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

rapid generation of both high and low δ18o large volume silicic magmas at the timber mountain oasis valley caldera complex nevada we present an oxygen isotope and pet rologic study of four voluminous zoned ash flow sheets of the southwestern nevada volcanic field swnvf topopah spring ts 1200 km 3 12 8 ma tiva canyon tc 1000 km in the basin and range province and it fa vors nearly total evacuation of the magma chamber during a single eruption each si licic magma unit was generated by a shal low influx of new mafic magma that melted 18 o 16 o depleted as in the case of at or 18 o 16 o enriched rm ts rocks the at tuff and associated pre and post caldera lavas are 2 5 lower in d 18 o than in the rm tuff and represent the largest known low d 18 o magma we find that all units of the at cycle contain isotopically zoned zir cons that have up to 2 core to rim zon ing and correspondingly smaller out of equilibrium quartz zircon and melt zircon fractionations air abraded cores of quartz and sphene do not preserve any d 18 o zon ing the higher d 18 o zircon cores in low d

covariance of initial 87sr 86sr ratios δ18o and sio2 in continental flood basalt suites the role of contamination and alteration when mutual positive correlations occur between delta sup 18 o sup 87 sr sup 86 sr initial ratio r sub o and sio sub 2 in continental flood basalt suites they are generally accepted as being due to crustal contamination in continental flood basalt suites that have undergone coupled assimilation fractional crystallization in which the contaminant is granitic continental crust r sub o reflects the degree of contamination and will correlate positively with sio sub 2 which reflects the degree of differentiation posteruptive alteration of a suite having a range of sio sub 2 values should result in a positive correlation between sio sub 2 and delta sup 18 o because the ability of a volcanic rock to concentrate sup 18 o depends in part on silica content suites that have undergone assimilation fractional crystallization followed by alteration after eruption may there for exhibit a positive correlation between delta sup 18 o sio sub 2 and r sub o the delta sup 18 o data in such suites may consequently suggest erroneously high degrees of contamination

crustal origin for the parana rhyolites discussion of description and petrogenesis of the parana rhyolites southern brazil by garland et al 1995 our criticisms of the garland et al 1995 paper are introduction three fold and concern 1 the interpretation of oxygen the south atlantic ocean divides what was once a single isotope data on the rhyolites 2 radiogenic isotope large igneous province into the larger parana flood basalt fingerprinting of potential source material 3 the major province of south america and the smaller etendeka element composition of melts produced from potential flood basalt province of namibia the term flood basalt sources in the lower crust province is in this case something of a misnomer because a substantial thickness of silicic volcanic rocks is preserved on both sides of the atlantic the parana rhyolites can be divided into two main types the chapeco high ti oxygen isotopes type which have a high concentration of high field one of the problems of using oxygen isotopes to solve strength elements and low sr isotope ratios sr sr at problems of magma genesis is that oxygen isotope ratios 130 ma 0 705 0 708 and the palmas low ti type are changed during low temperature alteration processes which has lower concentrations of high field strength it is well known that large shifts in do value can occur elements and elevated sr isotope ratios sr sr at 130 in glassy volcanic rocks such as the parana rhyolites e g ma 0 714 0 727 mantovani et al 1985 bellieni et al taylor 1968 cerling et al 1985 these problems can 1986 peate et al 1992 garland et al 1995 suggested be circumvented by analysing fresh phenocrysts such as that the former were produced by partial melting of pyroxene which are unaffected by the alteration process high ti parana basalts whereas the more voluminous harris et al 1990 used the pyroxene do values to palmas rhyolites were produced by prolonged asestimate the magma do values of the palmas low ti similation accompanying crystal fractionation of low ti type rhyolite of the parana to be between 10 1 and parana basalt magmas previous workers have suggested 10 7 with the equivalent low ti magma type in the that the palmas type rhyolites are derived from melting etendeka having do values between 9 7 and 11 2 of pre mesozoic middle to lower crust piccirillo et al these high values led harris et al 1990 to suggest an 1988 harris et al 1990 and we believe that garland et origin by crustal melting al 1995 have overlooked several important lines of garland et al 1995 acknowledged the high do evidence in rejecting the crustal melting hypothesis in values and went on to state that the high do values this comment we will draw on data from the etendeka were used by harris et al 1990 to invoke an entirely province of namibia which was once part of the same crustal origin but it has been argued elsewhere that post eruptive alteration can lead to an increase in do continental flood basalt province

