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

the occurrence of giant bivalve alatoconchidae from the yabeina zone upper guadalupian permian in european tethys abstract the permian gigantic bivalve family alatoconchidae forms a member of the tropical shallow marine fauna intimately associated with the tethyan warm water adapted verbeekinidae fusulines such as neoschwagerina and yabeina the velebit mountains in central croatia is one of the nine areas in the world that yielded alatoconchidae as first reported early in the 1970 s in addition to the neoschwagerina zone wordian middle guadalupian originally described we newly found alatoconchidae from the yabeina zone capitanian upper guadalupian from the brusane area in the central velebit mountains alatoconchidae occur in three stratigraphic levels of the velebit formation composed of black fusuline algal wackestone packstone with yabeina although fragmented the alatoconchidae shell reach sizes over 40 cm long and possess unique morphology with a lateral flange plus a prominent prismatic external layer in the double layered shell structure our results confirm for the first time in europe and thus in the western tethyan domain that the stratigraphic range of the gigantic bivalve family extends up to the capitanian as well as in the superocean panthalassa as no occurrence has been reported from the overlying lopingian upper permian alatoconchidae likely became extinct globally at the end of the guadalupian i e through the guadalupian lopingian boundary mass extinction

yvonaspis nouveau genre d osteostraci vertebrata agnatha du devonien emsien eifelien des gres de gaspe quebec canada all osteostracan species of the gaspe sandstones devonian of canada formerly attributed to the genus cephalaspis are referred here to the new genus yvonaspis the latter is characterized by the very elongated shape of the naso hypophysal opening this monophyletic genus may be the sister group of parameteoraspis among cephalaspidians

interior of the moon the presence of garnet in the primitive deep lunar mantle new trace element data have been generated by inductively coupled plasma mass spectrometry icp ms for 65 high and low ti mare basalts from the apollo 11 12 14 15 and 17 landing sites these data demonstrate that ratios of the high field strength elements and yttrium are generally chondritic in the mare source regions for the low ti basalts but nonchondritic in the high ti basalts and potassium rare earth elements and phosphorus kreep such differences can be explained by addition and subtraction respectively of ilmenite this interpretation is consistent with derivation of the basalts from lunar magma ocean lmo cumulates through which late stage ilmenite has sunk and mixed with earlier magma ocean products compositional differences between the mare basalts and volcanic glass beads suggest that at least some of the latter were derived from a garnet bearing source region siderophile and chalcophile element data suggest that the garnet bearing source escaped the lmo melting event and thus represents primitive moon material the remaining glasses are generally very low ti vlt and appear to have been derived from early lmo cumulates i e olivine and orthopyroxene similar to the vlt basalts the remaining glasses were derived from below the lmo cumulates and as the magmas rose they underwent sequential assimilation to varying degrees initially of an ilmenite rich cumulate followed by a kreep component this study presents geochemical evidence for the existence of garnet in a mare source and demonstrates the likelihood that the lmo was not whole moon thus preserving primitive lunar material which has acted as the source region for at least some of the volcanic glasses

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

the skaergaard liquid line of descent revisited there is a fundamental conflict between the suggestion that the iron content of skaergaard liquids increases during fe ti oxide fractionation and the observation that at the same time oxygen fugacity f text o text 2 drops by two log units below the fayalite magnetite quartz oxygen buffer fmq a new petrographic study of average skaergaard gabbros shows that the total modal content of fe ti oxides is about 22 in the early lzc and markedly decreases to below 5 in the uzc forward modeling based on these modal constraints as well as experimental results on skaergaard related dikes predicts that fractionation of troctolitic lza gabbros drives the derivative liquid towards a high iron content strong iron enrichment continues together with a small decline in silica during lzb crystallization due to the appearance of augite as a fractionating phase the fractionation of fe ti oxides in the lzc initially suppresses iron enrichment and reverses the silica trend to one of slight enrichment however continued evolution into the uz produces liquids with maximum uzc feo content of 23 25 wt and sio2 content of 53 wt feo is total iron as feo the maximum in feo is dependent on several factors of which the fe ti oxide mode has the strongest effect the f text o text 2 during crystallization of the lzc is widely thought to have been at or slightly below the fayalite magnetite quartz oxygen buffer fmq under closed system evolution incorporation of ferric iron into augite during formation of the lzb restricts the increase in f text o text 2 to about 0 1 log units above fmq 0 1 fmq likewise crystallization of the lzc through the uza involving fe ti oxide minerals leads to a decline in f text o text 2 of less than 0 1 fmq crystallization of the uzb c gabbros results in oxidation to a maximum of 0 5 fmq this behavior can account for the iron rich character of the uz gabbros as well as the low modal content of fe ti oxides thus evolved skaergaard liquids are high in iron and contain a modest amount of sio2 our modeling result do not account for a strong drop in f text o text 2 through the layered series such a drop would require an unacceptably high proportion of fe ti oxides and high magnetite content in the fractionating assemblage

