a carbon isotope reference curve for ca 1700 1575 ma mcarthur and mount isa basins northern australia abstract shallow marine paleo and mesoproterozoic sedimentary successions are widely distributed in several major basins across northern australia the successions are only gently deformed and their stratigraphy is relatively continuous thus offering an ideal opportunity to document secular variations in carbon isotopes marine carbonate intervals from two of these major basins the mcarthur and mount isa basins have been sampled to document secular variation in 13ccarb from approximately 1700 to 1575 ma in all cases the samples have been tied to a well established sequence stratigraphy which along with u pb shrimp zircon dates provides a time resolution of the order of 1 m y the data presented here thus provide the most comprehensive and best dated 13ccarb stratigraphy yet obtained from such ancient rocks diagenesis occurred early in the carbonate rocks from both basins with the result that fluid movements were restricted and primary carbon isotopic signatures were retained the 13ccarb values from both basins vary within a very narrow range around a mean of 0 6 with extreme values seldom lying further than 1 from the mean that is the curves are essentially flat the results of this study combined with earlier studies on younger rocks imply that the global ocean reached a state of equilibrium in the mid paleoproterozoic and remained stable for much of the following billion years current models of the ocean suggest that to maintain the carbon mass balance relatively low levels of tectonic activity would be required which in turn suggests that the availability of nutrients such as phosphorus was stable and low prolonged nutrient stability may therefore have exerted a major influence upon the evolution of the biosphere over this time interval

dissolution of corundum and andalusite in h2o saturated haplogranitic melts at 800 c and 200 mpa constraints on diffusivities and the generation of peraluminous melts of h2o in the aluminum alkali eigenvector could not be obtained the mechanisms and kinetics of equilibration between peraluminous from our results we propose that some h2o may provide charge minerals and granitic melt were investigated experimentally by balance for excess aluminum multicomponent diffusion models the dissolution of corundum and andalusite into h2o saturated yield eigenvalues for the corresponding aluminum alkali eigenvector metaluminous haplogranitic melt at 800 c and 200 mpa mineral of 0 3 10 to 1 9 10 cm s calculated effective and haplogranitic glass rods were juxtaposed inside platinum binary diffusion coefficients for al2o3 are higher 1 0 10 to capsules and then subjected to experimental conditions for times 3 6 10 cm s these diffusion coefficients are used to ranging from 12 to 2900 h upon melting the mineral melt calculate equilibration times between peraluminous minerals and interface retreats with the square root of time the composition of granitic melts in a model system in which two grains of corundum the melt at the interface changes with time but its asi aluminum or andalusite 1 cm apart are connected via a narrow melt column saturation index molar al2o3 cao na2o k2o at the conditions of the experiments diffusive equilibrium through remains constant at 1 20 after 480 h this value is close to initially metaluminous h2o saturated melt would be attained within the asi of an h2o saturated haplogranitic melt in equilibrium with 10 10 years at h2o activity well below saturation of the corundum or andalusite which may indicate that the composition of melt lower diffusivity of all components may increase the time to the melt at the interface follows the liquidus of these minerals in equilibration to 10 10 years approaching possible time frames the system sodium and potassium diffuse rapidly uphill across the for the generation and extraction of crustal melts entire length of the charge towards the interface resulting in a rapid and uniform increase in asi throughout the entire melt column this uphill diffusion of alkalis is due to coupling with excess aluminum entering the melt at the interface and involves long range