effective integration of core and log data abstract the primary purpose of core to log data integration is to reduce the uncertainty associated with formation evaluation in so doing advantage is taken of both the higher precision of core data and the larger scale of investigation of log data it is particularly important that when tying logs back to core the calibration algorithm is as well defined as possible a basic requirement is the definiton of a common reference depth scale a second requirement is to reconcile the different vertical resolutions by the depth averaging of core data the signal enhancement of log data or both essential to this process is the adoption of key intervals as control zones for data integration these procedures can result in a reduced uncertainty which transmits through to reservoir appraisal through a better petrophysical definiton of constituent lithological units important new developments will facilitate the integration of core and log data where core recovery is incomplete these include the ultrasonic monitoring of core entry into the core barrel the multi sensor scanning of whole cores and the interactive display of both core and log data in interrogative workstation mode implementation of the optimum data integration strategy and of the new upcoming developments will require cross discipline collaboration between those responsible for core acquisition core analysis and wireline log interpretation a key factor will be the ability to develop interpretative algorithms for cross scale application

geodynamics of rapid voluminous felsic magmatism through time abstract two end member geodynamic settings produce the observed examples of rapid voluminous felsic rhyolitic magmatism through time the first is driven by mantle plume head arrival underneath a continent and has operated in an identifiable and regular manner since at least 2 45 ga this style produces high temperature 1100 c low aspect ratio rheoignimbrites and lavas that exhibit high sio 2 al 2 o 3 ratios high k 2 o na 2 o ratios and where available data exists high ga al 2 o 3 ratios 1 5 with high f in thousands of parts per million and low water content f concentration is significant as it depolymerizes the silicate melt influencing the magmas physical behavior during development and emplacement these rhyolites are erupted as part of rapidly emplaced 10 15 myr mafic lips and are formed primarily by efficient assimilation fractional crystallization processes from a mafic mantle parent the second is driven by lithospheric extension during continental rifting or back arc evolution and is exclusive to the phanerozoic slips silicic large igneous provinces develop over periods 2 al 2 o 3 ratios and lower k 2 o na 2 o ratios rapid voluminous felsic magmatism represents both extreme examples of continental growth since the archean and also dramatic periods of crustal recycling and maturation during the phanerozoic

large volume low δ18o rhyolites of the central snake river plain idaho usa the miocene bruneau jarbidge and adjacent volcanic fields of the central snake river plain southwest idaho are dominated by high temperature rhyolitic tuffs and lavas having an aggregate volume estimated as 7000 km3 samples from units representing at least 50 of this volume are strongly depleted in 18o with magmatic feldspar 18ovsmow vienna standard mean ocean water values between 1 4 and 3 8 the magnitude of the 18o depletion and the complete lack of any rhyolites with normal values 7 10 combine to suggest that assimilation or melting of a caldera block altered by near contemporaneous hydrothermal activity is unlikely instead we envisage generation of the high temperature rhyolites by shallow melting of idaho batholith rocks under the influence of the yellowstone hotspot affected by eocene meteoric hydrothermal events the seeming worldwide scarcity of strongly 18o depleted rhyolites may simply reflect a similar scarcity of suitable crustal protoliths

