comment on 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps by knight et al earth planet sci lett 208 2003 85 99 abstract knight et al presented age and chemical data on two sets of lava flows from the rajahmundry area on either bank of the godavari river the age and petrogenesis of these flows and their possible link to sections of the main deccan province are of importance to the understanding of many aspects of flood basalt volcanism i comment on a the use of geochemical fingerprints for lava identification correlation at rajahmundry superceding apparent field relations b their 40 ar 39 ar data and its refinement based on statistical tests and the alteration state of the samples c correlation of age data and the magnetic polarity of the lavas to the geomagnetic polarity time scale and d the possibility that both lavas at rajahmundry were formed by intracanyon flows derived from 1000 km away

laser probe 40ar 39ar dating studies on sub milligram whole rock basalt samples the age of the steens mountain geomagnetic polarity transition revisited the steens mountain geomagnetic polarity transition is arguably the most comprehensively studied geomagnetic field reversal trapped in igneous rocks over twenty years ago this reversal was dated at 15 5 0 3 ma by the kar method we have now tested the feasibility of carrying out 40ar 39ar dating on 1 mg samples of fine grained whole rock basalt utilizing a laser probe i e on samples 1000 times smaller than have been traditionally analyzed twenty samples taken from lava flows below within and above the reversal n r in steens mountain oregon have been studied by this laser incremental heating technique eighteen of these specimens yield acceptable plateau ages with a weighted mean age of 6 2 0 1 ma standard error on the mean this result is in excellent agreement with the results obtained on two bulk samples of whole rock basalt that average 16 1 0 1 ma an attempt to confirm these ages by fusion of plagioclase laths taken from a few of the individual lava flows was hampered by their low k content 0 2 0 3 our best estimate for the age of six plagioclase laths each 1 mg weight is 16 5 0 5 ma we suggest the techniques utilized herein could be extended to carry out dating of basaltic material of age 1 ma

evaluation of recent tectonomagmatic discrimination diagrams and their application to the origin of basic magmas in southern mexico and central america discrimination diagrams to decipher tectonic settings have been in use for nearly 40 years although old diagrams have been extensively used the recent ones based on discriminant functions of ratio variables with or without log transformation proposed during 2004 2010 for the discrimination of four tectonic settings of island arc continental rift ocean island and mid ocean ridge were newly evaluated to show their high success rates of 57 3 100 and 58 5 100 for major element and immobile element based diagrams respectively for the continental arc of the andes evaluated for its similarity to island arc these four sets of diagrams showed success rates of 62 1 83 8 these four sets of five diagrams per set were therefore used to infer tectonic setting of the mexican volcanic belt mvb los tuxtlas volcanic field ltvf and central american volcanic arc cava using this approach the mvb especially its western central and eastern parts and the ltvf of southern mexico show a dominantly continental rift setting and the cava shows an arc setting the west central part of the mvb is consistent with dual tectonics of arc and rift these results confirm the application of an unusual mantle upwelling rift model for the mexican on land volcanism whereas the conventional plate tectonic subduction model seems to be applicable for the cava from guatemala to north western costa rica

volcan las navajas a pliocene pleistocene trachyte peralkaline rhyolite volcano in the northwestern mexican volcanic belt volcan las navajas a pliocene pleistocene volcano located in the northwestern portion of the mexican volcanic belt erupted lavas ranging in composition from alkali basalt through peralkaline rhyolite and is the only volcano in mainland mexico known to have erupted pantellerites las navajas is located near the northwestern end of the tepic zacoalco rift and covers a 200 m thick pile of alkaline basaltic lavas one of which has been dated at 4 3 ma the eruptive history of the volcano can be divided into three stages separated by episodes of caldera formation during the first stage a broad shield volcano made up of alkali basalts mugearites benmoreites trachytes and peralkaline rhyolites was constructed eruption of a chemically zoned ash flow then caused collapse of the structure to form the first caldera the second stage consisted of eruptions of glassy pantellerite lavas that partially filled the caldera and overflowed its walls this stage ended about 200 000 years ago with the eruption of pumice falls and ash flows which led to the collapse of the southern portion of the volcano to form the second caldera during the third stage two benmoreite cinder cones and a benmoreite lava flow were emplaced on the northwestern flank of the volcano finally the calc alkaline volcano sanganguey was built on the southern flank of las lavajas alkaline volcanism continued in the area with eruptions of alkali basalt from cinder cones located along nw trending fractures through the area although other mildly peralkaline rhyolites are found in the rift zones of western mexico only las navajas produced pantellerites greater volumes of basic alkaline magma have erupted in the las navajas region than in the other areas of peralkaline volcanism in mexico a factor which may be necessary to provide the initial volume of material and heat to drive the differentiation process to such extreme peralkaline compositions

multicomponent magnetizations from the mississippian mauch chunk formation of the central appalachians and their tectonic implications previous paleomagnetic study of mississippian red beds of the mauch chunk formation from the central appalachians reported shallow inclination directions a positive fold test was obtained after thermal demagnetization typically to only 550 c the near equatorial position for north america indicated by these results has been used to support the idea of a circa 15 latitudinal tectonic offset of the acadia displaced terrain in the northern appalachians the present study of 153 samples from 29 sites occupied in the mauch chunk formation of eastern and southern pennsylvania reveals multicomponent magnetizations a characteristic component declination d 161 0 inclination i 27 8 alpha95 a95 7 9 with unblocking temperatures usually concentrated above 650 c and with dual polarity 10 sites normal 13 sites reversed is isolated in 23 sites and passes a fold test at the 99 confidence level it is interpreted as an early acquired hematitic detrital or chemical magnetization of pre alleghanian orogeny and pre kiaman reverse polarity interval age a southern hemisphere paleolatitude of 15 for the sampling area is indicated another component has generally lower unblocking temperatures which however can extend up to 650 c or more and therefore is also likely to be carried by hematite this secondary magnetization d 170 6 i 5 3 a95 5 0 isolated in 25 sites is of uniform reverse polarity and application of fold tests suggests that it is of synfolding origin a near equatorial paleolatitude 3 n at the time of acquisition is obtained consistent with permo carboniferous kiaman directions from north america incomplete removal of the secondary component can explain the previous results from the mauch chunk comparison of the present paleomagnetic results with those from similar age rocks in the canadian maritimes does not support a latitudinal offset of acadia with respect to interior north america in the early carboniferous in fact according to these and other paleomagnetic data thought to be representative of this age the atlantic bordering continents already were near to a pangea configuration the alleghanian and particularly the hercynian orogenies therefore may not be associated with the closure of a large latitudinal ocean later in the carboniferous and permian alleghanian deformation however may have resulted in the partial bending of the pennsylvanian salient on the basis of apparently systematic deviations in paleomagnetic declinations

limits on the age of the deccan traps of india from paleomagnetic and plate reconstruction data and their uncertainties revised limits on the age of the deccan traps of india are provided by recent improvements to reconstructions of india relative to surrounding plates since late cretaceous time realistic estimates of the uncertainties in the plate reconstructions and recent paleomagnetic data from the deccans traps and from north america we estimate the age of the deccan traps by rotating its paleomagnetic pole into the north american reference frame different assumed ages for the deccan traps result in very different positions for its paleomagnetic pole relative to north america comparing these positions with the north american apparent polar wander path and accounting for uncertainties in both the paleomagnetic poles and in the reconstructions we place limits on the age of the deccan traps of 62 72 ma significantly younger than found in several prior studies the 62 72 ma limits are consistent with ages obtained independently from radiometric dating and paleontology and are consistent with the hypothesis that the deccan traps were extruded rapidly during reversed polarity chron 29r which contains the cretaceous tertiary boundary

paleomagnetism of the upper ordovician juniata formation of the central appalachians revisited again two components of magnetization were isolated in the upper ordovician juniata formation sampled in the area of the pennsylvania salient the thermally distributed reversed polarity b component was most likely acquired during alleghenian deformation and although it is poorly grouped it is similar to other appalachian synfolding magnetizations the pre alleghenian age c magnetization is entirely of normal polarity and shows a difference in declinations between the mean magnetizations isolated on the northern and southern limbs of the salient of 24 23 this anomaly is consistent with the sense and magnitude of declination anomalies observed in pre alleghenian magnetizations isolated in other throughgoing appalachian red beds of silurian devonian and early carboniferous age the mean inclination of 44 7 suggests a paleolatitude of about 26 s for the central appalachians in the late ordovician this paleolatitude fits a trend of southward motion of north america from the ordovician to the early devonian followed by northward drift through the remainder of the paleozoic

40ar 39ar and k ar geochronology of the dykes from the south indian granulite terrain abstract 40ar 39ar and conventional k ar data on mafic dykes and a felsic dyke together with previous data have been used to constrain the ages of the dyke magmatism in the south indian granulite terrain the age of the mainly ene wsw trending agali anaikatti swarm is estimated to be 1980 25 ma with a very limited occurrence of upper cretaceous 80 90 ma dykes in the area the nw se dharmapuri swarm is dated as at least 1800 ma possibly 1855 9 ma both the nw se and the ne sw dyke swarms of tiruvannamalai are dated at 1650 10 ma only a few dykes in north kerala are of middle proterozoic ca 1700 1650 ma age furthermore the 40ar 39ar results of dharmapuri dykes exhibit age spectrum characteristic of partial degassing they provide strong evidence of latest proterozoic overprinting by a thermal event probably related to the pan african orogeny most of the phanerozoic dykes are confined to the coastal region in kerala province the dolerites of central kerala and the majority of north kerala dolerites intruded during the 70 65 ma period whereas a large gabbro and a felsic dyke in central and north kerala respectively are of upper cretaceous age ca 85 ma and the mean 81 1 5 ma age from these dykes may represent cooling age the oldest phanerozoic upper jurassic 144 6 ma dyke intrusions have a restricted occurrence in south kerala very close to the southern tip of india the proterozoic dykes are interpreted as being lateral intrusions from under plating igneous bodies beneath the eastern shield region the geological record of the terrain does not show indications of significant pre magmatic extension and rifting but dyke magmatism may have led to extension rifting and the consequent development of the india antarctica sri lanka proterozoic high grade terrain the data are also interpreted to suggest that the south indian granulite terrain is an early proterozoic tectono metamorphic terrane with strong thermal imprints of neoproterozoic early palaeozoic age it is difficult to reconcile its extensions into proterozoic sri lanka or antarctica where tectono metamorphism is dated to be of neoproterozoic age the late phanerozoic dykes are predominantly related to the deccan trap event and are associated with mantle plume decompressional tectonics that gave rise to ocean floor spreading between the seychelles and the indian continent the few older ca 145 ma dykes in south kerala may be related to the rift tectonics that preceded the separation of australia antarctica and india