the role of sulphate reducing bacteria and cyanobacteria in dolomite formation in distal ephemeral lakes of the coorong region south australia abstract the dolomitic distal ephemeral lakes of the coorong region of south australia are dynamic systems in which the concentrations of solutes generally increase during late spring and summer when the rate of evaporation greatly exceeds aqueous input during late evaporation active bacterial sulphate reduction occurs in uppermost anoxic lake sediments as rising salinity and then h2s toxicity kills off the macrobiota these hypersaline lakewaters are characterised by high ph and elevated concentrations of carbonate and sulphate ions magnesium concentrated in cyanobacterial cells and sheaths is released into the strongly electrolytic alkaline lake brines from lake margin stromatolites and mats during desiccation and may explain the remarkable increase in mg levels recorded at this time continued evaporation of the lakewaters produces a halite crust overlying the dolomite sulphate free residual puddles and the absence of gypsum indicates that all sulphate is removed by sulphate reducing bacteria dolomite forms in situ as a primary precipitate from these microbially mediated lake porewaters and both the morphology and size distribution of dolomite grains is identical to bacterial cells it is not proven exactly when dolomite precipitation occurs but the submicron sized grains and multiple defects in a disordered lattice indicate that precipitation is rapid and follows bacterial removal of sulphate which is known to inhibit dolomite formation bacterial sulphate reduction and cyanobacterial degradation may thus lead to the removal of all kinetic inhibitors to dolomite formation and provide a mechanism for dolomite precipitation around bacterial cells in shallow subsurface sediments during the late stages of evaporation in adjacent non dolomitic lakes sulphate is present throughout evaporation so that other minerals principally aragonite and hydromagnesite are precipitated

magmatic and postmagmatic processes in tin mineralized granites topaz bearing leucogranite in the eurajoki rapakivi granite stock finland the 1 57 ga eurajoki stock in southwestern finland is composed been overemphasized at the expense of exsolution and reof biotite hornblende fayalite granite biotite granites and late stage crystallization topaz bearing granite as well as related topaz bearing rhyolite ongonite dykes miarolitic cavities indicate the presence of a separate fluid phase during late stages of crystallization of the topazbearing granite and greisen type sn be w zn mineralization is closely associated with it subsolidus reactions have modified the

relationships between cyclicity and stromatolite form in the late proterozoic bitter springs formation australia stromatolite biostromes and bioherms in the lower two units of the late proterozoic loves creek member of the bitter springs formation represent shallowing upward and deepening upward sequences in the central unit stromatolite form is governed by relative position in an asymmetric shallowing upward sequence ooid and or peloid intraclast grainstones and small irregular bulbous and columnar stromatolites characterize the basal transgressive portion of cycles domal columnar and stratiform stromatolites comprise the bulk of the cycle these forms accreted in a gradually shallowing epeiric sea domal stromatolites predominate in the deeper parts of cycles here synoptic relief gradually increases upwards columnar and stratiform stromatolites predominate in the shallower parts of cycles where synoptic relief rapidly diminishes upwards in thin bedded dolo mudstones at the tops of cycles the co occurrence of desiccation cracks tepee structures scalloped dissolution surfaces gypsum moulds and anhydrite nodule pseudomorphs provides evidence for subaerial exposure in contrast stromatolites in a unit at the base of the loves creek member accreted during a gradual rise in sealevel stratiform columnar and domal stromatolitic building blocks of the shallowing upward cycle are present in this deepening sequence but only the lower half of the shallowing upward cycle is represented synoptic relief of the stromatolitic laminae gradually increases upward throughout the basal stromatolitic unit recognition of a deepening upward stromatolite sequence at the base of the loves creek member and a disconformity surface between this sequence and the underlying gillen member permits palaeoenvironmental re interpretation of the loves creek member as a single large scale sea level cycle