formation of low δ18o rhyolites after caldera collapse at yellowstone wyoming usa we present a new model for the genesis of low 18o rhyolites of the yellowstone caldera based on analyses of zircons and individual quartz phenocrysts low 18o rhyolites were erupted soon after the massive caldera forming lava creek tuff eruption 602 ka 1000 km3 and contain xenocrysts of quartz and zircon inherited from precaldera rhyolites these zircons are isotopically zoned and out of equilibrium with their host low 18o melts and quartz diffusion modeling predicts that magmatic disequilibria of oxygen isotopes persists for as much as tens of thousands of years following nearly total remelting of the hydrothermally altered igneous roots of the depressed cauldron in which the alteration resistant quartz and zircon initially retained their 18o values these results link melting to caldera collapse rule out rapid or catastrophic magma meteoric water interaction and indicate wholesale melting rather than assimilation or partial melting

chemical stratigraphy of the parana lavas south america classification of magma types and their spatial distribution a new classification scheme has been developed to assign the lava flows of the parana continental flood basalt province south america into geochemically distinct magma types with six basaltic major and trace element abundances and or ratios by mapping out the spatial distribution of these magma types within the lava sequences it has been possible to determine the internal stratigraphy of the lava pile on a regional scale previous studies on road profiles traversing the well exposed coastal serra geral escarpment of southern brazil are summarised together with results from some new sampled sections more widespread stratigraphical investigations of the parana lavas have been hampered by the lack of sufficient topographic relief and the cover of sedimentary rocks however access to drill core chippings from nine boreholes in the central parana region has provided a unique opportunity to investigate the stratigraphy of the otherwise inaccessible deeper levels of the lava pile and to map out stratigraphic variations in three dimensions the borehole samples have indicated cated a stacking of units of different magma types all overlapping towards the north which suggests that the main locus of magmatism moved northwards with time within the parana basin this migration could be related to the northward propagation of rifting during the initiation of the south atlantic ocean maps of the surface distribution of samples of each magma type show a pattern consistent with the stratigraphy inferred from the boreholes although suggesting that the shift in magmatism may have been towards the northwest on the basis of geochemical similarities between magma types and their inferred stratigraphical relationships it is proposed that the parana can be divided into two principal magmatic centres 1 an older one in the south comprising the gramado esmeralda and urubici magma types and 2 a younger one developed about 750 km to the north formed by the pitanga paranapanema and ribeira magma types

low δ18o rhyolites from yellowstone magmatic evolution based on analyses of zircons and individual phenocrysts the yellowstone plateau volcanic field is one of the largest centers exchange time to form zoned zircons is between a few hundred and a few thousand years which reflects the residence time of lowo of rhyolitic magmatism on earth major caldera forming eruptions are followed by unusual lowo rhyolites new oxygen isotope magmas after formation and before eruption petrologic and geochemical data from rhyolites belonging to the 2 0 my eruptive history of yellowstone are presented with emphasis on the genesis of lowo magmas erupted after the huckleberry ridge tuff 2 0 ma 2500 km and lava creek tuff 0 6 ma

porosity and permeability prediction from wireline logs using artificial neural networks a north sea case study estimations of porosity and permeability from well logs are important yet difficult tasks encountered in geophysical formation evaluation and reservoir engineering motivated by recent results of artificial neural network ann modelling offshore eastern canada we have developed neural nets for converting well logs in the north sea to porosity and permeability we use two separate back propagation anns bp anns to model porosity and permeability the porosity ann is a simple three layer network using sonic density and resistivity logs for input the permeability ann is slightly more complex with four inputs density gamma ray neutron porosity and sonic and more neurons in the hidden layer to account for the increased complexity in the relationships the networks initially developed for basin scale problems perform sufficiently accurately to meet normal requirements in reservoir engineering when applied to jurassic reservoirs in the viking graben area the mean difference between the predicted porosity and helium porosity from core plugs is less than 0 01 fractional units for the permeability network a mean difference of approximately 400 md is mainly due to minor core log depth mismatch in the heterogeneous parts of the reservoir and lack of adequate overburden corrections to the core permeability a major advantage is that no a priori knowledge of the rock material and pore fluids is required real time conversion based on measurements while drilling mwd is thus an obvious application