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

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

ultra high temperature metamorphism and its significance in the central indian tectonic zone abstract in the present study from the southern margin of the central indian tectonic zone it is demonstrated how the metamorphic p t path of ultrahigh temperature granulite terranes can be reconstructed using the metamorphic transition in corundum granulites from early biotite melting to later fmas solid solid reaction the extreme metamorphism in these rocks caused two stage biotite melting resulting in initial porphyroblastic garnet 1 and later sapphirine spinel 1 incongruent solid mineral assemblages during this process the leucocratic and melanocratic layers in the corundum granulites evolved from an initial silica oversaturated to a later silica undersaturated domain in the melanocratic layer this allowed localized concentration of sapphirine spinel 1 and residual sillimanite 1 producing an extremely restitic assemblage at the culmination of peak metamorphism bm 1 bm 1 is constrained at 1000 c at relatively deep crustal levels p 9 kbar from the stability of ferroaugite in a co metamorphosed iron formation granulite during subsequent metamorphism bm 2 the reaction path and history in the corundum granulites shifted to the restitic domain allowing reacting sapphirine spinel 1 and sillimanite to produce coronal garnet 2 corundum assemblage via a fmas univariant reaction in the final stages of reaction history biotite 2 sillimanite 2 spinel 2 assemblage was produced after garnet 2 corundum due to localized melt crystal interaction the metamorphic sequence when interpreted with the help of a newly constructed qualitative kfmash petrogenetic grid reveals successive stages of heating increasing pressure and cooling around the kfmash invariant point opx crd which is consistent with a counterclockwise metamorphic p t path the near isobaric nature of post peak cooling t 250 300 c is also evident from multistage pyroxene exsolution and by the appearance of lamellar and coronal garnets in the iron formation granulites this study provides the first tight constraint for ultrahigh t metamorphism along a counter clockwise p t trajectory in the central indian tectonic zone and has important bearing for terrane correlations in this part of east gondwanaland in addition the new kfmash grid allows evaluation of metamorphic phase relations in ultrahigh t corundum bearing and corundum absent aluminous granulites

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

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

implications of simple flexure theory for the formation of saucer shaped sills saucer shaped dolerite sills have been identified in several sedimentary basins they are characteristically sub circular in plan with an inner sill connected to a steeply inclined arcuate sheet extending upwards from the edge of the inner sill and in some cases have raggedly developed flat outer rims by applying elastic plate theory to the overburden we obtain an expression for the maximum radius of a circular sill in terms of emplacement depth excess magma pressure elastic constants of the overburden and the tensile strain at failure this expression predicts that the radius of the flat base of a saucer shaped sill increases with emplacement depth as observed in the western karoo basin south africa some saucer shaped sills display an elliptical morphology from consideration of the flexural strains around the periphery of a horizontal elliptical sill we infer that saucer shaped elliptical sills have been fed from below by feeder dykes oriented along the major axis of the ellipse flat outer rims would have developed at the final stage of each intrusive episode when the excess magma pressure was decreasing towards equilibrium with the overburden load consequently flexural strains would also have decreased so continued intrusion might preferentially occur along a bedding plane

uranium decay series the discovery of the 238u decay chain of course started with the seminal work of marie curie in identifying and separating 226ra through the work of the curies and others all the members of the 238u decay chain were identified an important milestone for geochronometrists was the discovery of 230th called ionium by bertram boltwood the yale scientist who also made the first age determinations on minerals using the u pb dating method boltwood in 1906 established the antiquity of rocks and even identified a mineral from sri lanka then ceylon as having an age of 2 1 billion years the application of the 238u decay chain to the dating of deep sea sediments was by piggott and urry in 1942 using the ionium method of dating actually they measured 226ra itself through 222rn assuming secular equilibrium had been established between 230