cyclicity and paleoenvironmental dynamics rocknest platform northwest canada the superbly exposed rocknest formation northwest territories canada contains 140 to 160 shallowing upward cycles 1 to 15 m thick that contain distinctive marker beds and can be correlated for 200 km parallel to strike and 120 km perpendicular to strike cycles are grouped according to cycle base lithology which reflects paleogeographic position on the platform shale based cycles of the distal inner shelf pass westward into dolosiltite based cycles then into tufa based cycles of the shoal complex and then into grainstone based cycles of the outer shelf cycles are bounded by erosional surfaces that become less pronounced away from the shoal complex cycles formed in response to small asymmetric rapid rise slow fall eustatic oscillations in sea level 10 m during sea level rise rapid transgression and submergence of tidal flats occurred along the eastern margin of the shoal complex followed by eastward expansion of the shoal complex and slow progradation of tidal flats over lagoonal facies during sea level fall a glacio eustatic model for cyclicity accounts for the data favoring asymmetric eustatic sea level oscillations subaerial exposure and vadose diagenesis of platform during sea level falls arrested progradation and incomplete shoaling of many cycles and cycle periods of 20 000 to 100 000 yr 18 000 to 30 000 yr for rocknest inner shelf slopes ranged from 8 cm km average at high sea level stand to 2 cm km average at low sea level stand tidal flat progradation rates were 7 to 9 km 1 000 yr interaction of inner shelf slope sea level fall rate and sedimentation rate generates complete condensed or expanded cycle profiles one dimensional computer modeling reveals that sedimentation rates of shoal complex facies were 250 cm 1 000 yr that lag times in sedimentation following transgression may have ranged from zero on the shoal complex to 2 000 yr on the distal inner shelf and that subtidal sedimentation rates may have ranged from 80 cm 1 000 yr for the proximal inner shelf to 20 cm 1 000 yr for the distal inner shelf

silicate melt inclusion evidence for extreme pre eruptive enrichment and post eruptive depletion of lithium in silicic volcanic rocks of the western united states implications for the origin of lithium rich brines to evaluate whether anatectic and or highly fractionated lithophile element enriched rhyolite tuffs deposited in arid lacustrine basins lose enough lithium during eruption lithification and weathering to generate significant li brine resources pre eruptive melt compositions preserved in inclusions and the magnitude of post eruptive li depletions evident in host rhyolites were documented at six sites in the western united states each rhyolite is a member of the bimodal basalt rhyolite assemblage associated with extensional tectonics that produced the basin and range province and rio grande rift an evolving pattern of closed drainage basins and geothermal energy or mineral resources results from the 0 8 ma bishop tuff geothermal in california 1 3 to 1 6 ma cerro toledo and upper bandelier tephra geothermal and 27 9 ma taylor creek rhyolite sn in new mexico 21 7 ma spor mountain tuff be u f and 24 6 ma pine grove tuff mo in utah and 27 6 ma hideaway park tuff mo in colorado support the following conclusions melt inclusions in quartz phenocrysts from rhyolite tuffs associated with hydrothermal deposits of sn mo and be are extremely enriched in li 1 000s of ppm those from spor mountain have the highest li abundance yet recorded max 5 200 ppm median 3 750 ppm forty five to 98 of the li present in pre eruptive magma was lost to the environment from these rhyolite tuffs the amount of li lost from the small volumes 1 10 km3 of li enriched rhyolite deposited in closed basins is sufficient to produce world class li brine resources after each eruption meteoric water leaches li from tuff which drains into playas where it is concentrated by evaporation the localized occurrence of li enriched rhyolites may explain why brines in arid lacustrine basins seldom have economic concentrations of li considering that hydrothermal deposits of sn mo be u and f may indicate potential for li brines in nearby basins we surmise that the world s largest li brine resource in the salar de uyuni 10 mt received li from nearby rhyolite tuffs in the bolivian tin belt