low 18o silicic magmas why are they so rare low 180 silicic magmas are reported from only a small number of localities e g yellowstone and iceland yet petrologic evidence points to upper crustal assimilation coupled with fractional crystallization afc during magma genesis for nearly all silicic magmas the rarity of 10w l o magmas in intracontinental caldera settings is remarkable given the evidence of intense 10w l o meteoric hydrothermal alteration in the subvolcanic remnants of larger caldera systems in the platoro caldera complex regional ignimbrites 150 1000 km3 have plagioclase 6180 values of 6 8 0 1 whereas the middle tuff a small volume est 50 100 km3 post caldera collapse pyroclastic sequence has plagioclase 8 80 values between 5 5 and 6 8 o on average the plagioclase phenocrysts from the middle tuff are depleted by only 0 3 0 relative to those in the regional tuffs at yellowstone small volume post caldera collapse intracaldera rhyolites are up to 5 5 o depleted relative to the regional ignimbrites two important differences between the middle tuff and the yellowstone 10w 180 rhyolites elucidate the problem middle tuff magmas reached water saturation and erupted explosively whereas most of the 10w l 80 yellowstone rhyolites erupted effusively as domes or flows and are nearly devoid of hydrous phenocrysts comparing the two eruptive types indicates that assimilation of 10w 180 material combined with fractional crystallization drives silicic melts to water oversaturation water saturated magmas either erupt explosively or quench as subsurface porphyrins bejiire the magmatic 180 can be dramatically lowered partial melting of low 180 subvolcanic rocks by near anhydrous magmas at yellowstone produced small volume 10w 180 magmas directly thereby circumventing the water saturation barrier encountered through normal afc processes

geochemical and mineralogical evidence for the occurrence of at least three distinct magma types in the famous region bulk rock major and trace element variations in selected basalts from the famous area in conjunction with a detailed study of the chemical compositions of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationship among the various lava types the distribution of nio in olivine and cr spinel phenocrysts distinguishes the picritic basalts plagioclase phyric basalts and plagioclase pyroxene basalts from the olivine basalts for a given mg mg fe2 atomic ratio of the mineral the nio content of these phenocrysts in the former three basalt types is low relative to that in the phenocrysts in the olivine basalts the zr nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase pyroxene basalts and in addition distinguishes the picritic basalts from the other basalt types these differences indicate that the different magma groups could not have been processed through the same magma chamber and preclude any direct inter relationship via open or closed system fractional crystallization

determination of electrofacies using wireline logs based on multivariate statistical analysis for the kareem formation gulf of suez egypt beyond the traditional use of well logs electrofacies were introduced to define a set of log responses that can be used to characterize a bed and permitted it to be distinguished from the other rock units classification of a formation into electrofacies can be used in reservoir characterization and discrimination due to the hydrocarbon potential of the kareem formation we used the conventional well logs of four boreholes and a traditional core analyses data from one of these wells to identify electrofacies of the kareem formation in the southern gulf of suez multivariate statistical analyses such as principal component analysis pca and cluster analysis were used for this purpose the multivariate statistical analyses were applied to the raw log data the normalized data and the standardized log data three pc components were obtained from the analyses and six distinctive electrofacies were extracted from the dendrogram plot these electrofacies showed good correlation with the lithofacies that were described from core analysis in addition applying the reservoir zonation of the studied formation using the stratigraphic modified lorenz sml plot and the sum of squared errors sse methods led to its discrimination into four hydraulic flow units the deduced flow units were validated against the available core data

