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

petrogenesis of the swaziland and northern natal rhyolites of the lebombo rifted volcanic margin south east africa thejozini and mbuluzi rhyolites and oribi beds of the southern lebombo monocline southeastern africa have geochemical characteristics that indicate they were derived by partial melting of a mixture of high ti zr and low ti zr sabie river basalt formation types compositional variations within the different rhyolite types can largely be explained by subsequent fractional crystallization the srand nd isotope composition of the rhyolites is unique amongst gondwana silicic large igneous provinces having end values close to bulk earth 0 94 to 0 35 and low but more variable initial sr sr ratios 0 7034 0 7080 quartz phenocryst do values indicate that the rhyolite magmas had do values between 5 3 and 6 7o consistent with derivation from a basaltic protolith with do values between 4 8 and 6 2o the low do rhyolites 56 0o come from the same stratigraphic horizon and are overlain and underlain by rhyolites with more normal do magma values these low do rhyolites cannot have been produced by fractional crystallization or partial melting of mantle derived basaltic material the rhyolites have low water contents making it unlikely that the low do values are the result of post emplacement alteration modification of the source by fluid rock interaction at elevated temperatures is the most plausible mechanism for lowering the do magma value it is proposed that the low do rhyolites were derived by melting of earlier altered rhyolite in calderas situated to the east which were not preserved after gondwana break up

uplift and erosional history of the deccan traps india evidence from laterites and drainage patterns of the western ghats and konkan coast abstract the western ghats escarpment in the western coastal region of peninsular india is locally capped by plateau remnants on which a regional high level laterite carapace is preserved geochemical fingerprinting studies show that the laterite has been developed from a protolith of panhala formation basalts the youngest known formation of the deccan sequence the high level laterite is in detail structurally concordant with its underlying basalts and hence it is concluded that it represents the original top of the lava pile a younger low level laterite carapace is developed on the konkan plain below the escarpment where it lies discordantly on basalts of the ambenali and poladpur formations from which it developed in situ the low level laterite rises topographically northwards in response to differential uplift probably at a maximum in the nasik region the main structures of the ghats region excluding the coastal monocline are the nasik dome and the mahabaleshwar anticline these have no effect on regional drainage patterns and it is argued that they are largely secondary features developing in response to post volcanic uplift

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

40ar 39ar dating of mineral separates and whole rocks from the western ghats lava pile further constraints on duration and age of the deccan traps abstract there has been an ongoing debate for almost two decades on the age and duration of deccan trap volcanism some of the best available ages have been determined using the 40 ar 39 ar method but differences in sample location laboratory procedure type of analyzed material and ages of monitors may have been responsible for much of the apparent spread in the data we have collected and analyzed samples from close to the bottom and top of the main lava pile in the western ghats near bombay we confirm previous findings that alteration and ar recoil strongly affect these rocks particularly measurements made on whole rocks when mineral separates mostly plagioclase are analyzed the spread is much reduced we find a mean age of 65 4 0 7 ma for five flows near the base of the section jahwar and igatpuri formations and 65 2 0 4 ma for a dyke cross cutting the poladpur formation below the topmost mahabaleshwar formation this implies that at least 1800 m out of the 2500 m composite trap section were erupted in a short interval less than 1 ma long and close to 65 5 ma i e close to the cretaceous tertiary boundary we find that older ages near 67 ma found by several authors are essentially based on whole rock samples which display disturbed spectra when this can be checked i e when the spectra are published