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

the nature of mesoproterozoic central indian tectonic zone with exhumed and reworked older granulites abstract the ese wnw trending central indian tectonic zone citz divides the indian subcontinent into southern and northern crustal provinces its narrow northern belt consists of closed early proterozoic mahakoshal rift zone which is presently confined between two moho reaching faults that characterize the son narmada sona lineament the mesoproterozoic sausar mobile belt smb the chhotanagpur granite gneiss complex cggc and the gneissic complex of northeast india represent the wider southern belt the mahakoshal rift belt has a thick pile of mafic ultramafic rocks and possibly oceanic components the basin closed due to a south directed subduction creating a magmatic arc that was sutured to its southern boundary during 1 8 1 7 ga a south dipping ductile shear zone known as the son narmada south fault affects both its supracrustals and intrusive granitoids the son narmada south fault generally delimits the northern boundary of the gondwana basins whereas the son narmada north fault delineates the northern boundary of the mahakoshal mobile belt and the southern boundary of the meso neoproterozoic vindhyan basin the smb representing the southern belt of the citz in the central sector comprises strongly folded and metamorphosed non volcanic and mn rich sausar sediments intermixed with reworked basement gneisses migmatites and granulites representing the pre sausar assemblage the sausar orogeny closed during terminal mesoproterozoic ca 1 0 ga on the other hand metamorphism in the basement gneiss migmatites and northern and southern granulite belts pre dates the sausar orogeny the earliest structure of the smb truncates and rotates the n s oriented structures of the southward located sakoli and the dongargarh belts of neoarchaean palaeoproterozoic age the northern granulite belt of the smb and the cggc and the high grade supracrustals from the sonapahar area in ne india all indicate very high p t conditions during 1 7 1 5 ga peak metamorphism which was followed by decompressional cooling the setting resembles continental collision which was followed by tectonic exhumation and denudation on the other hand the southern granulite belt from the smb documents a late archaean 2 6 ga event and a later 1 4 ga event the dalma volcanic belt located close to the southern tectonized boundary of the cggc belt and possibly the assemblage of copious mafic enclaves within the ca 550 ma granite plutons from ne india represent back arc like setting during 1 6 1 5 ga the setting is caused due to the southward subduction of the north indian block beneath the southern block leading to continental collision the metasedimentary belt located further to the south of the dalma volcanic belt and flanking the archaean singbhum craton to the north on the other hand is palaeoproterozoic in age but has been metamorphosed during ca 1 6 ga because of the northern collision related orogeny reworking and incorporation of basement gneisses and granulites from the mid crust level to the younger sausar cover sediments denote renewed spell of thrust tectonics that remobilized the collision zone rocks of the earlier orogeny the tectonic transport was from the north to the south with north dipping thrusts thus there was a reversal in the direction of convergence from the preceding orogeny the sausar orogeny closed during ca 1 0 ga it caused anatectic event producing granitoid plutons migmatites and charnockites in the cggc belt and migmatites in the gneissic complex in ne india