integration of outcrop and borehole image logs for high resolution facies interpretation example from a fluvial fan in the ebro basin spain electrical borehole image logs yield high resolution information about variations in micro resistivity along the borehole wall to interpret these variations in terms of sedimentary structures and lithofacies types calibration with real rock is needed normally the only real rock available is core and this only provides one dimensional information in this paper the interpretation of fluvial facies types from borehole image logs was established by direct comparison with outcrops four fluvial facies associations were established in an outcrop study of a low net to gross fluvial succession i meandering rivers ii braided rivers iii crevasse deltas and iv crevasse splays the lithofacies characteristics and palaeocurrent distributions of each fluvial facies association were established two 200 m deep wells were drilled behind the cliff face outcrops one well was cored to a depth of 150 m and borehole image logs were recorded in both wells the wells were correlated with the outcrop the borehole image logs were analysed by their vertical colour succession and the dipmeter pattern image log facies were established and these were interpreted in terms of the fluvial facies associations encountered in the corresponding outcrops the study of borehole image logs yields a set of diagnostic criteria for a detailed fluvial facies interpretation and the establishment of depositional trends and thus provides a powerful tool for the direct interpretation of fluvial facies in a reservoir setting

diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks recently obtained data on oxygen diffusion in feldspars quartz and hornblende permit the prediction of the apparent18o16o temperatures that would be measured in a rock that consisted only of those three minerals and cooled slowly from high temperature the computed temperatures would be based on the differences in the18o16o ratios between coexisting pairs of minerals the present calculation takes into account the diffusion rates for oxygen as a function of temperature the cooling rate of the rock the mineral grain sizes and the mode of the rock for mineral grains 1 mm in radius and a cooling rate of 10 c m y the minimum difference in apparent temperature between quartz feldspar and feldspar hornblende pairs will be 115 c despite the assumption of a normal uneventful slow cooling history to room temperature further the apparent quartz hornblende temperature will range over 30 c 590 620 c depending on the mode of the rock for a cooling rate of 1000 c m y the apparent difference in temperature can be as much as 400 c consequently consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high grade metamorphic rocks or igneous rocks which are cooled slowly departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature or a complex history for the rock for some minerals including hornblende the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens the choice of the specimens used for such calibrations needs to be re evaluated in light of these findings this may result in a change in the equilibrium equation constants an example from the literature the san jose tonalite baja california mexico was modelled and yields 18o concentrations in the minerals that correspond closely with the measured values this suggests that the model used is appropriate that the rock has had a simple thermal history and that it cooled at 100 200 c m y over the temperature range 800 500 c the set of paleotemperatures obtained for a rock will in general yield neither the mineral closure temperatures nor the formation or crystallization temperatures on the other hand the cooling rate of the rock may be derived from the data this in turn may have important tectonic implications with regard to denudation and uplift rates

upper palaeozoic basins of the british isles caledonide inheritance versus hercynian plate margin processes hypotheses to explain the origin and location of upper palaeozoic sedimentary basins are presented group a basins were those old red sandstone basins whose location and subsidence were controlled by caledonide features including active and remnant fore arc basins midland valley and major transcurrent movements orcadian basin newer granite plutonic magmas in the southern uplands subduction complex post dated subduction and resulted from post collision cratonization processes as yet little understood group b basins were those whose initial positions in space were controlled by palaeo plate factors in particular the location of granitoid plutons but whose subsidence was controlled by active hercynian neoplate processes some group a basins were regenerated by this process midland valley a new hypothesis is presented to account for the origin of a carboniferous rift to sag trend similar to that proven in modern continental margin basins such as the north sea the hypothesis is an adaptation of mckenzie9s lithospheric stretching model some of whose major features are well shown in the carboniferous history of the north and central british isles group b processes were controlled by ligurian bretonic plate consumption in the s brittany massif central vosges areas group c basins in the rheno hercynian zone also owe their location and origin to back arc rifting and continental crustal oceanization processes caused by liguro bretonic subduction