electrical resistivity cross section across northern part of saurashtra region an insight to crystallized magma and fluids abstract saurashtra region forms the western part of the deccan volcanic province dvp in india and is bounded by the cambay rift basin crb in the east narmada rift basin nrb in the south and kachchh rift basin krb in the north this region is uplifted due to different stages of inter continental breakup madagascar india seychelles india different stages of rifting reactivation of fault zones and deccan volcanism the major portion of saurashtra region is covered by deccan basalts underlain by mesozoic sediments detailed magnetotelluric mt and long period magnetotelluric lmt soundings have been carried out in the northern part of saurashtra with an aim to bring out the geoelectrical configuration of different basins jamnagar jasdan and cambay along the profile 2d inversion carried out for different modes transverse electric te transverse magnetic mode tm and te tm independently the inversion of different modes shows distinct electrical signatures due to the 3d nature of the data we have considered the tm mode of interpretation because it is less prone to 3d distortions the final model represents large scale heterogeneities in the crust due to the presence of different resistive and conductive blocks high conductive lithosphere denotes the thermal influx of the mantle material to the base of the crust exchange of heat with the lower crust during basaltic magmatic underplating released carbonate fluids and or dehydration of minerals resulting in high conductivity anomalies a prominent conductive feature on the east side of the profile at upper crustal depths may be due to partial melts associated with mafic ultra mafic intrusions related to the reunion plume activity different resistive blocks in conjunction with seismic tomography studies represent recrystallized older precambrian crust that forms the uplifted blocks of jamnagar jasdan and western part of cambay basins

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

on b be ratios in the mexican volcanic belt boron and beryllium concentrations were measured in a diverse suite of well characterized rocks from the mexican volcanic belt mvb low b and high be result in relatively low b be ratios in the mvb compared to other arcs nevertheless b systematics resemble those of other arcs and provide insights into mantle processes in the mvb b enrichment depends first on magma type and second on edifice type and location b be values are highest 5 15 in andesites and dacites erupted from calcalkaline strato volcanoes located along the volcanic front such as volcan colima and v san juan rocks from strato volcanoes located behind the volcanic front generally have lower values 1 5 b be values are also elevated in differentiated members of rock suites that show evidence for significant crustal assimilation in the westernmost mvb west of the michoacan guanajuato volcanic field mgvf cindercone ejecta including basalts lamprophyres and basanites contain low b be values 5 the lamprophyres and basanites have very low b be despite high ba ce and other common measures of subduction signature in the mgvf where cinder cones occur exclusively b be values in primitive calc alkaline basalts are distinctly higher than those from alkaline basalts 3 8 vs 1 3 indicating that high b be is a mantle derived feature and not an artifact of crustal assimilation comparison among various elemental ratios indicates that cs and u show enrichment patterns similar to b all are enriched in calc alkaline rocks but not in lamprophyres or basanites in contrast ba k and sr are enriched in both calc alkaline rocks and the lamprophyres and basanites multi stage processes and differing melting mechanisms are inferred to explain the variable characteristics of mvb volcanic rocks first slab derived fluids rich in fluid mobile elements including b and ba infiltrate the mantle wedge these fluids cause fluid fluxed melting that produces calc alkaline magmas enriched in all fluid mobile elements these lavas erupt from large volcanic front strato volcanoes the slab derived fluids also metasomatize portions of the mantle wedge producing phlogopite and or amphibole these phases have high partition coefficients for ba sr k but may have low partition coefficients for b cs u accordingly subduction zone metasomatism produces a mantle wedge enriched in ba sr k but not necessarily in b cs u decompression melting of this type of metasomatized mantle will consume the hydrous phases and produce magmas such as lamprophyres or basanites with high ba ce sr la and k la but low b be cs la and u la this interpretation implies two types of subduction zone signatures one involving enriched ba sr k elements that have longer residence times in the mantle wedge and another involving enriched b cs u which all partition so strongly into fluids or melts that they have short residence times in the mantle and are only enriched in magmas generated by fluid fluxed melting or that have assimilated crustal material assimilation of granites and crustal rocks can also enrich differentiated lavas in b

mesozoic igneous activity in the maranha o province northern brazil 40ar 39ar evidence for separate episodes of basaltic magmatism low and high ti basaltic rocks of the maranha o province in northern brazil appear to be related to the opening of the atlantic ocean 40ar 39ar dating was carried out on powdered whole rock basalts both washed and unwashed in dilute nitric acid to document this relationship acid treatment does not alter the argon retention properties of retentive phases but is effective in removing alteration phases high ca k degassed at low laboratory temperatures our results identify that at least two periods of volcanic activity formed the maranha o province low ti basalts from the western sector are 200 190 ma and are related to the opening of the north atlantic high ti basalts from the eastern sector are 129 124 ma slightly younger than compositionally similar rocks in the serra geral flood basalt province of southern brazil and are related to the opening of the south atlantic other flood basalts such as the columbia river deccan rajmahal ferrar and siberian provinces were each predominantly formed in 1 2 ma the maranha o basalts remnants of separate igneous events spanning 70 ma should not be classed as a single flood basalt province the temporal complexities noted in the maranha o basaltic rocks appear to be related to the tracks of hotspots that led to the rifting of the atlantic ocean two sets of mesozoic basaltic rocks occur in southern africa india and south america resulting from events leading to the fragmentation and dispersal of sections of gondwanaland in each sub continent the two provinces were formed by events separated by 50 70 ma of these pairs only the maranha o and serra geral show geochemical similarities indicating compositionally distinct upper mantle source regions under sections of brazil separated by 3000 km

parana flood basalt volcanism primarily limited to 1 myr beginning at 135 ma new 40 ar 39 ar ages for rocks from rio grande do sul and critical evaluation of published radiometric data abstract 40 ar 39 ar step heating analyses were carried out on seven rocks five basalts an andesite and a rhyolite from the southern parana province 28 s 30 s they yield plateau isochron ages of 135 134 ma in good agreement with published step heating data on rocks from the same area critical review of laser spot isochron ages for rocks from the province ranging from 140 to 130 ma are shown to be unreliable estimates of crystallization ages as the rocks were substantially altered step heating results on three of these rocks thought to yield good plateau ages are shown to be incorrect as a result of a technicality in dating procedures followed u pb ages on zircon and baddeleyite separated from a variety of rock types 30 s 23 s fall in the range 135 to 134 ma all reliable 40 ar 39 ar and u pb ages indicate volcanism was sharply focused initiated at 135 ma and 1 myr in duration no variation of age with either latitude or longitude is noted scrutiny of published 40 ar 39 ar ages on the florianopolis dykes shows they cannot be used as reliable crystallization ages u pb work shows that this dyke swarm was formed coevally with the main part of the parana province most of the published 40 ar 39 ar ages on the ponta grossa dyke swarm are unreliable a few ages appear reliable and suggest the magmatic event in this area may have postdated the main parana pulse by 1 2 myr a single 40 ar 39 ar age from a high nb basalt in the southernmost part 34 s of the parana at 135 ma highlights the need for further radiometric work on other areas of this flood basalt province the parana province postdates the time of the jurassic cretaceous bound ary by 10 myr

the petrologic history of the sanganguey volcanic field nayarit mexico comparisons in a suite of crystal rich and crystal poor lavas abstract to evaluate if intermediate magmas erupting from volcan sanganguey mexico and the surrounding volcanic field are formed by mixing of basalts and rhyolites or if they initially exist as intermediate liquids a detailed petrological study is presented for eight andesite and dacite magmas six of the samples erupted from the central edifice four andesites and two dacites are crystal rich 50 vol whereas the remaining two samples one andesite and one dacite erupted from monogenetic vents in the peripheral volcanic field and are crystal poor 5 vol despite the variation in crystallinity all samples are multiply saturated in five to seven mineral phases plagioclase orthopyroxene titanomagnetite ilmenite apatite clinopyroxene hornblende in all samples plagioclase spans a 30 40 mol an range in composition and orthopyroxene spans a range in mg of 5 10 pre eruptive temperatures and oxygen fugacites relative to the nno buffer range from 853 24 to 1085 16 c and 0 1 0 1 to 0 9 0 1 nno on the basis of fe ti two oxide thermometry application of the plagioclase liquid hygrometer to the samples reveals maximum h 2 o contents that range from 1 7 6 2 wt comparison with phase equilibrium experiments demonstrates that all plagioclase and orthopyroxene compositions in the crystal poor samples could have grown from their respective whole rock compositions comparison of crystal rich samples with phase equilibrium experiments reveals the presence of sodic xenocrysts which reflect resorption textures and an estimated excess plagioclase crystal cargo of 6 vol the excess plagioclase crystal cargo is not distinguishable from phenocrystic plagioclase based on composition or texture suggesting that they were also grown in intermediate melts and are therefore described as antecrystic no calcic plagioclase xenocrysts an 79 typical of hydrous arc basalts are observed thus it is likely that the excess plagioclase in the crystal rich samples were originally formed in intermediate magmas for the crystal poor samples we propose that the mechanism producing the complex phenocryst assemblages is degassing cooling as it may shift equilibrium plagioclase compositions kinetically inhibit crystal growth and increase melt viscosity leading to complex textures notably the hypothesis of degassing cooling induced crystallization requires that the intermediate melts initially exist as liquids prior to crystallization supporting the hypothesis that intermediate melts are generated in the deep crust and arrive in the upper crust as liquids for the crystal rich samples degassing cooling may also be the mechanism generating a portion of the compositional and textural variation in the mineral assemblages and some incorporation of antecrysts or xenocrysts must occur as evidenced by an excess plagioclase crystal cargo however we find no definitive evidence supporting the incorporation of crystals initially grown in basalts or rhyolites given the similarities in phase assemblage mineral compositions mineral textures and intensive variables between the crystal poor and rich samples we conclude that the melts arriving into the upper crust beneath volcan sanganguey and the surrounding peripheral volcanic field are intermediate in composition and are initially formed as liquids in the deep crust