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

bimodal magmatism produced by delamination geochemical evidence from late palaeozoic volcanic rocks from the yili block western tianshan northwestern china

epicontinental permian cretaceous seaways in central india the debate for the narmada versus godavari rifts for the cretaceous tertiary incursion abstract this study presents an overview of mesozoic transgressions that influenced the palaeogeography of central india and offers an analysis of the existing evidence for the interpretation of the depositional record of infra and inter trappean sequences these data are then used to understand the respective palaeogeographic settings of the narmada and the godavari master valleys leading to a reappraisal of the paths suggested thus far for a cretaceous tertiary k t seaway major rift related discontinuities have been subjected to reactivation episodes during the mesozoic these tectonic events caused the initiation of several pericratonic basins the riftogenic valley systems have been affected by transgressions from the western margin of india through the narmada valley and or from the eastern margin through the godavari valley however for the most part these continent scale valley systems have hosted mainly mesozoic fluviatile and lacustrine sediment successions besides a marine incursion from the west in the lower reaches of narmada valley is widely recognised on the basis of the late cretaceous bagh beds additionally the maastrichtian palaeogeography of these valley systems of central india have been strongly influenced by the uplift resulting from the thermo magmatic flux associated with the deccan volcanism this resulted in major topographic readjustments within the godavari and narmada tapti palaeovalleys resulting in turn in major basinal depressions as revealed from the reconstruction of the palaeo digital elevation model significantly removed from the eastern and western coastal margins of india isolated occurrences of marine biota such as foraminifera marine algae and palm fossils have been found in infra and inter trappean maastrichtian and danian sequences of central india their presence in these inland basins is enigmatic and three hypotheses have been proposed to explain the presence of marine environments in these basins a trans deccan strait connecting both the margins of india through pre existing rift systems b an 800 km long seaway through the narmada valley and c a marine incursion through the godavari valley this palaeogeographic analysis supports a marine incursion through the godavari rift and presents arguments against the other two

magnetostratigraphy and 40ar 39ar geochronology of the malwa plateau region northern deccan traps central western india significance and correlation with the main deccan large igneous province sequences abstract basaltic lava flows of malwa plateau together with the mandla area and the toranmal section constitute the northern deccan province in central western india with the present outcrop area of 80 000 km2 the malwa flows form an extensive but relatively thin sequence we sampled deccan basalts along two 100 km each n s and e w transects along the western and southern rims of the malwa plateau the 67 12 0 44 ma 40ar 39ar age obtained in the present study clearly shows that the eruption of the malwa flows started during the magnetochron c30n by mapping the magnetic c30n c29r transition on the western and southern edge of the plateau the total volume and eruption rate can be calculated 42 000 km3 0 03 km3 a furthermore the results point towards the narmada tapi rift zone as the possible source area for the malwa lava flows the malwa sequence is older and partly synchronous with the thick sequences of the western ghats since their eruption centers are spatially independent they should not be correlated with younger although geochemically similar formations of the western ghats other northern deccan flows show clear similarities with the malwa flows and can be magnetostratigraphically and geochemically correlated we therefore suggest a new subgroup for the northern deccan province the narmada subgroup defined on the geochemical and magnetic characteristics of the malwa mandla and toranmal flows the introduction of this new subgroup clarifies the position of the northern deccan province in the deccan stratigraphy and is a step towards a better understanding of the deccan eruption history

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

were the deccan flood basalts derived in part from ancient oceanic crust within the indian continental lithosphere abstract deep mantle plumes supposedly incorporate deeply subducted eclogitized oceanic crust and continental flood basalts cfbs are now thought by some to be derived from such eclogite bearing peridotite plumes eclogite peridotite mixtures have much lower solidi and produce much greater melt fractions for a given temperature than peridotite fe rich eclogite or pyroxenite bearing sources have been inferred for many cfbs however plumes with considerable amounts of eclogite should have difficulty in upwelling owing to the high density of eclogite besides cfbs are always located along pre existing lithospheric structures suture zones edges of thick cratons and commonly associated with lithospheric rifting and continental breakup india s major late mesozoic cfb the deccan traps erupted through rift zones and a new continental margin that had developed along ancient suture zones traversing the subcontinent many deccan basalts are too fe rich to have been in equilibrium with a peridotite mantle source and have commonly been considered to be significantly fractionated derivatives of picritic liquids however it is possible to view them as relatively less evolved liquids derived from a source with extra fertility i e an fe rich source a new non plume plate tectonic model for icelandic hotspot volcanism involves melting of a shallowly recycled slab of eclogitized iapetus oceanic crust formerly trapped along the caledonian suture the model explains the geochemical petrological characteristics of icelandic basalts and is consistent with passive upper mantle upwelling under iceland inferred from recent seismic tomography based on the petrological and geochemical features of the deccan flood basalts of the type section in the western ghats i propose that old eclogitized oceanic crust trapped in the ancient indian suture zones could have produced voluminous basaltic melts during the deccan event