volcano stratigraphy and 40ar 39ar geochronology of the macusani ignimbrite field monitor of the miocene geodynamic evolution of the andes of southeast peru abstract a total of 1400 m of ignimbritic ash flow tuffs distributed in five cross sections and extending from the pre neogene base to the uppermost flows were studied in the miocene macusani ignimbrite basin southeast peru volcano stratigraphic interpolation and precise 40ar 39ar dating of sanidine biotite and rhyolitic glass from the ash flow tuffs give rise to a southwest northeast correlation chart subdivided into six volcanic cycles or fundamental cooling units i e 1 10 0 0 5 2 7 8 8 0 0 1 3 7 5 0 1 4 7 3 0 1 5 6 8 7 0 o 1 and 6 6 7 o 1 ma these delimit two brief e 10 1 and 7 1 ma which are sensibly synchronous with the quechua 2 and quechua 3 compressional events characterizing the tectonic regime in the central andes this close relationship between tectonic pulses and felsic magmatic activity adds a supplementary constraint to models of the miocene tectono magmatic evolution of the cordillera oriental the stratigraphic relationships of the uranium mineralization of the macusani field are defined for the first time the stratiform stratabound occurrences are restricted to three main cooling units dated at 7 8 7 5 and 6 9 6 8 ma which constitute a maximum age for uranium deposition

phase relations of peraluminous granitic rocks and their petrogenetic implications peraluminous granitic rocks are magmatic rocks that contain quartz po tassic feldspar and or sodic plagioclase and one or more aluminum rich minerals thus these rocks contain more aluminum than could be accom modated by feldspar in a cipw normative calculation and show normative corundum peraluminous granites are defined by their chemical nature but their recognition is often based on petrography this discrepancy thus may lead to inconsistency although peraluminous granitic rocks are com monly true granite streckeisen 1973 they range from tonalite to granite of special interest are strongly peraluminous as defi ned below granite granodiorite or in rare instances trondhjemite these rocks contain alumi nous minerals almandinic garnet cordierite tourmaline muscovite that are thought to be stable in silicate magmas only if these magmas are themselves peraluminous the origin of peraluminous granite and related intrusive rocks is of much current interest many of these form shallow level intrusions and they can reach enormous sizes examples are the batholiths in the lachlan fold belt in southeastern australia white chappell 1983 the cornub ian batholith in england exley stone 1966 the south mountain batholith in nova scotia mckenzie clarke 1975 the manaslu batho lith in the himalayas lefort 1981 much of the idaho batholith

crystallization of accessory phases in magmas by local saturation adjacent to phenocrysts abstract accessory minerals commonly occur attached to or included in the major crystalline phases of felsic and some intermediate igneous rocks apatite is particularly common as inclusions but fe ti oxides pyrrhotite zircon monazite chevkinite and xenotime are also known from silicic rocks accessories may nucleate near the host crystal liquid interface as a result of local saturation owing to formation of a differentiated chemical boundary layer in which accessory mineral solubility would be lower than in the surrounding liquid differentiation of this boundary layer would be greatest adjacent to ferromagnesian phenocrysts especially fe ti oxides it is with oxides that accessories are most commonly associated in rocks a boundary layer may develop if the crystal grows more rapidly than diffusion can transport incorporated and rejected elements to and from the phenocryst diffusion must dominate over convection as a mode of mass transfer near the advancing crystal liquid interface in order for a boundary layer to exist accumulation of essential structural constituent elements of accessory minerals owing to their slow diffusion in evolved silicate melt also may force local saturation but this is not a process that applies to all cases local saturation is an attractive mechanism for enhancing fractionation during crystallization differentiation if accessory minerals attached to or included in phenocrysts formed because of local saturation their host phenocrysts must have grown rapidly when accessories nucleated in comparison to lifetimes of magma reservoirs some inconsistencies remain in a local saturation origin for accessory phases that cannot be evaluated without additional information