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

high precision pb sr nd hf isotopic characterization of usgs bhvo 1 and bhvo 2 reference materials the recent development of multiple collector inductively coupled plasma mass spectrometry mc icp ms and increasing use of the technique have created the need for well characterized rock standards especially for isotopic systems where no internal fractionation correction can be applied this paper presents a careful leaching experiment on the u s geological survey usgs reference materials bhvo 1 and bhvo 2 hawaiian basalts and documents the evidence for contamination of the rock powders during processing this contamination accounts for the difference in pb isotopic ratios of bhvo 1 and bhvo 2 as well as for their lack of homogeneity both in pb isotopic compositions and in some trace element contents copyright 2005 by the american geophysical union

infrastructure and mechanics of volcanic systems in iceland the divergent plate boundary in iceland consists of more than two dozen systems where most of the volcano tectonic activity takes place at the surface the volcanic systems are characterised by 5 20 km wide and 40 100 km long swarms of tension fractures 102 m long normal faults 103 m long and volcanic fissures the holocene fissure swarms are confined to less than 10 000 year old basaltic lava flows mostly pahoehoe occurring near the centre of the active rift zone in any particular swarm the number of tension fractures exceeds that of normal faults all tension fractures and normal faults are vertical at the surface indicating that the surface parts were generated by an absolute tension in addition to a fissure swarm many volcanic systems have a central volcano some of which have developed collapse calderas in the late tertiary and pleistocene lava pile of iceland extinct volcanic systems are represented by local sheet swarms and regional dyke swarms the sheet swarms are normally circular or slightly elliptical several kilometres in radius and are confined to the extinct central volcanoes many swarms are associated with large plutons exposed at 1 2 km depth beneath the initial top of the rift zone and presumably the uppermost parts of extinct crustal magma chambers and in short traverses up to 90 of the rock may consist of sheets the sheets have a very variable strike dip on average 45 65 mostly towards the centre of the associated volcano and have an average thickness of about 1 m the regional dykes occur outside central volcanoes in swarms that are commonly 50 km long and 5 10 km wide in several kilometre long traverses commonly 1 5 of the rock consist of dykes but occasionally as much as 15 28 most regional dykes are subparallel and subvertical the average dyke thickness in the pleistocene swarms is less than 2 m but 4 6 m in the tertiary swarms while active the volcanic systems in the rift zone are supplied with magma from reservoirs located at the depth of 8 12 km at the boundary between the crust and upper mantle the reservoirs are partially molten with totally molten top regions and of cross sectional areas similar to those of the volcanic systems that they feed some active volcanic systems especially those that develop central volcanoes high temperature areas and calderas have in addition to the deep seated magma reservoirs shallow crustal magma chambers located at 1 3 km depth which in turn are fed by the deeper magma reservoirs it is proposed that the regional dyke swarms are supplied with magma largely from the deep seated reservoirs whereas the local sheet swarms are mainly fed from the associated crustal magma chambers because the volume of a crustal chamber is less than that of its deeper source reservoir a single magma flow dyke intrusion from the reservoir may trigger tens of magma flows sheet intrusions from the chamber which is one explanation for the enormous number of sheets associated with many central volcanoes the sheets follow the stress trajectories of the local stress fields around the source chambers whereas the regional stress field associated with the divergent plate movements controls the emplacement of the regional dykes it is suggested that many dykes develop as self affine some as self similar structures when applied to the krafla volcanic system in northern iceland model calculations suggest that a magma flow from the krafla reservoir with regional dyke formation should occur on average once every several hundred years and that tens of sheets might be injected from the shallow krafla chamber and triggered by a single magma flow from the krafla reservoir these results are in broad agreement with the available data

a reappraisal of the coastal panvel flexure deccan traps as a listric fault controlled reverse drag structure abstract the panvel monoclinal flexure of the deccan traps flood basalt province along the rifted west coast of india is neither a monocline formed due to simple downbending of the lava pile e g due to sag nor due to compressional folding nor merely an extensional fault structure as previously proposed but is interpreted here as a reverse drag structure on an east dipping listric master fault that should lie offshore bombay with numerous subsidiary antithetic west dipping and sympathetic east dipping faults the proposed model satisfactorily explains a wide range of observations such as the systematic westerly increase in dip of the basalts their occurrence at shallow levels on the continental shelf syn volcanic subsidence and sedimentation and oil reserves of the shelf innumerable graben systems of the continental interiors in the world are indeed characterized by listric faults and reverse drag structures and the reverse drag model appears highly applicable to most flood basalt provinces with coastal flexures and seaward dipping reflector sequences at rifted continental margins namely karoo parana east greenland west greenland and afar

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