comments on deccan flood basalts at the cretaceous tertiary boundary by v courtillot j besse d vandamme r montigny j j jaeger and h cappetta the age of the flood basalts of the deccan area and the length of the interval during which the outpourings took place are not known precisely the authors have tried to date those basalts and estimate how long the volcanic activity lasted on the basis of new paleomagnetic paleontological and geochronological data of the deccan effusives they came to the preliminary conclusion that the volcanic activity may have lasted less than 1 ma and that the majority of the lavas poured out during chron 29r unfortunately the authors were not familiar with our paper on paleomagnetism and radiomettic age determinations of deccan trap lavas 2 in our paper we presented the paleomagnetic data derived from 223 individual lava flows from 16 sections all over the deccan area and gave the k ar age determinations for 13 selected unaltered lava flows we agree that the enormous volume of deccan extrusives poured out in a relatively short interval and that the volcanic activity was probably concerned with the spreading in the central indian ocean in late cretaceous and early tertiary time

the deccan tholeiite lavas and dykes of ghatkopar powai area mumbai panvel flexure zone geochemistry stratigraphic status and tectonic significance abstract mumbai city situated on the western indian coast is well known for exposures of late stage deccan pillow basalts and spilites pyroclastic rocks rhyolite lavas and trachyte intrusions these rock units and a little studied sequence of tholeiitic flows and dykes in the eastern part of mumbai city constitute the west dipping limb of a regional tectonic structure called the panvel flexure here we present field petrographic major and trace element and sr nd isotopic data on these tholeiitic flows and dykes best exposed in the ghatkopar powai area the flows closely resemble the mahabaleshwar formation of the thick western ghats sequence to the east in sr nd isotopic ratios and multielement patterns but have other geochemical characteristics e g incompatible trace element ratios unlike the mahabaleshwar or any other formation the flows may have originated from a nearby eruptive center possibly offshore of mumbai two dykes resemble the ambenali formation of the western ghats in all geochemical characteristics though they may not represent feeders of the ambenali formation lavas most dykes are distinct from any of the western ghats stratigraphic units some show partial e g sr nd isotopic similarities to the mahabaleshwar formation and these include several dykes with unusual concave downward ree patterns suggesting residual amphibole and thus a lithospheric source the flows and dykes are inferred to have undergone little or no contamination by lower continental crust most dykes are almost vertical suggesting emplacement after the formation of the panvel flexure and indicate considerable east west lithospheric extension during this late but magmatically vigorous stage of deccan volcanism

relationships between crustal contamination and crystallisation in continental flood basalt magmas with special reference to the deccan traps of the western ghats india the upper part of the deccan traps sequence bushe to mahabaleshwar formations shows a statistically significant tendency for the most mafic lavas to be the most contaminated by crustal materials this is the reverse of the relationship shown by suites evolving by contamination accompanied by fractional crystallisation afc the observed correlations e g between mg number and sr isotope initial ratios are partly an accidental consequence of the fact that the most mafic lavas are more abundant in the lower part of the sequence while contaminant availability declines in the upper part it is probable however that the correlations are augmented by increased contamination of hotter magma batches during ascent through dykes a process during which fractional crystallisation is suppressed by magmatic turbulence the absence of afc relationships suggests that most of the contamination took place during the ascent stage rather than in a magma chamber other continental flood basalt provinces such as the parana and etendeka do show afc relationships and it is speculated that this may be a result of magma chamber contamination coupled with flow rates which prevent contamination during ascent