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

carbon isotopes and the rise of atmospheric oxygen new data for the isotopic composition of carbon in carbonate sediments deposited between 2 6 and 1 6 ga indicate that the value of 13c in these sediments underwent a very large positive excursion between 2 22 and 2 06 ga a reassessment of the earlier 13c data for carbonate sediments shows that this excursion was probably worldwide and that it was preceded and followed by several hundred million years during which the 13c of carbonate sediments differed little from that of modern carbonates the large 13c excursion between 2 22 and 2 06 ga was probably related to an abnormally high rate of organic carbon deposition which generated an abnormally high rate of o2 production we estimate that the total excess o2 produced during the excursion was between 12 and 22 times the present atmospheric o2 inventory the 13c data therefore suggest that the o2 content of the atmosphere increased very significantly between 2 22 and 2 06 ga this inference is supported strongly by several other lines of evidence

overview of the meso to neoproterozoic evolution of the vindhyan basin central india abstract despite its origin in a stable intracratonic basin the vindhyan supergroup central india embodies tectonic driven depositional cycles of various orders a marked change in sedimentation pattern was coupled with a transient plate margin compression in the otherwise extensional regime an unconformity laterally correlatable with a conformity that divides the supergroup in two sequences arose in consequence maximum flooding surfaces mfs within the sequences can generally be correlated with intrabasinal volcanism that drastically diminished through the initial rift to later sag stage of basin evolution concomitantly with the change in tectonic milieu paleoseismic deformation structures and related deposits also became drastically reduced in scale nevertheless metre and decametre scale depositional cycles correlate well with the subtle record of paleoseismicity intermittent tilting of the depositional substratum is demonstrable in systematic bed dip changes in metre scale fining upward depositional cycles in the rift stage in the sag stage it is reflected merely in consistent deflection of paleocurrent directions at the top of the decametre scale parasequence sets towards the top of the supergroup progressive thickening and decreasing frequency of parasequences indicates a declining rate of basin subsidence progradation despite punctuation thus resulted in basin filling

pb pb age of earliest megascopic eukaryotic alga bearing rohtas formation vindhyan supergroup india implications for precambrian atmospheric oxygen evolution abstract though considered to be the earliest known well preserved megascopic body fossil of an eukaryotic alga grypania spiralis has not yet been dated directly and reliably in any of its four known locations in the world we report a pb pb isochron age of 1599 48 ma for the grypania bearing rohtas formation of the vindhyan supergroup central india besides being about 500 myears older than the previously inferred age for this formation this pb pb age together with close age constraints on the same fossil bearing horizons in jixian china and montana usa indicates that grypania bloomed on a global scale between 1 6 and 1 5 ga being an obligate aerobe the appearance of a much older grypania within one of the principal bif deposits near michigan usa has been linked to a significant jump in atmospheric oxygen between 2200 and 1900 ma inferred from geochemical evidence the long gap of 400 myears between the older and younger grypania occurrences could be due to reduction of atmospheric oxygen below the grypania survival threshold as a result of extensive bifs between about 1950 and 1850 ma

the miocene pliocene macusani volcanics se peru i mineralogy and magmatic evolution of a two mica aluminosilicate bearing ignimbrite suite

two mica rhyolitic tephra in the east pisco basin peru new age and dispersion constraints for the eruptions of the eastern cordillera of central andes wo mica biotite and muscovite volcanics are particularly rare in the geological record one of the several dozen volcanic ash layers from central andes volcanoes found in the upper miocene marine succession of the pisco formation ica desert peru contains juvenile biotite and muscovite sillimanite andalusite feldspars and rhyolitic glass 39ar 40ar dating on biotite and muscovite concordantly constrain an age of 7 96 ma for this two mica ash layer a second tephra in the pisco formation has a similar biotite composition and an age between 7 45 and 6 93 ma the peculiar mineral assemblage and the chemical composition of biotite indicate a strongly peraluminous composition of the erupted magmas and together with the 39ar 40ar ages suggest to consider a correlation of these ash layers to the eruptions of the miocene macusani peru or morococala bolivia volcanic complexes in the eastern cordillera of central andes the major and trace element composition of glass supports the correlation with macusani a provenance from morococala seems less likely given the large distances involved these results provide new data on the volcanic activity of the eastern cordillera revealing ash that dispersed to over 500 km to the west in the forearc marine basins this finding highlights that the exhumed forearc east pisco basin is highly promising as an archive of distal ash for the reconstruction of the volcanic activity of central andes during the miocene silicic flare ups