variation in quartz arenite deformation mechanisms between a roof sequence and duplexes abstract microstructural abundances and histories in quartz arenite of the lower silurian tuscarora sandstone were used to determine the nature and role of microscale deformation in a cover sequence and underlying thrust system and to assess the degree to which the cover sequence accommodated emplacement of the thrust system sandstone samples are located across the transition from the central to southern appalachian foreland thrust system where the thrusts change from blind to emergent southwards and where southern deformation of early alleghanian age has been previously shown to be overprinted by central deformation of alleghanian age microstructures observed with transmitted light and cathodoluminescence microscopy indicate that grain scale deformation occurred by dislocation flow pressure solution and microfracturing with the last being generally the most important the sequence of deformation mechanisms is the same for the cover sequence and the thrust system pressure solution during sedimentary compaction dislocation flow during layer parallel shortening and localized microfracturing with limited pressure solution near major thrust ramps and in steep fold limbs a greater abundance of dislocation flow microstructures in the cover sequence from layer parallel shortening indicates grain scale accommodation in the tuscarora sandstone of some shortening associated with emplacement of the thrust system the transition zone between the central and southern appalachians contains the greatest occurrence of every microstructure which is consistent with the area having been affected by diachronous central and southern alleghanian deformations

regional trends in the timing of alleghanian remagnetization in the appalachians pole positions related to remagnetized components isolated in appalachian limestone and redbed rock units range over about 60 m y of the permian carboniferous apparent polar wander path for north america apparent ages of remagnetization are older in the southern appalachians and younger to the north if the remagnetizations are associated with fluids expelled during the alleghany orogeny then the apparent remagnetization age trend could describe the timing of thrust sheet emplacement

unraveling the mineralogical complexity of sediment iron speciation using sequential extractions author s slotznick sp sperling ea tosca nj miller aj clayton ke van helmond nagm slomp cp swanson hysell nl abstract 2019 american geophysical union all rights reserved iron speciation is one of the most widely applied proxies used to reconstruct oxygen levels and redox conditions in past aqueous environments the iron speciation proxy estimates proportions of different reactive iron species in fine grained sedimentary rocks which are mapped to redox conditions based on empirical calibrations from modern sediments it is based on a standardized extraction technique of sequentially applying acetate hydroxlamine hcl dithionite and oxalate solutions to a powdered sample in order to dissolve iron phases and quantify the amount of iron carried by carbonates easily reducible oxyhydroxides ferric iron oxyhydr oxides and magnetite respectively although tested on pure minerals and mixtures assessments of whether this sequential extraction process accurately dissolves the targeted minerals in natural sediments and sedimentary rocks are lacking in our study residues from each sequential extraction step were analyzed using rock magnetic and x ray diffraction experiments to identify and quantify the iron bearing minerals that were dissolved the dithionite extraction robustly removes the targeted mineralogy as magnetic data show it to solubilize nearly all of the goethite however magnetic quantification of magnetite was orders of magnitude less than the iron measured in the oxalate extraction x ray diffraction data suggest that dissolution of iron bearing clays specifically berthierine chamosite could explain this disparity our data compilation shows higher values of iron from the oxalate extraction in precambrian sedimentary rock samples suggesting a significant temporal shift in iron cycling recognition of heterogeneity in chemical extraction efficiency and targeting is vital for holistic multiproxy interpretation of past oxygen levels and communication between disciplines

neogene volcanism at the front of the central mexican volcanic belt basaltic andesites to dacites with contemporaneous shoshonites and high tio2 lava as part of the continuing study of the young mexican volcanic belt designed to document the ages and types of volcanism from the gulf of california to the valley of mexico a 50 km wide segment of the central part of the belt has been mapped and five types of lava have been found pliocene 3 77 ma shoshonite lava flows k 2 o 2 wt sio 2 52 58 wt form eroded plateaus more than 50 km behind the present volcanic front but in the past 1 m y shoshonites have erupted closer to the volcanic front 300 km from the middle america trench the shoshonite lava type is the most enriched in ba sr and zr of the suite and plagioclase phenocrysts are absent presumably because of high contents of dissolved water 3 5 wt quaternary shield volcanoes and several cinder cones with small volume lava flows are composed of high tio 2 lavas 1 2 wt which have 51 57 wt sio 2 9 ppm and zr 160 230 ppm pliocene high tio 2 lava is found within the eroded plateaus located 50 km behind the present volcanic front quaternary basaltic andesite 52 57 wt sio 2 with up to 10 wt mgo is found at the volcanic front along with more siliceous andesite 57 63 wt sio 2 representatives of the siliceous andesites 57 63 wt sio 2 are free of plagioclase phenocrysts low in al 2 o 3 15 7 wt but rich in mgo 5 wt experiments reported elsewhere suggest that the magmas contained 3 7 wt dissolved water this andesite erupted along a normal fault between 0 3 and 0 005 ma and it is associated with dacite similarly lacking plagioclase phenocrysts but having comparatively abundant pyroxene other dacite in the zitacuaro area is richly porphyritic having plagioclase and hornblende this dacite forms clusters of steep sided domes or widespread pyroclastic deposits and the latest eruptions dated by radiocarbon and k ar occurred between 0 03 0 05 ma estimates of the volume of magma erupted in the zitacuaro valle de bravo region in the past 1 m y 1 8 km 3 m y 1 km 1 show that this area is somewhat less productive per 1 km of arc than those to the west 3 5 km 3 m y 1 km 1 in the michoacan guanajuato volcanic field mgvf a large volume of dacite has erupted in the zitacuaro valle de bravo region which is rare in the mgvf the cone density in the zitacuaro valle de bravo region 2 1 100 km 2 is only slightly lower than the 2 6 cones 100 km 2 found in the mgvf

tectonic climatic supercycle in the billion year plate tectonic eon permian pangean icehouse alternates with cretaceous dispersed continents greenhouse abstract the earth alternates in a supercycle 400 m y long from a single continent pangea and ocean panthalassa with an icehouse climate to many continents and oceans with a greenhouse climate the supercycle is driven by the heat that accumulates beneath the insulator of pangea and the greenhouse is made by the excess co2 vented from the mantle during the faster rate of plate activity in the current supercycle a since the mid carboniferous 320 ma the growth and decay of pangean heat is observed in globally synchronous cratonic sedimentary sequences that represent in ascending order 1 a stratigraphic gap 2 sagging 3 rifting 4 fast and 5 slow seafloor spreading stages 4 and 5 of the previous supercycle b are observed back to the start of the phanerozoic in the proterozoic earlier stages of b are postulated back to 720 ma and a third supercycle c to 1100 ma the big glaciations of the latest proterozoic 700 600 ma and late palaeozoic 320 260 ma are confined to the icehouse state during stages 1 3 the sturtian 800 ma latest ordovician 440 ma and quaternary 2 0 ma glaciations fall within greenhouse states during stage 5 of slow seafloor spreading the next icehouse glaciation is expected 80 ma hence the problem of long term past and future climate change is soluble by global studies urgently needed are 1 global stock takes of a the rate of granite emplacement which serves as a proxy via plate activity for the mantle source of atmospheric co2 and b carbonate sinks and other sedimentary indicators of co2 and 2 the extension of the sea level curve back past 570 ma to the start of the plate tectonic eon 1100 ma ago from such studies environmental baselines for past climatic states can be drawn for gauging present and future background variations

diagenetic alteration of chamositic clay minerals to ferric oxide in oolitic ironstone abstract in the clinton oolitic ironstone silurian of pennsylvania ooids that were initially composed of chamositic clay minerals were diagenetically altered to ferric oxide evidence for this alteration includes the preservation of intermediate stages in a variety of partly altered ooids finely laminated chamositic ooids in marginal zones of ferric oxide storm beds bed parallel transitions of ooid compositions in bioturbated oolite and spastolithic shapes of ferric oxide ooids electron microprobe analysis demonstrates that this oxic alteration did not leave significant abundances of residual silicon and aluminum based on textural characteristics and on a permian age of remagnetization the most likely time for the alteration was during early phases of the alleghany orogeny recognitio that the ooids were initially chamositic clay minerals resolves at least part of the oxidation reduction paradox maynard 1986 for ferrous phyllosilicate could presumably accumulate in the low oxygen conditions associated with the medial silurian oceanic anoxic event

does deccan volcanic sequence contain more reversals than the three chron n r n flow magnetostratigraphy a palaeomagnetic evidence from the dyke swarm near mumbai

seismogenesis of the lower crustal intraplate earthquakes occurring in kachchh gujarat india abstract large intraplate continental earthquakes like the 1811 12 new madrid m w 8 0 and the 2001 bhuj mw7 7 were highly destructive because they occurred in strong crust but the mechanisms underlying their seismogenesis are not understood here we show using local earthquake velocity tomography and joint inversion of receiver functions and surface wave group velocity dispersion that the crust and uppermost mantle beneath the 2001 bhuj earthquake region of western india is far more complex than hitherto known through previous studies a new image of the crust and underlying mantle lithosphere indicates the presence of a 18 km thick high velocity vp 7 15 8 11 km s differentiated crustal and mantle magmatic layer above a hot and thin lithosphere only 70 km in the epicentral region of 2001 bhuj earthquake this magmatic layer begins at the depth of 24 km and continues down to 42 km depth below this region brittle ductile transition reaches as deep as the moho 34 km due to the possible presence of olivine rich mafic magma our 1 d velocity structure envisages an initial phase of plume activity deccan plume at 65 m y ago resulting in basaltic magma in the eclogitic layers at sub lithospheric levels wherein they were subjected to crystallization under ultra high pressure conditions our study also delineates an updoming of moho 4 7 km as well as asthenosphere 6 10 km below the kachchh rift zone relative to surrounding areas suggesting the presence of a confined body of partial melts below the lithosphere asthenosphere boundary restructuring of this warm and thin lithosphere may have been caused due to rifting at 184 and 88 m y ago and tholeiitic and alkalic volcanism related to the deccan traps k t boundary event at 65 m y ago recent study of isotopic ratios proposed that the alkalic basalts found in kachchh are generated from a co 2 rich lherzolite partial melts in the asthenosphere that ascended along deep lithospheric rift faults into the lithosphere it appears that such kind of crust mantle structure deepening of brittle ductile transition and a high input of volatiles containing co 2 emanating from mantle control the seismogenesis of lower crustal earthquakes in the kachchh continental rift zone