a mountain front model for the variscan deformation of the south wales coalfield access to the considerable geological database of british coal together with seismic evidence of subsurface relationships has enabled the development of a structural model for the south wales coalfield detailed site investigations are used as key examples the simple broad synclinal nature of the coalfield masks a more complex structural system thrusting of variscan age is largely confined to the lower and middle coal measures and throughout the central and northern coalfield is dominantly a forethrust system however along the south crop a major backthrust system is developed which in places penetrates the upper coal measures the south crop shows features analogous with a mountain front and is interpreted as a triangle zone in which pre existing basement faults have facilitated uplift the lack of thrusting within the upper coal measures in contrast to the underlying lower and middle coal measures suggests they have acted as a passive roof to the thrust system geographical variation in the intensity of the deformation can be related to the proximity of and buttressing against the caledonian massif although broadly analogous with current models of foreland basin development and deformation the south wales coalfield demonstrates the importance of pre existing massifs and lineaments in determining the precise evolution of an area

oxygen isotope geochemistry of the silicic volcanic rocks of the etendeka parana province source constraints oxygen isotope ratios of pyroxene phenocrysts in the silicic volcanic rocks from the cretaceous etendeka parana flood basalt province namibia south america are believed to reflect the delta sup 18 o values of the original magmas the authors recognize a high delta sup 18 o value type delta sup 18 o pyroxene approximately 10 per thousand found in the south of both regions and a low delta sup 18 o value type delta sup 18 o pyroxene approximately 6 5 per thousand found in the north other differences between thee two rhyolite types include higher concentrations of incompatible elements and lower initial sup 87 sr sup 86 sr ratios in the low delta sup 18 o value type the authors suggest that the regional distribution of rhyolite types reflects differences in source composition which can best be explained if the sources are lower crustal late proterozoic mobile belt material high delta sup 18 o and archean lower crust low delta sup 18 o

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

o isotope study of the bushveld complex granites and granophyres constraints on source composition and assimilation the 2 05 ma bushveld magmatic event culminated in the production of490 000 km 3 of granite and granophyres in these granitic rocks high temperature equilibrium o isotope fractionations are generally preserved between quartz and zircon but not between quartz and feldspar or between biotite and amphibole quartz separated from four granite samples shows no significant difference in core and rim d 18 o values which indicates that quartz is not significantly zoned and provides further evidence that it is unaffected by alteration quartz can therefore be used as a proxy for the magma d 18 o value leading to estimates of 6 9o for both the granites assuming quartz magma 1 11o and granophyres assuming quartz magma 0 62o similar magma d 18 o values 6 6o were obtained using zircon d 18 o values assuming zircon magma 1 3o the initial nd isotope ratio of the granitic rocks ranges from 0 509676 to 0 509822 with an average value of 0 509655 n 12 this corresponds to average end values of 5 9 and 4 8 for the granites and granophyres respectively the similarity in isotope composition between the granites and granophyres and between the granitic rocks from each of the three major lobes of the bushveld complex is consistent with a common origin the d 18 o values of the granitic rocks suggest derivation from mantle derived magmas by either fractional crystallization or partial melting but this hypothesis is incompatible with their crustal end values average 5 5 the associated rustenburg layered suite rls rocks have average d 18 o values of 7 1o which is within error of the average estimate for the bushveld granitic rocks and similar end values however granitic magma derived from the same paretal magmas that produced the rls would have had an average magma d 18 o of about 7 9o 1 o higher than observed we therefore suggest that the granitic magmas were produced by fractional crystallization of rls magma or by partial melting of solidified rls magma at depth followed by assimilation at a shallower level of a significant quantity of hydrothermally altered low d 18 o material from the since eroded volcanic edifice

production of jurassic rhyolite by anatexis of the lower crust of patagonia abstract the mid jurassic marifil and chon aike volcanic rocks of eastern patagonia are part of one of the largest silicic igneous provinces known rb sr geochronology indicates eruptive ages of 175 190 ma for the marifil complex mostly toarcian aalenian the majority of the rocks are isotopically uniform with initial 87sr 86sr 0 7067 0 0003 and endt 4 2 primary magmas of andesitic composition were generated by partial melting of mafic grenvillian lower crust indentified by depleted mantle model ages of 1150 1600 ma lower crustal pyroxene granulite xenoliths can be modelled as residual jurassic source although they may alternatively be co genetic cumulates the dacite rhyolite suite formed by crystal liquid fractionation processes from the primary andesites involving multistage crystallization and re melting during magma ascent with the later stages of evolution being explicable by fractional crystallization of plagioclase amphibole and accessory minerals the province represents large scale lower crustal reworking associated with the unique tectonic and igneous environment of gondwana break up

