra region in the northwestern deccan continental flood basalt province india is notable for compositionally diverse volcano plutonic complexes and abundant rhyolites and granophyres a lava flow sequence of rhyolite pitchstone basaltic andesite is exposed in osham hill in western saurashtra the osham silicic lavas are ba poor and with intermediate zr contents compared to other deccan rhyolites the osham silicic lavas are enriched in the light rare earth elements and have end t 65 ma values between 3 1 and 6 5 and initial 87sr 86sr ratios of 0 70709 0 70927 the osham basaltic andesites have initial end values between 2 2 and 1 3 and initial 87sr 86sr ratios of 0 70729 0 70887 large ion lithophile element concentrations and sr isotopic ratios may have been affected somewhat by weathering notably the sr isotopic ratios of the silicic and mafic rocks overlap however the nd isotopic data indicate that the silicic lavas are significantly more contaminated by continental lithosphere than the mafic lavas we suggest that the osham basaltic andesites were derived by olivine gabbro fractionation from low ti picritic rocks of the type found throughout saurashtra the isotopic compositions and the similar al2o3 contents of the osham silicic and mafic lavas rule out an origin of the silicic lavas by fractional crystallization of mafic liquids with or without crustal assimilation as previously proposed for some icelandic rhyolites and supported here by melts modelling the osham silicic lavas may have been derived by partial melting of hot mafic intrusions emplaced at various crustal depths due to heating by repetitively injected basalts the absence of mixing or mingling between the rhyolitic and basaltic andesite lavas of osham hill suggests that they reached the surface via separate pathways

basalts of the northeastern deccan traps india isotopic and elemental geochemistry and relation to southwestern deccan stratigraphy the northeastern deccan traps include flows with strong isotopic and elemental affinities to lavas in the southwest as far as 900 km away however many of the northeastern flows show a negative correlation of 206pb 204pb with end t and positive correlation of 206pb 204pb with 87sr 86sr t distinct from any of the isotopic fields for the southwestern formations this array from end t 5 87sr 86sr t 0 7041 206pb 204pb 17 9 to end t 0 87sr 86sr 0 7068 206pb 204pb 20 5 overlaps the ambenali formation fm field and runs toward that of the common signature peng et al 1994 that appears to have been an important magma type in several of the lower formations of the southwest this array appears to represent mixing between an ambenali type and 1 a common signature end member or 2 an end member with even higher 206pb 204pb and 87sr 86sr and lower end than the common signature such mixing has been postulated but not documented previously for the deccan lavas isotopically and chemically indistinguishable from the ambenali fm form the tops of sections near jabalpur and chikaldara beneath them are flows that chemically resemble the poladpur fm which sits immediately below the ambenali in the southwest and several lavas similar to the khandala fm beneath the poladpur and intervening bushe fm in the southwest given the general stratigraphic correspondence with the southwestern sequence many of these northeastern flows may be petrogenetically related to the respective southwestern formations lavas with broadly poladpurand khandala like elemental compositions are also abundant in a thick section south of mhow several other mhow area flows resemble the distinctive bushe fm however nearly all of the poladpur and khandala like northeastern deccan lavas analyzed have higher 206pb 204pb man any southwestern poladpur or khandala fm basalts if they indeed are related petrogenetically to these formations many of the northeastern magmas interacted less with low 206pb 204pb continental material and thus must have erupted from different feeder systems

modeling the dispersal of levantine intermediate water and its role in mediterranean deep water formation this paper demonstrates the importance of levantine intermediate water liw in the deep water formation process in the mediterranean using the modular ocean general circulation model at 0 25 resolution 19 vertical levels over the entire mediterranean with an open gibraltar strait liw formation is strongly prescribed in the rhodes gyre region by haney 1971 relaxation while in other regions surface salinity relaxation is much reduced by applying the mixed thermohaline surface boundary conditions isopycnal diagnostics are used to trace water mass movements and volume fluxes are monitored at straits low viscosity and diffusion are used to permit baroclinic eddies to play a role in water mass dispersal the overall water budget is measured by an average flux at gibraltar of 0 8 sv of which 0 7 sv is exchanged with the eastern basin at sicily liw density around 28 95 spreads rapidly after formation throughout the entire levantine due to baroclinic eddies toward the west liw accumulates in the northern and central ionian with some entering the adriatic through otranto and some mixing southward in eddies and exiting to the western mediterranean through sicily liw is converted to deep water in the south adriatic at an average rate of 0 4 sv water exchange through the otranto strait appears to be buoyancy driven with a strong bias to the end of winter march april while at sicily the exchange has a strong symmetric seasonal cycle with maximum transport of 1 1 sv in december indicating the effects of wind driving liw pathways in the west are complex and variable in the tyrrhenian intermediate water becomes uniform on isopycnal surfaces due to eddy stirring west of sardinia two liw boundary currents are formed in the balearic basin one flows northward up the west coast of sardinia and corsica and one westward along the northern african coast the northward current is consistent with observations while the westward current is intermittent for the first 10 years often breaking up into eddies which enter the basin interior some observations of high salinity waters near the african coast may support this interpretation liw retains a subsurface salinity maximum of 38 4 38 5 practical salinity units psu when reaching the northwestern mediterranean contrasting with surface waters fresher than 38 0 psu west mediterranean deep water is formed below 1500 m depth with climatological characteristics when it is mixed and cooled during winter convection in lions gyre

pressure temperature paths from garnet zoning evidence for multiple episodes of thrust burial in the hinterland of the sevier orogenic belt garnets are present in two horizons of the schist of stevens spring from the basin creek area of the grouse creek mountains in northwest utah the two horizons possess different bulk compositions which resulted in garnet growth by different reactions along the same pressure temperature p t path garnet in the upper horizon grew from the breakdown of chlorite at upper greenschist facies conditions and garnet in the lower horizon grew from the breakdown of staurolite at upper amphibolite facies conditions from the upper horizon five garnets from three samples were analyzed all display growth zoning ragged morphologies associated with secondary rim consumption and yield garnet biotite geothermometry temperatures of 460 490 c from the lower horizon one garnet from each of two samples was analyzed these also display growth zoning but differ from garnet in the upper horizon in that they are dominantly idioblastic and yield garnet biotite geothermometry temperatures of 635 c garnet biotite geothermometry calculated for every point along detailed compositional traverses across the garnets revealed localized reequilibration along rims cracks and inclusions in both generations of garnet garnets from the upper horizon display prograde reequilibration and the garnets from the lower horizon display retrograde reequilibration numerical simulations of garnet growth using the gibbs method with duhem s theorem were carried out to determine p t paths the p t path defined by the modeling of five garnets from the upper horizon is an isothermal pressure increase of 1 7 kbar the p t path defined by the modeling of two garnets from the lower horizon has a steep p t trajectory d p d t 32 bars c and a total pressure change of 0 9 kbar both paths are indicative of thrust burial however the two paths cannot be reconciled as products of a single thrust episode these data are interpreted to indicate two different episodes of thrust burial during the sevier orogeny separated by 150 c of heating and partial exhumation