contemporaneous calc alkaline and alkaline volcanism at sanganguey volcano nayarit mexico sanganguey volcano located in the northwestern segment of the mexican volcanic belt is a calc alkaline stratovolcano that has erupted a chemically continuous suite of andesites and dacites during the past 200 000 yr alkalic volcanism however appears to have been predominant in the area since the pliocene alkalic basalts and their derivatives erupted along extensional fractures trending northwest through sanganguey and continued to erupt through the flanks of the stratocone following cessation of calc alkaline activity the calc alkaline andesite dacite volcanism was focused beneath sanganguey and appears to have been controlled by crystal fractionation in a shallow magma chamber no basalts that erupted in the area have chemical compositions that could be parental to the sanganguey calc alkaline suite the most basic calc alkaline rocks basaltic andesites have incompatible trace element concentrations nearly identical to the most basic alkaline rocks implying that the two suites were derived from distinct parental magmas alternatively it is suggested that stagnation of mantle derived alkalic basalts provided heat input to mobilize lower crustal rocks resulting in the generation of calc alkaline magmas

volcano stratigraphy and physical characterization of the san mateo pomez unit cerritos member temascalcingo volcano central mexico abstract the temascalcingo volcano is a dacitic stratovolcano located at the eastern part of the acambay graben a tectonic basin in the central sector of the trans mexican volcanic belt tmvb which is an active mostly calc alkaline volcanic arc that crosses mexico from the pacific ocean to the gulf of mexico this study characterises the explosive activity of the san mateo pumice eruption that is the most recent and largest paroxysmal episode of this volcano and the last unit of the cerritos member indeed this eruption is associated with an undated debris avalanche event consisting of a major sector collapse developed on the western flank of the volcano stratigraphic data suggest a highly explosive eruption that generated a relatively thick up to 10 m and widespread pumice fallout deposit trending to the ne and an aerial distribution of at least 100 km2 this subplinian type eruption of vei 4 lasted about 9 h and produced a plume height up to 12 km above the vent i e 14 km above sea level this type of explosive eruption is relatively common in the tmvb so a comprehensive geological study with the corresponding numerical simulations like the one that was carried out contributes to the understanding of the explosive volcanism of modest size stratovolcanoes that are common in the tmvb

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

40ar 39ar age constraints for the jaramillo normal subchron and the matuyama brunhes geomagnetic boundary our mid pleistocene 4oar 39ar age recalibration of the geomagnetic polarity timescale is nearly in accord with the oxygen isotope climate record calibration of the astronomical timescale proposed by johnson 1982 and shackleton et al 1990 4oar 39ar ages of a normally magnetized rhyolite dome in the valles caldera northern mexico yielded a weighted mean age of 1 004 0 019 ma a k ar age of 0 909 0 019 ma for this rock by doell and dalrymple 1966 was the linchpin for the recognition and calibration of the jaramillo normal subchron jns other 4oar 39ar ages from the vailes caldera and 4oar 39ar ages of ivory coast tektites indicate that the jns began at about 1 11 ma and ended before 0 92 ma probably near 0 97 ma the matuyama brunhes boundary occurred between 0 79 ma and 0 76 ma on the basis of 4oar 39ar sanidine ages from 1 three reversely magnetized rhyolite domes of the valles caldera 0 793 0 018 ma 0 794 0 007 ma and 0 812 0 023 ma and pumice 0 789 0 006 ma from the reversely magnetized oldest toba tuff of sumatra and 2 pumice 0 764 0 005 ma and 0 757 0 009 ma from the lower and upper units of the normally magnetized bishop tuff the age of the boundary may be close to 0 77 ma as deduced from rates of sedimentation in ancient lake bonneville utah increasingly aware that the polarity of earth s magnetic field has changed repeatedly through geologic time and isotopic dating of these polarity reversals resulted in a geomagnetic polarity time scale gpts for the late cenozoic this combined paleomagnetic and isotopic research played a pivotal role in confirming plate tectonic theory glen 19821 which revolutionized the earth sciences continual refinement of isotopic ages which define the gpts has increased the usefulness of geomagnetic polarity reversals as marker hori zons in many types of geologic studies we present 4oar 39ar data and ages for some key volcanic units that support changes in the isotopic age chronology of the gpts using volcanic rocks erupted near the time of the geologically youngest two geomagnetic polarity reversals figure 1 the older of the two reversals originally called by doell and dalrymple 1966 the jaramillo event is now named the jaramillo normal subchron jns it is recorded in middle pleistocene rocks as a brief episode of normal polar ity within the late matuyama chron the younger of the two called the matuyama brunhes boundary mbb is regis tered in middle pleistocene rocks worldwide and consists of

palaeomagnetism and 36ar40ar vs 39ar40ar isotope correlation ages of dyke swarms in central kerala india tectonic implications palaeomagnetic results and whole rock 36ar40ar vs 39ar40ar isotope correlation ages are presented for dyke swarms occurring within the granulite terrain in central kerala west coast of india palaeomagnetic data show that the nnw sse leucogabbro dykes sampled are normally magnetised d 307 and i 57 whereas most nw se dolerite dykes are reversely magnetised d 163 and i 61 one dolerite dyke shows normal polarity the present 40ar39ar isotope ages for dolerite dykes and previously reported k ar ages for leucogabbros suggest 81 2 ma for acquisition of the leucogabbro directions and 69 1 ma for acquisition of the reverse magnetic directions within the dolerites based on palaeomagnetic and isotopic age data the dolerite dykes appears to represent feeder systems for the deccan volcanic sequences we suggest that the deccan eruptions were widespread and probably extended southward at least into central kerala the entire magmatic episode may have occurred between ca 70 and 65 ma during the 31r period probably corresponding to the cretaceous tertiary boundary the leucogabbros may correspond to the 34n superchron and represent a plume related magmatic episode precursory to the main volcanic event these data suggest that an early intraplate plume tectonic setting since ca 80 ma preceded lithospheric thinning asthenospheric upwelling and consequent massive flood volcanism during the period 70 65 ma this culminated in ocean floor spreading between the seychelles mascarene and continental india

estimation of lava extrusion and magma production rates for two flood basalt provinces utilizing the statistical approach of cox and dairymple 1967 it is shown that the geomagnetic polarity time scale of harland et al 1982 may contain errors of 0 85 1 0 m y for middle late miocene times a revised time scale based on the work of mcdougall et al 1984 is correlated with the magnetostratigarphic units of the columbia river basalts united states lava extrusion rates reached a peak of 275 km3 kyr for a period of 0 5 m y around 15 8 ma during the columbia volcanic episode it is suggested that even higher rates of lava extrusion may have occurred in the deccan province central india 65 ma a simple energy budget calculation indicates that the reunion hotspot could provide enough heat to melt the total volume of rock needed to produce the deccan traps during the 5 m y it would have taken the subcontinent to traverse the hotspot

remagnetization mechanisms in triassic red beds from south china paleogeographic reconstructions based on paleomagnetic data rely on the reliability of the natural remanent magnetization nrm as a primary geomagnetic signal remagnetizations however can be common in many rock types including late paleozoic and mesozoic red beds and they complicate paleogeographic interpretations extracting the primary nrm from partially remagnetized rocks and understanding the remagnetization mechanism are important in these contexts we carried out a systematic paleomagnetic study of red bed samples from the triassic huangmaqing formation nanjing 32 0 n 118 9 e south china two nrm components carried by secondary and primary hematite are isolated in 47 of the 94 samples studied where the latter component has a direction in stratigraphic coordinates of d 29 2 i 34 6 95 10 9 47 samples from 6 sites that yields a paleopole of 60 8 n 228 1 e dp dm 12 5 7 2 which is consistent with triassic pole positions for the south china block a secondary chemical remanent magnetization crm d 227 1 i 80 8 95 7 3 is documented in all 94 samples from 10 sites and is carried by pigmentary hematite that is inferred to have been generated by magnetite oxidation during orogenic activity this secondary component has steep inclinations and is interpreted to have been influenced by a combination of the remanence carried by original parent magnetite the orogenic stress field and the prevailing geomagnetic field direction during deformation this crm direction is recorded commonly by red beds from the south china block and is significant for regional tectonic studies in the area

isotopic and trace element constraints on the origin and evolution of alkaline and calc alkaline magmas in the northwestern mexican volcanic belt a wide variety of rock types has been produced by pliocene to recent volcanism in the northwestern portion of the mexican volcanic belt composite cones erupted rocks typical of calc alkaline volcanic belts associated with subduction monogenetic cinder cones surrounding the composite cones erupted mildly alkaline basalts and related rocks one larger center las navajas produced basalts trachytes and peralkaline rhyolites such alkaline rocks are typically associated with crustal extension related to rifting both subduction and rifting appear to be taking place in western mexico the most basic calc alkaline rocks although somewhat fractionated show depletion in high field strength elements hfse and enrichment in alkaline earth elements relative to other incompatible elements a characteristic typical of magmas in subduction related volcanic arcs the basic alkaline rocks show no depletion in hfse and show higher concentrations of incompatible elements than the basic calc alkaline rocks the calc alkaline rocks show a tight cluster of 87sr 86sr and 143nd 144nd ratios both of which correlate weakly with sio2 basic alkaline rocks have lower sr and higher nd isotopic ratios whereas the more differentiated alkaline rocks have isotopic ratios that overlap with and extend beyond the range of values found in calc alkaline rocks trace element data indicate that calc alkaline magmas could not have been derived by crystal fractionation from the alkaline magmas nor by assimilation of crustal materials by alkaline magmas the evolution of both calc alkaline and alkaline rocks requires a process of crystal fractionation accompanied by assimilation afc of crustal rocks to account for changes in isotopic ratios and trace element concentrations in the more differentiated members of each suite assuming a contaminant located in the lower or middle crust afc modeling shows that the amounts of assimilation required for alkaline magmas are somewhat higher than those for calc alkaline magmas calc alkaline and alkaline rocks appear to have resulted from melting of two distinct sources alkaline from an oib type source and calc alkaline from a mixed mantle and slab derived source