structural synthesis of s staffordshire uk implications for the variscan evolution of the pennine basin the presence of strata of westphalian a to possibly stephanian age in south staffordshire located at the southern margin of the pennine basin provides a rare control on the timing of late carboniferous deformation in central england during westphalian a c times sedimentation in south staffordshire was controlled by thermal subsidence with a superimposed influence of synsedimentary normal and reverse faulting alkaline basic igneous activity occurred during westphalian c times synchronous with uplift of the southern margin of the pennine basin uplift culminated during westphalian c d times in a phase of variscan inversion along basement lineaments and the development of a regional unconformity renewed subsidence and deposition of a relatively thick sedimentary sequence of westphalian d to possibly stephanian age was followed by a final phase of variscan deformation comprising possibly two pulses of uplift tilting and erosion observations presented for south staffordshire in conjunction with studies from other parts of the pennine basin indicate a complex interrelationship between rifting thermal subsidence and variscan basin inversion pulses of n s extension and e w to nw se compression are interpreted as the product of regional strike slip deformation superimposed upon a regime of thermal subsidence alkaline igneous activity is considered to be the product of mantle partial melting in response to limited decompression due to transtensional displacements on crustal thickness faults

intraplate magmatism related to opening of the southern iapetus ocean cambrian wichita igneous province in the southern oklahoma rift zone abstract southern oklahoma and adjacent parts of texas contain an extensive igneous province emplaced during early cambrian rifting within the southern oklahoma rift zone the rift zone was initiated in association with the opening of the southern iapetus ocean during rodinia supercontinent breakup and later became the site of a series of linked uplifts and basins as a result of late paleozoic inversion igneous rocks within the rift are referred to as the wichita province and are present mostly in the subsurface although critical exposures occur in the wichita and arbuckle mountains in southwestern and southern oklahoma wells drilled into basement in the region provide a wealth of information on the distribution and relations of the major igneous units in the upper crust and geophysical data provide important constraints on deeper levels of the rift zone the upper parts of the igneous rift fill comprise the carlton rhyolite group which has an estimated subsurface areal extent of 40 000 km 2 and the related wichita sheet granites which intrude the lower parts of the rhyolite succession these rocks have a type characteristics and were emplaced after intrusion tilting and uplift of a large tholeiitic layered mafic complex smaller bodies of gabbro also intruded the layered complex after it was tilted u pb zircon and 40 ar 39 ar geochronology indicates the felsic rocks and at least some of the mafic units were emplaced in a relatively narrow time frame at 539 530 ma basalts and intermediate lavas are present only in the subsurface our new work shows these lavas to have tholeiitic to mildly alkaline compositions and to be more extensive than previously realized forming thick successions that both underlie and are intercalated with the rhyolites diabase dikes were intruded into older crust during initial opening of the rift and a suite of late diabase intrusions also cuts the rhyolites and granites indicating that mafic magma was supplied to the rift throughout its igneous history geophysical data show that an enormous mass of mafic rock occupies deeper parts of the rift extending to depths of at least 10 km beneath the surface the total volume of igneous rock emplaced within the rift is estimated to be in excess of 250 000 km 3 detailed studies of carlton rhyolite exposures in the wichita mountains complemented by work on drill cuttings from basement wells indicate that lava flows up to 400 m thick make up major parts of the rhyolitic succession the generally limited exposures of these rhyolite lavas in the wichita mountains are interpreted to be remnants of laterally extensive flow units similar to those documented from other a type felsic provinces trace element abundances define three distinct geochemical groups of rhyolite suggesting derivation from three different sources or magma reservoirs flows belonging to the different geochemical groups are intercalated in the wichita mountains exposures which may indicate that laterally extensive flows from separate magma chambers came to rest on top of one another during accumulation of the rhyolitic volcanic pile