the occurrence drivers and implications of submesoscale eddies on the martha s vineyard inner shelf abstractthe occurrence drivers and implications of small scale o 2 5 km diameter coherent vortices referred to as submesoscale eddies over the inner shelf south of martha s vineyard massachusetts are examined using high frequency hf radar based high resolution 400 m observations of surface currents within the 300 km2 study area eddies occurred at rates of 1 and 4 day 1 in winter and summer respectively most were less than 5 h in duration smaller than 4 km in diameter and rotated less than once over their lifespan 60 of the eddies formed along the eastern edge of study area adjacent to wasque shoal and moved westward into the interior often with relative vorticity greater than f eddy generation was linked to vortex stretching on the ebb and flood tide as well as the interaction of the spatially variable tide and the wind driven currents however these features had complex patterns of surface divergence and stretching eddies located away from wasque shoal were related to the movem

apatite liquid partition coefficients for the rare earth elements and strontium abstract sixteen sets of apatite liquid partition coefficients dap liq for the rare earth elements ree la sm dy lu and six values for sr were experimentally determined in natural systems ranging from basanite to granite the apatite melt glass assemblages were obtained from starting glasses artificially enriched in ree sr and fluorapatite components these were run under dry and hydrous conditions of 7 5 20 kbar and 950 1120 c in a solid media piston cylinder apparatus an sem equipped electron microprobe was used for subsequent measurement of ree and sr concentrations in coexisting apatites and quenched glasses the resulting partition coefficient patterns resemble previously determined apatite phenocryst groundmass concentration ratios in the following respects 1 the rare earth patterns are uniformly concave downward i e the middle ree are more compatible in apatite than the light and heavy ree 2 dreeap liq is much higher for silicic melts than for basic ones and 3 strontium and therefore eu2 is less concentrated by apatite than are the trivalent ree the effects of both temperature and melt composition on dreeap liq are systematic and pronounced at 950 c for example a change in melt sio2 content from 50 to 68 wt causes the average ree partition coefficient to increase from 7 to 30 a 130 c increase in temperature on the other hand results in a two fold decrease in dreeap liq partitioning of sr is insenstitive to changes in melt composition and temperature and neither the sr nor the ree partition coefficients appear to be affected by variations in pressure or h2o content of the melt the experimentally determined partition coefficients can be used not only in trace element modelling but also to distinguish apatite phenocrysts from xenocrysts in rocks reported apatite megacryst host basalt ree concentration ratios 12 for example are considerably higher than the equilibrium partition coefficients which suggest that in this particular case the apatite is actually xenocrystic a reversal experiment incorporated in our study yielded diffusion profiles of ree in apatite from which we extracted a ree ca interdiffusion coefficient of 2 4 10 14 cm2 s at 1120 c extrapolated downward to crustal temperatures this low value suggests that complete ree equilibrium between felsic partial melts and residual apatite is rarely established