chemical remanent magnetization due to deep burial diagenesis in oolitic hematite bearing ironstones of alabama abstract oolitic hematite bearing ironstones of the silurian red mountain formation of alabama are shown to carry a single component remanence stable enough to have survived major folding of probable permian age nevertheless the remanence direction ten sites yielding a paleopole at 38 0 n 132 4 e with dm 3 6 dp 1 9 its reverse polarity and a negative intraformational conglomerate test show that the remanence was very likely acquired during the pennsylvanian some 130 ma after deposition this remanence is likely a chemical remanent magnetization crm acquired during diagenesis induced by heating due to deep burial under a pennsylvanian clastic wedge two possible mechanisms for acquisition of crm during deep burial diagenesis are considered in hypothesis i the oolitic hematite transformed from original geothite when heated to about 80 c acquiring crm in hypothesis ii the oolitic hematite originated from ferrihydrite and was too fine grained to acquire stable crm until heat raised the solubility of hematite allowing grain growth hypothesis i explains the timing of remanence acquisition better but there is some evidence that oolitic goethites may be stable to considerably more than 80 c hypothesis ii has some difficulty explaining preliminary paleomagnetic results from oolitic hematite bearing ironstones of the silurian clinton group new york state we prefer hypothesis i but both hypotheses remain plausible both hypotheses warn that continental red beds may also acquire crm during diagenesis induced by deep burial heating long after deposition but before folding

triassic paleomagnetism from the western pyrenees revisited implications for the iberian eurasian mesozoic plate boundary since the pioneering studies of van der voo tectonophysics 7 1969 5 and van der voo and boessenkool j geophys res 78 1973 5118 paleomagnetism of permo triassic redbeds and volcanics from the western pyrenees has furnished important contributions for delineating the mesozoic boundary between the iberian and eurasian plates in this paper we present a new paleomagnetic study focussed on triassic red beds 23 sites of the paleozoic basque massifs pbm the aim of this study is to complement previous studies done in those massifs to better constrain the complex kinematics of the western pyrenees two stable magnetic components have been isolated 1 a dual polarity pre folding magnetisation carried by specular hematite and 2 a secondary normal polarity component also carried by hematite our data confirm both the origin and the rotation pattern of the primary remanence described in previous works nevertheless field tests performed on the secondary component do not confirm the earlier interpretations by schott and peres tectonophysics 156 1988 75 as they indicate a synfolding nature of the remagnetisation instead of a post folding origin we consider that the secondary component is better explained if a cretaceous age is considered the presence of such remagnetisation in the western pyrenees strengthens the widespread occurrence of similar remagnetisation events reported in northern iberia in connection with the extensional tectonic events that occurred during cretaceous times a comparison of the rotations recorded by the triassic component and by the remagnetisation indicate that the paleozoic units underwent variable tectonic rotations before the remagnetisation was acquired most likely in connection with the counterclockwise rotation of iberia with respect to eurasia these results favour that the mesozoic plate boundary between the iberian and eurasian plates was a wide domain of distributed deformation and therefore contradict previous interpretations claiming for a discrete plate boundary

on the age and morphology of the reunion event data pertinent to the age of the normal polarity reunion event or subchron and whether it is a split feature are critically examined new 40 ar 39 ar age determinations on lavas from two sequences on reunion island along with available potassium argon k ar 40 ar 39 ar and astrochronological analyses indicate an age of 2 15 2 13 ma although the possibility cannot be completely ruled out at present there is no convincing evidence supporting the contention of a short duration of normal polarity some 70 90 kyr later

estimating the effect of tree uprooting on variation of soil horizon depth by confronting pedogenetic simulations to measurements in a belgian loess area 1 spatial patterns of soil often do not reflect those of topographic controls we attempted to identify possible causes of this by comparing observed and simulated soil horizon depths observed depths of e bt bc c1 and c2 horizons in loess derived soils in belgium showed a weak to absent relation to terrain attributes in a sloping area we applied the soil genesis model soilgen2 16 onto 108 1 1 m2 locations in a 1329 ha area to find possible causes two scenarios were simulated model 1 simulated soil development under undisturbed conditions taking slope aspect and loess thickness as the only sources of variations model 2 additionally included a stochastic submodel to generate tree uprooting events based on the exposure of trees to the wind outputs of both models were converted to depths of transitions between horizons using an algorithm calibrated to horizon depths observed in the field model 1 showed strong correlations between terrain attributes and depths for all horizons although surprisingly regression kriging was not able to model all variations model 2 showed a weak to absent correlation for the upper horizons but still a strong correlation for the deeper horizons bc c1 and c2 for the upper horizons the spatial variation strongly resembled that of the measurements this is a strong indication that bioturbation in the course of soil formation due to treefalls influences spatial patterns of horizon depths

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

gravity and magnetic signatures of volcanic plugs related to deccan volcanism in saurashtra india and their physical and geochemical properties abstract the bouguer anomaly and the total intensity magnetic maps of saurashtra have delineated six circular gravity highs and magnetic anomalies of 40 60 mgal 10 5 m s 2 and 800 1000 nt respectively three of them in western saurashtra coincide with known volcanic plugs associated with deccan volcanic province dvp while the other three in se saurashtra coincide with rather concealed plugs exposed partially the dvp represents different phases of eruption during 65 5 2 5 ma from the reunion plume the geochemical data of the exposed rock samples from these plugs exhibit a wide variation in source composition which varies from ultramafic mafic to felsic composition of volcanic plugs in western saurashtra and an alkaline composition for those in se saurashtra detailed studies of granophyres and alkaline rocks from these volcanic plugs reveal a calc alkaline differentiation trend and a continental tectonic setting of emplacement the alkaline plugs of se saurashtra are associated with ne sw oriented structural trends related to the gulf of cambay and the cambay rift basin along the track of the reunion plume this indicates a deeper source for these plugs compared to those in the western part and may represent the primary source magma the junagadh plug with well differentiated ring complexes in western saurashtra shows well defined centers of magnetic anomaly while the magnetic anomalies due to other plugs are diffused though of the same amplitude this implies that other plugs are also associated with mafic ultramafic components which may not be differentiated and may be present at subsurface levels paleomagnetic measurements on surface rock samples from dvp in saurashtra suggest a susceptibility of 5 5 10 2 si units with an average koenigsberger ratio q n of almost one and average direction of remanent magnetization of d 147 4 and i 56 1 the virtual geomagnetic pole vgp position computed from the mean direction of magnetization for the volcanic plugs and deccan basalt of saurashtra is 30 n and 74 w which is close to the vgp position corresponding to the early phases of deccan eruption modeling of gravity and magnetic anomalies along two representative profiles across junagadh and barda volcanic plugs suggest a bulk density of 2900 and 2880 kg m 3 respectively and susceptibility of 3 14 10 2 si units with a q n ratio of 0 56 which are within the range of their values obtained from laboratory measurements on exposed rock samples the same order of gravity and magnetic anomalies observed over the volcanic plugs of saurashtra indicates almost similar bulk physical properties for them the inferred directions of magnetization from magnetic anomalies however are d 337 and 340 and i 38 and 50 which represent the bulk direction of magnetization and also indicate a reversal of the magnetic field during the eruption of these plugs some of these plugs are associated with seismic activities of magnitude 4 at their contacts based on this analysis other circular semi circular gravity highs of nw india can be qualitatively attributed to similar subsurface volcanic plugs

an assessment of 40ar39ar dating for the whole rock volcanic samples from the luzon arc near taiwan from electron microprobe petrographic and image analyses of samples from the northern tip of the luzon arc the effects of various alteration reactions phenocryst and groundmass assemblages on the whole rock 40ar39ar step heating spectra have been delineated the alteration of primary hornblende to an assemblage of zoisite chlorite sphene the palagonitization of glass and the crystallization of phillipsite tends to cause discordant 40ar39ar dates in the low temperature steps of the age spectra discordant 40ar39ar dates in the high temperature steps appear to result from degassing of inherited or trapped 40ar from coarse grained phenocrysts samples in which most of the k resides in groundmass plagioclase tend to yield meaningful plateau dates whereas samples in which most of the k resides primarily in palagonitized glass tend to yield meaningless plateau and isochron dates substantial loss of 39ar 13 40 occurred from irradiated samples prior to 40ar39ar step heating analysis which appears to be correlated with the abundance of groundmass and the degree of alteration this argon loss is attributed to thermally induced argon diffusion from the fine grained groundmass plagioclase glass and alteration phases during the pre step heating bakeout

diagenetic magnetite carries ancient yet secondary remanence in some paleozoic sedimentary carbonates many sedimentary carbonate rocks carry stable magnetizations that can be shown to reside in magnetite when such magnetizations are observed it is often argued or demonstrated that the magnetite was incorporated into the sediment during deposition however paleomagnetic and rock magnetic studies in conjunction with analyses of magnetic extracts from the helderberg and bonneterre carbonates united states indicate that the magnetite present in these rocks is most likely of diagenetic i e postdepositional origin