origin of k feldspar megacrysts in rhyolites from the emeishan large igneous province southwest china abstract silicic rocks occur in the uppermost units of the longest volcanic succession 5000 m thick in the binchuan area of the permian emeishan flood basalt province of sw china they are predominantly rhyolites and to a lesser extent trachytes both containing potassium feldspar megacrysts as the dominant phenocryst phase up to approximately 20 mm in size these megacrysts contain domains of albite arranged in vein like networks likely formed by post magmatic alteration crystal size distributions csd suggest that these megacrysts grew in a stable magmatic system consistent with relatively uniform core to rim compositional k 2 o 14 16 wt and isotopic profiles 208 pb 206 pb ratios 2 06 2 08 0 005 both whole rock trace elements and pb isotope ratios of these silicic rocks are similar to the emeishan basalts suggesting a common source for both mafic and felsic units and a limited role of crustal melting in genesis of the felsic units major and trace element models further indicate that these rocks could not have formed exclusively by re melting of old crust or solidified basaltic rock but must have formed through crystal fractionation from the flood basalts or possibly partial melting of basaltic rock followed by fractional crystallization k feldspar bearing rhyolites are also observed in the last stages of other large igneous provinces we suggest that they represent final melt fractions and their appearance in the magmatic system coincides with waning of voluminous magmatic activities

combined gravity and magnetic modeling over pavagadh and phenaimata igneous complexes gujarat india inference on emplacement history of deccan volcanism abstract a large number of igneous intrusions related to the deccan magmatism are exposed in the western and central part of the indian shield gravity and magnetic g m surveys over some of these igneous intrusive bodies depict gravity high and bipolar magnetic anomalies as the most characteristic signatures the present g m survey was carried out over the pavagadh and phenaimata igneous intrusives associated with the phenaimata complex bouguer gravity anomaly shows an elliptical shaped relative gravity high of about 40 mgal and bipolar magnetic anomaly varies from south to north between 800 nt and 1200 nt the joint g m modeling reveals the presence of a dense mafic body 2 86 g cm3 this body is characterized by a remanant magnetization the related inclination i 44 and declination d 160 may correspond with the 29r polarity chron of deccan magnetostratigraphy remanant magnetization together with age data suggest that the phenaimata igneous intrusive emplaced during the end of the main magmatism phase of deccan over the pavagadh a circular gravity and magnetic lows of about 15 mgal and 500 nt respectively is reported for the first time which is surrounded by a gravity and magnetic high of about 30 mgal and 350 nt respectively the joint g m modeling over the pavagadh intrusive reveals the presence of a deep seated cone shaped high density 3 0 g cm3 gabbroic body which might extend up to a great depth its top surface reaches up to a depth of about 10 0 km overlying this body is a low density 2 40 g cm3 rhyolite which extends up to the surface and is the source for low gravity anomaly it is surrounded by another high density 2 89 and 3 02 g cm3 mafic bodies with reverse remanant magnetization direction i 38 and d 152 the modeled direction of remanant magnetization for the rhyolite i 32 and d 336 and deeper gabbroic i 32 and d 340 bodies show normal polarity measured magnetization direction for the mafic body surrounding the rhyolite relates to the middle reverse polarity 29r chron inferred declination and inclination may then correspond to upper normal 29n middle reverse 29r and lower normal 30n polarity chrons therefore the magma forming the pavagadh igneous complex was emplaced covering the major span of deccan eruption g m model suggests that the magma chambers developed within the higher crustal levels and rhyolite originated from the underlying mafic magma through assimilation and fractional crystallization afc