constraints on the mantle sources of the deccan traps from the petrology and geochemistry of the basalts of gujarat state western india the late cretaceous early tertiary flood basalts in the gujarat area of the northwestern deccan traps kathiawar peninsula pavagadh hills and rajpipla exhibit a wide range of compositions from picrite basalts to rhyolites moreover the basaltic rocks have clearly distinct tio2 contents at any given degree of differentiation and strongly resemble the low titanium and hightitanium basalts found in most of the gondwana continental flood basalt cfb suites four magma groups are petrologically and geochemically distinguished 1 a low ti group characterized by rocks with varying sio2 saturation and with tio2 1 8 wt in picrites nb 19 p p m zr av 6 5 and tt v av 47 3 an intermediate ti group with tio2 contents slightly lower than the high ti rocks at the same degree of evolution and with correspondingly lower incompatible trace element contents and ratios in particular k2o nb ba and zr y av 5 2 4 a potassium rich group kt broadly similar in geochemical character to the high ti group but showing more extreme k rb and ba enrichment av k20 na20 l ba y 20 the most primitive low ti and high ti picrites when corrected for low pressure olivine fractionation show distinct major and trace element geochemistry in particular for cao ai2o3 cao tio2 and al2o3 tio2 and moderate but significant variations in their sio2 and fe2ost contents these characteristics strongly suggest the involvement of different mantle sources more depleted for the low ti picrites and richer in cpxfor the high ti picrites but with broadly the same pressures of equilibration 27 14 kbar this in turn suggests a strong lateral heterogeneity in the gujarat trap mantle low ti picrites and related differentiates in kathiawar are reported systematically for the first time here and suggest the existence of hfse depleted mantle in the northwestern deccan traps with extension at least to the seychelles islands and to the area of the bushe formation near bombay in the pre drift position before the development of the carlsberg ridge the absence of correlations between lile hfse ratios and sio2 argues against crustal contamination processes acting on the low ti picrites possibly owing to their probably rapid uprise to the surface consequently the mantle region of this rock group was probably re enriched by small amounts of ule rich materials the substantially higher trace element enrichment of the least differentiated high ti picrites relative to the basalts of the ambe noli and mahableshwar formations of the western ghats testifies also to the presence of more incompatible element rich oib4ike mantle sources in northern and northwestern gujarat these sources were geochemicaily similar to the present day reunion mantle sources

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

aeolian sediment transport and deposition in a modern high latitude glacial marine environment aeolian sand and dust in polar regions are transported offshore over sea ice and released to the ocean during summer melt this process has long been considered an important contributor to polar sea floor sedimentation and as a source of bioavailable iron that triggers vast phytoplankton blooms reported here are aeolian sediment dispersal patterns and accumulation rates varying between 0 2 g m 2 yr 1 and 55 g m 2 yr 1 over 3000 km2 of sea ice in mcmurdo sound south west ross sea adjacent to the largest ice free area in antarctica sediment distribution and the abundance of southern mcmurdo volcanic group derived glass show that most sediment originates from the mcmurdo ice shelf and nearby coastal outcrops almost no sediment is derived from the extensive ice free areas of the mcmurdo dry valleys due to winnowed surficial layers shielding sand sized and silt sized material from wind erosion and because of the imposing topographic barrier of the north south aligned piedmont glaciers southerly winds of intermediate strength ca 20 m sec 1 are primarily responsible for transporting sediment northwards and offshore the results presented here indicate that sand sized sediment does not travel more than ca 5 km offshore but very fine sand and silt grains can travel 100 km from source for sites 10 km from the coast the mass accumulation rate is relatively uniform 1 14 0 57 g m 2 yr 1 three orders of magnitude above estimated global atmospheric dust values for the region this uniformity represents a sea floor sedimentation rate of only 0 2 cm kyr 1 well below the rates of 9 cm kyr 1 reported for biogenic dominated sedimentation measured over much of the ross sea these results show that even for this region of high windblown sediment flux aeolian processes are only a minor contributor to sea floor sedimentation excepting areas proximal to coastal sources