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

lithofacies of the silurian keefer sandstone east central appalachian basin usa abstract fourteen lithofacies of the keefer sandstone in the area of northeastern west virginia and western maryland were identified depositional environments of the lithofacies include storm dominated marine shelf basin barrier island and backbarrier tidal flat and lagoon coastal lithofacies document a dominant basinward progradation with some sea level rise which placed backbarrier lithofacies above barrier island lithofacies shelf lithofacies as stacked coarsening upward sequences document as many as four fluctuations within the major progradation coarsening upward sequences are typically capped by transgressive sands which represent material eroded from relatively landward environments and transported basinward during coastal retreat deposition of cycles within the keefer occurred under conditions of tectonic quiescence dominant steady or falling sea level followed by rise in sea level and in a climate which was warm and humid

from jurassic shores to cretaceous plutons geochemical evidence for paleoalteration environments of metavolcanic rocks eastern california volcanic and plutonic rocks exposed in east central california record a long history of metasomatism and or metamorphism within the mesozoic cordilleran continental arc we use whole rock and mineral elemental compositions along with standard and cathodoluminescence petrography to characterize alteration histories of late triassic to middle jurassic metavolcanic rocks in the ritter range white inyo mountains and alabama hills although alkali metasomatism is widespread and pervasive ratios and abundances of ce th tb and ta suggest that mafic protoliths from the white inyo mountains were shoshonitic whereas those from the ritter range were calc alkaline alkali exchange apparently modified the compositions of many metavolcanic rocks much of this metasomatism may have occurred at low temperature t conditions and attended or shortly postdated deposition of the volcanic protoliths high 18 o values for k rich metatuffs from

basalt weathering and pedogenesis across an environmental gradient in the southern cascade range california usa abstract basalt rocks occupy substantial land area and play a significant role in global weathering patterns and biogeochemical cycling the objective of this research was to quantify climatic controls of weathering and pedogenic processes on basalt derived soils across an environmental gradient on the western slope of the casacade range of california usa we hypothesized that climate controls mineral neogenesis with cool moist conditions favoring formation of short range order sro materials and warm dry conditions favoring smectite crystalline fe oxyhydroxides and kaolins four pedons were sampled across an elevation gradient 250 2500 m having large variation in mean annual soil temperature 5 17 c and mean annual precipitation 750 1350 mm the soil mineral assemblage was characterized by x ray diffraction selective dissolution total elemental analysis and light microscopy the degree of weathering and mineral assemblage exhibited a clear threshold at the permanent winter snowline 1200 m maximum soil development was noted just below the snowline with soils dominated by kaolinite and dehydrated halloysite crystalline fe oxyhydroxides 48 kg m 2 extensive loss of cations chemical index of alteration cia 95 and clay accumulation 447 kg m 2 in contrast the high elevation snow dominated pedons displayed less intense weathering e g cia

geochemical and isotope data from the acatlan volcanic field western trans mexican volcanic belt origin and evolution abstract the quaternary acatlan volcanic field avf is located at the western edge of the trans mexican volcanic belt tmvb this region is related to the subduction of the pacific cocos and rivera plates beneath the north american plate since the late miocene avf rocks are products of pleistocene volcanic activity and include lava flows domes erupted basaltic andesite trachyandesite trachydacite and rhyolite of calc alkaline affinity most rocks show depletion in high field strength elements and enrichment in large ion lithophile elements and light rare earth elements as is typical for magmas in subduction related volcanic arcs 87sr 86sr values range from 0 70361 to 0 70412 while nd values vary from 2 3 to 5 2 sr nd isotopic data plot along the mantle array on the other hand lead isotope compositions 206pb 204pb 18 62 18 75 207pb 204pb 15 57 15 64 and 208pb 204pb 38 37 38 67 give evidence for combined influences of the upper mantle fluxes derived from subducted sediments and the upper continental crust involved in magma genesis at avf additionally 18o whole rock analyses range from 6 35 in black pumice to 10 9 in white pumice of the acatlan ignimbrite a fairly good correlation is displayed between sr as well as o isotopes and sio2 emphasizing the effects of crustal contamination compositional and isotopic data suggest that the different avf series derived from distinct parental magmas which were generated by partial melting of a heterogeneous mantle source

influence of deccan volcanism synrift magmatism on the crust mantle structure and its implications for the seismogenesis of earthquakes occurring in the kachchh rift zone

the geology and petrology of volcan san juan nayarit mexico and the compositionally zoned tepic pumice abstract volcan san juan is the westernmost quaternary composite volcano in the mexican volcanic belt its activity is divided into three stages of decreasing eruptive volume during stage 1 activity the main cone of san juan and the adjacent cerro alto grew to a combined volume of 60 km3 through eruption of hornblende bearing andesites dacites and rhyodacites stage 1 included explosive eruptions of hornblende dacite and an andesite dated at 33 750 a years ago and 23 010 a stage 2 activity involved a major plinian eruption 14 770 a that produced the hypersthene hornblende bearing compositionally zoned rhyodacitic andesitic tepic pumice and led to formation of an oval shaped caldera 4 1 km in diameter tepic the capital city of nayarit state with a population of 250 000 lies at the ne foot of volcan san juan this location was buried under 2 9 m of rhyodacitic pumice fall deposits from the tepic pumice eruption erosion has since removed virtually all exposures of tepic pumice 15 km from the caldera the estimated volume of erupted tephra is 5 6 km3 equivalent to 1 2 km3 of magma modeling of the eruption dynamics indicates that as the eruption evolved the column height increased from 15 km to 24 km and maximum mass discharge rate increased from 3 106 kg s to 6 107 kg s during stage 3 activity a hornblende andesitic lava dome partially filled the caldera and sent block lava flows down the north slope of the cone total erupted volume was 0 55 km3 compared to other composite volcanoes of the western mexican volcanic belt the products from san juan have the highest sr contents and the lowest rb y yb and lu contents resulting in high la yb and sr y with low rb sr these observations indicate a strong slab melt component at san juan consistent with its proximity to the middle america trench where young hot lithosphere is subducting the san juan andesites dacites and rhyodacites also have relatively high 87sr 86sr 206pb 204pb 207pb 204pb and 208pb 204pb and low end values compared to these other volcanoes which appear to reflect significant crustal contamination of san juan s magmas

berthierine formation in reservoir rocks from the siri oilfield danish north sea as result of fluid rock interactions part i characterization abstract berthierine as the only authigenic fe rich clay mineral occurs in an oil bearing glauconite rich sandstone paleocene heimdal member of the lista formation danish north sea based on the results of thin section transmission and scanning electron microscopy five different morphological types of berthierine were identified 1 mainly as grain coatings partly together with authigenic quartz covering detrital grains 2 as pore filling clay in open pores 3 small crystals in transformed glauconite grains 4 small crystals associated with siderite in layers of mica micaceous glauconite and 5 as elongated crystals in siderite concretions although different in occurrence all berthierine types investigated by microprobe analysis are of similar chemical composition suggesting similar precipitation conditions investigations by transmission electron microscopy reveal that fine berthierine crystals display straight parallel layers with a typical lattice fringe spacing of 0 7 nm and that their occurrence is intimately connected to oil phases in pore space partly berthierine textures resemble mineralized oil water emulsions berthierine is more abundant in the oil leg than in the water leg but occurs especially abundant at the oil water contact the coexistence of berthierine together with quartz overgrowth but also its appearance in glauconite indicates that berthierine formed in situ and that it is a product of glauconite dissolution berthierine formation is coupled to the reduction of aqueous fe 3 into fe 2 derived from glauconite dissolution and is related to fluid rock interactions triggered by oil degradation which occurred in the oil filled zone and at the oil water contact the biomarker parameters pristane n c 17 and phytane n c 18 indicate an increased extent of oil degradation at the oil water contact on the one hand soluble oil degradation products methane carbon dioxide and hydrogen changed the hydrogeochemical conditions such as ph pe alkalinity etc and controlled dissolution of glauconite and feldspar on the other hand the changed hydrogeochemical conditions led to berthierine and quartz precipitation in water wet reservoir intervals

mid continent gravity high of central india and the gondwana tectonics abstract the most prominent features of the bouguer anomaly map of central india are the linear elongated gravity highs extending east west for almost 800 900 km from the west coast up to the southeast of jabalpur in central india this anomaly is located south of narmada and coincides with the tapti basin and adjoining satpura mountains this gravity high is best reflected in its eastern part where it is surrounded by the deep seated faults of the godavari satpura and mahanadi rift valleys of the gondwana permian jurassic period the wavelength characteristics and two dimensional modelling along a profile suggest that the causative source for the central and the eastern part of this bouguer high is located in the middle crust at a depth of approximately 12 13 km with a density of 2 9 g cm3 this density represents basic rocks which might be the result of extensional tectonics associated with the gondwana rift valleys deep seismic sounding dss profiles have also indicated a high velocity at very similar depths in this region the prominence of the eastern part of the bouguer high and its interrelationship with the gondwana rift valleys referred to above and to the damodar graben extending further eastward suggest that these rift valleys might have been initiated in central india south of narmada and propagated radially toward the east coast which eventually resulted in the separation of india from gondwanaland during the late jurassic the western part of the bouguer high may also represent mid crustal basic rocks associated with the rift tectonics concealed under the deccan trap this elongated bouguer high is compared to the mid continent gravity high of the united states of america which is also caused by basic intrusives related to an old rift system

could mars be dark and altered there is a long known dichotomy in the martian albedo with an associated but mostly assumed mineralogical split as well the bright red regions are inferred to be weathered oxidized dust and the dark grey regions unaltered volcanic material a number of recent analyses suggest this division is unnaturally simplistic and the association of many dark regions with the former presence of water requires a re examination of the spectra in light of potential alteration minerals i present an alternate interpretation of the reflectance spectral characteristics of some dark regions on mars that includes dark layer silicates if their presence is confirmed on mars this will have implications for sequestration of current and past volatile inventories clues to the extent and type of geochemical weathering and potential zones where bacterial life forms may have emerged

the eruptive history of the tequila volcanic field western mexico ages volumes and relative proportions of lava types the eruptive history of the tequila volcanic field 1600 km 2 in the western trans mexican volcanic belt is based on 40 ar 39 ar chronology and volume esti mates for eruptive units younger than 1 ma ages are reported for 49 volcanic units including volcn tequila an andesitic stratovolcano and peripheral domes flows and scoria cones volumes of volcanic units 1 ma were obtained with the aid of field mapping ortho aerial pho tographs digital elevation models dems and arcgis software between 1120 and 200 kyrs ago a bimodal distribution of rhyolite 35 km 3 and high ti basalt 39 km 3 dominated the volcanic field between 685 and 225 kyrs ago less than 3 km 3 of andesite and dacite erupted from more than 15 isolated vents these lavas are crystal poor and show little evidence of storage in an upper crustal chamber approximately 200 kyr ago 31 km 3 of andesite erupted to form the stratocone of volcn tequila the phenocryst assemblage of these la vas suggests storage within a chamber at 2 3 km depth after a hiatus of 110 kyrs 15 km 3 of andesite erupted along the w and se flanks of volcn tequila at 90 ka most likely from a second discrete magma chamber lo cated at 5 6 km depth the youngest volcanic feature 60 ka is the small andesitic volcano cerro tomasillo 2 km 3 over the last 1 myr a total of 128 22 km 3 of