the chogat chamardi subvolcanic complex saurashtra northwestern deccan traps geology petrochemistry and petrogenetic evolution abstract the saurashtra region in the northwestern part of the deccan continental flood basalt province india is notable for its many volcano plutonic complexes compositional diversity and the abundance of rhyolite and granophyre in this first detailed study of the chogat chamardi subvolcanic complex in eastern saurashtra we report microgranite and granophyre plutons gabbro plutons cut by basaltic andesite dykes as well as rhyolite plugs bulk rock geochemical including nd sr isotopic data suggest a strong contribution from ancient rb rich basement crust to the silicic magmas the mafic rocks are also crustally contaminated but less than the silicic rocks in conformity with observations elsewhere e g the skye and skaergaard granophyres the chogat chamardi silicic rocks have initial 87sr 86sr for t 65 ma as high as 0 72914 and initial end values as low as 13 9 trends defined by the chogat chamardi and other deccan rhyolitic suites on plots of sr concentration vs sr isotopic ratio are modeled with concurrent assimilation and fractional crystallization afc processes involving a basaltic magma and granitic basement rocks considering both nd and sr isotopic variations the chogat chamardi silicic rocks notably require crustal end members very different from those inferred in most previous isotopic studies of deccan rocks

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

geochemistry and paleomagnetism of late cretaceous mafic dikes in kerala southwest coast of india in relation to large igneous provinces and mantle plumes in the indian ocean region new geochemical and paleomagnetic results are presented on two late cretaceous dikes of the 85 90 ma leucogabbroic and doleritic dikes and the 65 70 ma dolerites in kerala india the dikes are rich in incompatible elements have fractionated patterns with light rare earth element enrichment and are akin geochemically to cretaceous basalts on the east coast of madagascar the magmas were formed at garnet lherzolite depths above the marion plume constituting part of a large igneous province in madagascar in contrast the 65 70 ma dolerites are moderately depleted in incompatible elements with almost flat rare earth element patterns and resemble the upper formations of the deccan traps and the tholeiitic dikes of the seychelles these dolerites were formed by melting of spinel lherzolite over the reunion plume paleomagnetic data from the dikes and the other coeval igneous units from south india provide the 90 ma pole latitude 24 longitude 293 a 95 5 9 n 18 sites for india the 65 70 ma dolerites possess both normal and reverse polarities and the mean pole latitude 36 longitude 283 a 95 5 7 n 10 sites compares well with the deccan superpole paleolatitude estimates indicate 5 southward migration for the marion plume and a northward migration for the reunion plume in conformity with global mantle circulation models however distinguishing migration of the reunion plume from the effects of true polar wander is difficult furthermore the geodynamic reconstructions extending the shear zones of southern madagascar into south india are not tenable

the mount pavagadh volcanic suite deccan traps geochemical stratigraphy and magmatic evolution abstract the patterns of eruption and dispersal of flood basalt lavas on the surface or as magmas in dykes and sills within the crust determine the volcanological and stratigraphic development of flood basalt provinces this is a geochemical and sr isotopic study of lavas of varied compositions that outcrop around mount pavagadh 829 m deccan traps an important outlier north of the main basalt outcrop most of the 550 m thick exposed section at pavagadh is made up of subalkalic basalts rich in the incompatible elements particularly nb ba and sr picrite and rhyolite dacite flows also occur the latter capping the sequence the relatively high initial 87sr 86sr ratios up to 0 7083 and chemical characteristics of the rhyolitic rocks of pavagadh are consistent with a small but significant involvement of the granitic basement crust in their genesis an assimilation fractional crystallization afc model involving the picrite lava and either a southern indian or a western indian granite as the contaminant explains the geochemical and sr isotopic variation in the basalts and the rhyolites quite well a systematic comparison of the basaltic lavas with binary plots normalized multielement patterns and discriminant function analysis to the well established lava stratigraphy of the western ghats 400 500 km to the south precludes any chemical genetic relationships between the two basalts exposed in sections closer to pavagadh 150 200 km in the toranmal navagam and barwani mhow areas have several flows with some similar chemical characteristics however the pavagadh sequence is significantly different from all of these sequences geochemically petrogenetically and in magnetic polarity to be considered independently built this result is significant in terms of eruptive models for the deccan traps as it is increasingly apparent that there were separate but possibly coeval eruptive centers with their own distinctive chemistries developed in various areas of this vast 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

physical volcanology of the pavagadh rhyolites northern deccan traps stratigraphic structural and textural record of explosive and effusive eruptions abstract mount pavagadh 840 m in the northern deccan traps india is mainly composed of mafic lava flows picrites ankaramites mugearites basalts and capped by rhyolites rhyolites also found at lower elevations on and around the mountain are thought to represent blocks of the eroded upper rhyolites or local eruptions in situ although the pavagadh sequence has been studied for 150 years the stratigraphy structures textures and physical volcanology of the pavagadh rhyolites have been poorly documented and are discussed here at length the upper rhyolite sequence contains tens of non welded crystal bearing glass and pumice rich ash beds of 47 m total thickness these represent a primary fallout ash succession derived from distant plinian eruptions possibly corresponding with massive ignimbrite deposits recently recognised in saurashtra several hundred kilometers to the southwest the fallout ash succession is overlain by a 50 m thick lava flow showing well developed flow banding and folding basal and upper breccias and a local basal vitrophyre the lava flow is overlain by an ignimbrite with a basal vitrophyre both these units showing excellent vitroclastic textures the upper rhyolites thus represent alternating explosive effusive explosive eruptions the lower level rhyolites in the area showing intense rheomorphic deformation in outcrop but lacking vitroclastic textures are probably also lavas however they are not blocks of the eroded upper rhyolite lava flow but from separate eruptions the pavagadh rhyolites were probably far more laterally extensive than their present outcrops with a minimum original volume of 9 km3 their source areas remain unknown geochemical and geochronological work on the pavagadh rhyolites is needed in the stratigraphic and volcanological context provided here to understand their petrogenesis and potential relationship with the cretaceous palaeogene k pg boundary mass extinction

petrogenesis of rhyolites and trachytes from the deccan trap sr nd and pb isotope and trace element evidence trachytes and rhyolites from salsette island north of bombay have distinctive trace element and isotope features which mark them out from typical crustal melts their highly incompatible trace element and sr nd and pb isotope ratios are similar to those of the associated deccan flood basalts thus the rhyolites and trachytes are closely related to the basalts and a striking compositional gap between 50 and 65 sio2 suggests that the high sio2 rocks evolved by 10 15 partial melting followed by variable amounts of fractional crystallisation the source material could have been basalt within the deccan trap or related gabbroic rocks in deep crustal sill complexes the rhyolites yield an rb sr whole rock age of 61 5 1 9 ma with a slightly high initial 87sr 86sr 0 7085 18 it is argued that crustal extension provides a suitable regime for the generation of acid magmas by partial melting of associated basic rocks

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

a post k t boundary early palaeocene age for deccan type feeder dykes goa india basaltic dykes exposed along the coast of goa represent the youngest phase of a number of different mafic suites intruding the complex precambrian terrain that forms the pre deccan basement of peninsular india these dykes crop out 50 80 km beyond the southern limits of the deccan continental flood basalt cfb province four of seven sampled dykes were dated using the whole rock 40ar 39ar step heating technique three 40ar 39ar determinations yield palaeocene plateau and isochron ages that clearly indicate an early post cretaceous i e danian age and as a weighted mean average of all four samples yields an age of 62 8 0 2 ma these intrusions are clearly significantly younger than the current estimates for the age range of deccan traps volcanism 65 69 ma the trace element signatures rare earth element patterns and 87sr 86sr ratios of these dykes are consistent with those of the nearby deccan basalts more specifically their geochemical signatures are similar to those of the basalt flows composing the volumetrically important ambenali mahabaleshwar formations which form the uppermost succession wai subgroup of the sw deccan these geochemical and age data demonstrate that deccan type magmatism continued to affect western india for at least 1 2 my after the k t boundary 65 0 0 1 ma and therefore their emplacement places important constraints upon the timing duration and evolution of this much debated cfb province the importance of these dykes is examined within the context of the deccan eruptions and the associated rifting of the western indian margin

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