modelling basalt weathering at elevated co2 concentrations implications for terminal to post magmatic rifting in the deccan traps kachchh india

alteration and submergence of basalts in kachchh gujarat india implications for the role of the deccan traps in the india seychelles break up

flood basalts and large igneous provinces from deep mantle plumes fact fiction and fallacy abstract flood basalts large igneous provinces and all intraplate magmatism are almost unanimously ascribed to features called mantle plumes in current geodynamics literature in the starting plume model flood basalts and any associated hotspot tracks have been ascribed to the melting of a large plume head and a narrow tail respectively and the deccan flood basalt province of india and the laccadives maldives chagos reunion island volcanic chain in the indian ocean are widely thought to be an excellent example supporting this model however the existing geodynamic thermal petrological geochemical and geochronological constraints on deccan volcanism do not require or support a mantle plume origin a new interpretation made here rules out any role for the reunion plume in deccan volcanism and even questions the notion that the reunion hotspot is the manifestation of an underlying reunion plume the laccadives reunion hotspot track is viewed here as caused by a southward propagating fracture based on the published radiometric age data and the ultimate cause of deccan volcanism is argued to be mega scale lithospheric rifting the appreciation of the great original areal extent of the deccan nearly as far as the southern tip of peninsular india solves the hitherto unsolved problem of the cause of the uplift of the 1500 km long western ghats escarpment this cause being combined surface erosion and magmatic underplating hypothesized mantle plumes do not appear responsible for most large igneous provinces instead their very existence is questionable no geological evidence of any kind geochemical petrological thermal topographic requires mantle plumes the self contradictions and special pleading that proliferate in plume explanations are unjustified and unnecessary if one accepts based on various grounds that a globe encircling enriched mantle layer the perisphere may overlie a deeper depleted mantle which is the source of morb the systematic geochemical variations along the laccadive reunion chain can be explained by this model and are not consistent with a plume tail experiencing entrainment from surrounding morb mantle as in the current plume model hotspot tracks may not reflect plate motion above stationary deep mantle upwellings but the stress state of the lithosphere as originally proposed decades ago the locations of large igneous provinces hotspots and hotspot tracks are strongly controlled by lithospheric architecture and history they are not placed randomly on the globe indifferent to the surface geology neither are large igneous provinces sudden chance events within the vastness of geological time which plume theories typically allege

redbeds and thermoviscous magnetization theory thermal demagnetization character istics of a brunhes age viscous overprint in appalachian redbeds and a thermoviscous component acquired in the laboratory at moderate tempera ture agree well with relaxation time blocking temperature relations for hematite proposed by pullaiah et al 1975 supporting evidence was obtained from redbeds associated with an igneous dike intrusion although the interpretation of these data are complicated by magnetochemical alterations paradoxically experimental data for magnetite in some limestones were shown to agree more closely with an alternative theory of walton 1980 the hematite in redbeds is predominantly singledomain sd whereas magnetite in the lime stones studied extends well into the multidomain md range thus experiment and theory could be reconciled if it is assumed that pullaiah eta applies strictly to sd material whereas waltoh s theory somehow describes thermoviscous effects dominated by larger md grains

constraints on the origin of alkaline and calc alkaline magmas from the tuxtla volcanic field veracruz mexico lavas erupted in the tuxtla volcanic field tvf over the last 7 ma include primitive basanites and alkali basalts mildly alkaline hy normative mugearites and benmoreites and calc alkaline basalts and basaltic andesites the primitive lavas are silica undersaturated with high concentrations of both incompatible and compatible trace elements variable la yb with constant yb at 6 to 8 times chondritic and low sr and o and variable pb and nd isotopic ratios the primitive magmas originated by increasing degrees of melting with pressure decreasing from greater than 30 kbar to 20 kbar in the garnet stability field another group of alkali basalts and hawaiites has lower ni and cr concentrations and higher fe mg ratios and was derived from the primitive group by crystal fractionation at pressures of several kbar incompatible trace elements in these silica undersaturated lavas show depletion in high field strength elements hfse relative to large ion lithophile elements similar to subduction related basalts ba nb ratios are nearly constant and thus the hfse depletion cannot be the result of a residual hfse bearing phase in the source but could be the result of generation from a source contaminated by fluids or melts from the subducted lithosphere the silica saturated mugearites and benmoreites and the calc alkaline basalts and basaltic andesites were erupted only between 3 3 and 1 0 ma these have incompatible element concentrations generally lower than in the silica undersaturated lavas and thus could not have been derived by crystal fractionation from the silica undersaturated alkaline magmas magmas parental to the silica saturated magmas originated by higher degrees of melting at lower pressures than the primitive magmas melting may have been promoted by an influx of fluid from the subducted lithosphere trace element and sr nd pb and o isotopic data suggest that three components are involved in the generation of tvf magmas the mantle a fluid from the subducted lithosphere and continental crust tvf alkaline lavas are similar to those erupted in the back arc region of the mvb and japan and show characteristics similar to alkaline magmas erupted in the southern andean volcanic arc these low degree melts reach the surface along with calc alkaline lavas in the tvf due to an extensional stress field that allows their passage to the surface

critical evaluation of the age of the deccan traps india implications for flood basalt volcanism and faunal extinctions attempts to evaluate the possible link between flood basalt volcanism and the faunal extinctions observed at the k t boundary must rely on precise knowledge of the amount of such volcanism that took place 65 ma the current geochronological data do not permit close resolution of either the timing or the duration of deccan volcanism the age of the bulk of the material is placed in the range 70 60 ma further resolution of this hypothesis awaits acquisition of high quality 40 ar 39 ar plateau ages on key sections of the deccan traps

primitive calc alkaline and alkaline rock types from the western mexican volcanic belt since the early pliocene small volumes of alkaline magmas have erupted in the western mexican volcanic belt mvb in close association with the volumetrically dominant calc alkaline magmas both suites include relatively rare primitive types characterized by high mg cr and ni contents primitive hypersthene normative basalts parental to the calc alkaline suite are found along the volcanic front but are absent at greater distances from the trench the volcanic front calc alkaline suites are typically hornblende bearing indicating relatively high water contents associated primitive alkaline magmas at volcanic front locations also contain hydrous minerals these nepheline normative suites include basanites phlogopite bearing minettes and other hornblende bearing lamprophyres these are probably the youngest and freshest lamprophyres yet discovered on earth and ideal samples for addressing the origin of this exotic class of rocks on extended morb normalized elemental plots all lamprophyres show patterns similar to the calc alkaline basalts but have overall enrichments of 2 to 25x in ti k p ba sr light rare earth elements and other incompatible elements the lamprophyres show the same strong relative enrichments of ba k and sr and depletions in ti and nb that characterize the calc alkaline rocks of western mexico and all subduction zones these similarities in relative element abundances support a common source region for all primitive magmas of the western mvb the primitive calc alkaline and alkaline rocks of western mexico show narrow overlapping ranges in sr nd and pb isotopic ratios also consistent with derivation of all magmas from a common source the presence of the relatively cold subducted cocos and rivera plates below the volcanic front demands that all primitive magmas originated in the depth range 30 to 75 km these magmas are enriched in ba k sr and other elements probably transported by hydrous fluids rising from the subducted slab several lines of evidence support the presence of phlogopite in the source region of western mvb magmas probably in the form of phlogopite rich veins cutting asthenospheric peridotite the first liquids formed upon partial melting of this veined source region will concentrate the vein component and generate the lamprophyres these lamprophyric melts rarely erupt in arc settings more commonly forming dikes in the arc crust eruption of lamprophyric magmas in the western mvb is favored by through crustal extensional fracture systems related to active rifting of the jalisco block from the n american plate larger volumes of calc alkaline basalt are generated by melting of the same source which dilutes the vein component with larger proportions of peridotitic wall rock

deposition and diagenesis of clinton ironstones silurian in the appalachian foreland basin of pennsylvania clinton ironstones mid silurian on the southeastern margin of the appalachian foreland basin in central pennsylvania consist largely of skeletal grainstone storm beds whose skeletal fragments are extensively impregnated and replaced by ferric oxide other forms of iron mineralization consist of ooids superficial ooids cavity fillings and cements composed of ferric oxide and or chamositic clay and two generations of iron rich carbonate spar true oolite is rare small proportions of ferruginized grains are present in coarse lags at tops of some small scale 1 to 5 m coarsening upward sequences which are the record of mid shelf shoals from which the skeletal grainstone storm beds were derived ooids and superficial ooids are likely to have formed in dysaerobic conditions where the pycnocline of a highly stratified sea intersected shoal flanks ferrous iron from anaerobic deeper waters precipitated on quartz and skeletal fragment nuclei in weakly reducing conditions with episodic gentle agitation several stages of early diagenetic ferruginization affected skeletal grainstone storm beds and sequence capping coarse lags chamositic clay precipitated in pores and cavities of skeletal fragments within quartz sand adjacent to crinoid ossicles and as bed parallel connectors of large skeletal fragments as diagenesis progressed with further burial many skeletal fragments were extensively replaced by ferric oxide most chamositic clay altered to ferric oxide other chamositic clay matured from berthierine to chamosite and clusters of bladed specularite replaced parts of primary grains and secondary carbonate cements paleomagnetic and fluid inclusion analysis indicates that many late diagenetic changes took place at elevated temperatures during deen burial in an early phase of the alleghanian orogeny

seismic signature of sub trappean gondwana basin in central india abstract the digitized and trace normalized record sections of original analog deep seismic sounding dss data collected along the multai pulgaon profile have been re interpreted to find basement and intra basement structure in the area between the satpura basin and the pranhita godavari gondwana graben in central india it is found that the data are best satisfied by a model with a low velocity zone in between the deccan traps and the basement the study delineates a basement fault that could be associated with the known gavaligarh salbardi lineament low velocity sediments vp 3 7 km s delineated from the present study could be gondwana sediments which have been reported in surrounding areas this finding indicates the possible extension of gondwana formations of the pranhita godavari graben to the satputra basin through the wardha valley

the eruptive history of the mascota volcanic field western mexico age and volume constraints on the origin of andesite among a diverse suite of lamprophyric and calc alkaline lavas the mascota volcanic field is located in the jalisco block of western mexico where the rivera plate subducts beneath the north american plate it spans an area of 2000 km2 and contains 87 small cones and lava flows of minette absarokite basic hornblende lamprophyre basaltic andesite and andesite there are no contemporary dacite or rhyolite lavas new 40ar 39ar ages are presented for 35 samples which are combined with nine dates from the literature to document the eruptive history of this volcanic field the oldest lavas 2 4 to 0 5 ma are found in the southern part of the field area whereas the youngest lavas predominantly 0 5 ma are found in the northern portion on the basis of these ages field mapping and the use of ortho aerial photographs and digital elevation models it is estimated that a combined volume of 6 8 3 1 km3 erupted in the last 2 4 myr which leads to an average eruption rate of 0 003 km3 kyr and an average volume per eruptive unit of 0 1 km3 the dominant lava type is andesite 2 1 0 9 km3 followed by absarokite 1 6 0 8 km3 basaltic andesite 1 2 0 5 km3 basic hornblende lamprophyre 1 0 0 4 km3 and minette 0 9 0 5 km3 thus the medium k andesite and basaltic andesite comprise approximately half 49 of the erupted magma with twice as much andesite as basaltic andesite and they occur in close spatial and temporal association with the highly potassic lamprophyric lavas there is no time progression to the type of magma erupted a wide variety of evidence indicate that the high mgo 8 9 wt basaltic andesites 52 53 wt sio2 were formed by h2o flux melting of the asthenopheric arc mantle wedge whereas the mafic minettes and absarokites were formed by partial melting induced by thermal erosion of depleted lithospheric mantle containing phlogopite bearing veins there is only limited differentiation of the potassic magmas with none more evolved than 55 4 wt sio2 and 4 4 wt mgo this may be attributable to rapid crystallization of the mantle derived melts in the deep crust owing to their low volumes thus the andesites 58 63 wt sio2 are notable for being both the most voluminous and the most evolved of all lava types in the mascota volcanic field which is not consistent with their extraction from extensively crystallized 60 70 low volume intrusions instead the evidence supports the origin of the andesites by partial melting of amphibolitized mafic lower crust driven by the emplacement of the minettes absarokites and the high mg basaltic andesites

remagnetization of the lower ordovician hongshiya formation of the southwestern yangtze block abstract large scale block migration has been proposed based on early and early late ordovician paleomagnetic data for the south china block scb however this is anomalous in terms of the previously reconstructed affinity between the scb and east gondwana a paleomagnetic and petrographic reassessment of the lower ordovician sedimentary rocks is therefore necessary to assess the reliability of the early ordovician paleopole of the scb consequently we obtained paleomagnetic data from 47 sites at five localities of the scb for most specimens detailed thermal demagnetization experiments yielded viscous components at 120 c while the stable characteristic remanent magnetization chrm was isolated up to 680 c the chrm passed the fold test and hematite is identified as the remanence carrier the samples give the site mean chrm direction ds is 313 0 61 9 ks 47 1 95 3 2 after tilt correction five localities 47 sites although the chrm direction passed the fold test that was likely acquired before the tertiary petrographic studies reveal the occurrence of widespread secondary hematite in the specimens indicating that the rocks were remagnetized after deposition the average paleomagnetic direction overlaps with the jurassic paleomagnetic direction from sampling areas of the southwestern scb implying that the remagnetization event occurred during the jurassic the early ordovician paleopole of the scb is therefore urgently needed for plate reconstruction of the scb within gondwana

the deccan trap cretaceous paleogene boundary connection new 40ar 39ar ages and critical assessment of existing argon data pertinent to this hypothesis abstract the cretaceous paleogene boundary kpgb was dated by the 40ar 39ar method herein from the western interior of north america at 65 48 0 12 ma 1 in good agreement with other recent published estimates for the deccan traps india new argon ages as well as others available in the literature are assessed for reliability based on a statistical reliability of plateau isochron sections and b freshness of material dated utilizing the alteration index method from tholeiitic lavas from the composite western ghats section cwgs only six ages are found to be reliable estimates of the time of crystallization these ages along with the magnetic polarity of the lavas agree with the geomagnetic polarity time scale gpts at 67 64 ma alkaline rocks from the anjar area of kutch provide three reliable ages that suggest a hiatus in lava extrusion around kpgb for the rajahmundry basalts the upper flow s age defines its formation during chron 29n a single age from the lower reversed polarity flow appears somewhat dichotomous when plotted against the gpts the reliable lava ages indicate the most voluminous reversed polarity sections of the cwgs were extruded at a time statistically indistinguishable from that of the kpgb the deccan trap kpgb faunal extinction hypothesis remains plausible but must compete with the latest report favoring a very close temporal connection 0 03 m y between the chixculub impact crater and the kpgb

worldwide distribution of continental rock lithology implications for the atmospheric soil co2 uptake by continental weathering and alkalinity river transport to the oceans 1 the silicate rock weathering followed by the formation of carbonate rocks in the ocean transfers co2 from the atmosphere to the lithosphere this co2 uptake plays a major role in the regulation of atmospheric co2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks this leads us to develop the first world lithological map with a grid resolution of 1 1 this paper analyzes the spatial distribution of the six main rock types by latitude continents and ocean drainage basins and for 49 large river basins coupling our digital map with the gem co2 model we have also calculated the amount of atmospheric soil co2 consumed by rock weathering and alkalinity river transport to the ocean among all silicate rocks shales and basalts appear to have a significant influence on the amount of co2 uptake by chemical weathering

paleomagnetism of a lateritic paleoweathering horizon and overlying paleoproterozoic red beds from south africa implications for the kaapvaal apparent polar wander path and a confirmation of atmospheric oxygen enrichment the olifantshoek group in southern africa contains paleoproterozoic red beds that are exceptionally well preserved lying unconformably atop a regionally extensive lateritic paleoweathering profile we studied the basal unit of this succession known as the gamagara or mapedi formation and the lateritized substrate so called drakenstein or wolhaarkop paleosol on which it developed two ancient magnetic components are observed one int usually with a distributed unblocking spectrum between 300 and 600 c but occasionally persisting to 675 c is directed shallowly southward or northward a mesoscale fold test at south sishen mine indicates that this component was acquired during deformation similarity of the direction to previous results suggests that it was acquired at 1240 ma during early namaqua orogenesis combining our int results with existing data from the namaqua eastern zone nez we calculate the nez pole at 44 9 n 021 5 e k 23 2 a 95 12 8 q 5 the most stable component from our data set hig always persisting as a nonzero endpoint to demagnetization at 665 680 c is observed in 32 samples from south sishen and beeshoek mines directed moderately east downward sishen or west upward beeshoek this component predates the mesoscale fold at sishen more importantly a conglomerate test at beeshoek indicates that hig was acquired prior to paleoproterozoic deposition of the gamagara mapedi formation the concordance between directions from the paleoweathering zone and immediately overlying red beds indicates that hig is a primary magnetic remanence for the basal gamagara mapedi bgm formation the dual polarity bgm paleomagnetic pole 02 2 n 081 9 e dp 7 2 dm 11 5 q 6 lies neatly between previous kaapvaal poles with ages of 2220 ongeluk lavas and 2060 ma bushveld complex our data thus support recent correlations of the gamagara mapedi formation with pre bushveld sediments of the pretoria group a pre bushveld age for bgm is also consistent with its substantial distance from a new albeit less reliable paleomagnetic pole from the 1930 ma hartley lavas higher within the olifantshoek succession 12 5 n 332 8 e k 18 6 a95 16 0 q 3 our conglomerate test at beeshoek confirms previous allegations that the intense hematitization observed in the drakenstein wolhaarkop paleosol occurred during paleoproterozoic weathering under a highly oxygenated atmosphere

origin of hematite carrying a late paleozoic remagnetization in a quartz sandstone bed from the silurian rose hill formation virginia usa abstract paleomagnetic and petrographic studies of a quartz sandstone bed from the silurian rose hill formation of the central appalachians show that hematite carrying a late paleozoic remagnetization is authigenic comparison of paleopole positions from the rose hill formation with late paleozoic poles for north america suggests a synfolding remagnetization the sandstone cement is characterized by pore lining hematite pore filling berthierine and intergrowths of hematite berthierine and quartz these cements postdate pressure solution of detrital quartz grains late paleozoic cementation is a consequence of diagenesis associated with alleghanian deformation detrital fe mg silicate grains are believed to have been a local source of fe for some authigenic hematite and berthierine although mass balance calculations suggest that an external source is also required some berthierine aggregates are pseudomorphic after detrital fe bearing silicate grains trace amounts of detrital tourmaline are the only remnants of the detrital fe mg silicates

reevaluation of plate motion models based on hotspot tracks in the atlantic and indian oceans abstract plate motion models based on hotspot tracks in the atlantic and indian oceans predict minimal movement less than a few millimeters per year between these hotspots and their counterparts in the pacific ocean for the past 100 m yr whereas plate circuit exercises indicate relative motions of 20 mm yr hotspot based models also suggest that the rajmahal traps india were located 1000 km away from the kerguelen hotspot at 115 ma and the deccan traps india were located a similar distance from the reunion hotspot at 65 ma this is at odds with conclusions derived from paleomagnetism plate circuits and geochemical parameters that suggest a genetic link between flood basalt provinces in india and hotspots in the indian ocean these divergent views may be explained by plume action 1000 km from its center or errors in the hotspot motion models the latter hypothesis is scrutinized in this article by examination of the radiometric ages for hotspot tracks in the atlantic and indian oceans the

revised ages for tuffs of the yellowstone plateau volcanic field assignment of the huckleberry ridge tuff to a new geomagnetic polarity event 40 ar 39 ar ages were determined on the three major ash flow tuffs of the yellowstone plateau volcanic field in the region of yellowstone national park in order to improve the precision of previously determined ages total fusion and incremental heating ages of sanidine yielded the following mean ages huckleberry ridge tuff 2 059 0 004 ma mesa falls tuff 1 285 0 004 ma and lava creek tuff 0 639 0 002 ma the huckleberry ridge tuff has a transitional magnetic direction and has previously been related to the reunion normal polarity subchron dating of the reunion event has been reviewed and its ages have been normalized to a common value for mineral standards the age of the huckleberry ridge tuff is significantly younger than lava flows of the reunion event on re union island supporting other evidence for a normal polarity event younger than the reunion event