environmental control on diverse stromatolite morphologies in the 3000 myr pongola supergroup south africa a unique outcrop of partly silicified dolomite in the white umfolozi section of the pongola supergroup south africa indicates that stromatolites were diverse and adapted to a range of shallow tidal depositional settings 3000 myr ago composite columnar stromatolitic bioherms 0 7 1 6m high and 0 4 1 0m in diameter formed along the margins of a tidal channel they were flanked away from the channel by flat stratiform and small domical stromatolites growing in low energy tidal flat environments conical stromatolites 0 05 0 30m high and 0 03 0 10m in diameter accreted in high energy coarse grained carbonate sand along the bottom of the tidal channel the stromatolites probably formed through the activities of filamentous oxygen producing photoautotrophic cyanobacteria

facies architecture and sequence development in a neoproterozoic carbonate ramp lakheri limestone member vindhyan supergroup central india abstract outcrop based study of high resolution sequence stratigraphy intra and interfacies sedimentologic clues in neoproterozoic lakheri limestone central india reveals definite signals for relative sea level variations and paleogeographic shifts between shallow beach shoreface and distal shelf depositional cycles in this carbonate formation are bound by unconformities or disconformities of ravinement origin on outcrop scale the unconformities are sharp and irregular and marked by definite depletion in 13 c values of limestones in the study area the entire lakheri limestone succession is divided into four depositional sequences bound by type i unconformities different variants of system tracts transgressive highstand and lowstand resulting from differences in paleogeographic locations basin floor physiography and variable rates of relative sea level change constitute the sequences developed on lakheri carbonate ramp a possible forcing of intrabasinal tectonics both extension and compression on meter to tens of meter thick lakheri sequences is inferred facies impersistence both along and across depositional strike of lakheri limestone is an additional complexity in depositional architecture of carbonate ramp sequence this possibly resulted from formation of depositional slope breaks through tectonic or depositional factors three dimensional reconstruction of lowstand wedge brings out the control of rugged physiography of the basin floor imparted through patchy biohermal growth on facies distribution in this neoproterozoic ramp succession

epidote phenocrysts in dacitic dikes boulder county colorado epidote ps21 crystallized early as elongate phenocrysts in late cretaceous rhyodacitic dikes in the vicinity of ward boulder county colorado other unusual phenocryst phases are garnet gr17 24 and muscovite in a xenolith containing kyanite corundum biotite and plagioclase magmatic garnet grew as a rim around xenocrystic pyrope rich py37 garnet the xenolith was derived from a granulite facies zone not represented at the present day erosion surface which is composed of upper amphibolite facies cordierite and sillimanite bearing gneisses the dike magmas were fed not from an immediately underlying batholith but from a magma chamber at a depth corresponding to a pressure of 8 13 kilobars phenocrysts cystallized in the temperature range 800 to 700 c under h2o and o2 activities greater than normal for silicic magmas this occurrence shows convincingly not only that epidote can be magmatic but that it is a possible early crystallization phase in silicic magmas

carbon and oxygen isotopic variations in peritidal stromatolite cycles paleoproterozoic kajrahat limestone vindhyan basin of central india abstract variations of carbon and oxygen isotopic ratios in response to cyclical sea level fluctuations have been documented from a paleoproterozoic peritidal stromatolite succession the upper division of the kajrahat limestone vindhyan supergroup of central india consists of several shallowing upward stromatolite cycles identified by regular and systematic changes in stromatolite size normally larger stromatolites are followed upward in the succession by smaller stromatolites and microbial laminites that occupy the top of the cycle desiccation cracks are found in all the facies indicating subaerial exposure we investigated the stable isotope compositional variations across nine complete stromatolite cycles showing frequent subaerial emergence carbon and oxygen isotopic values of the limestones in general are comparable to contemporary marine values available from earlier studies but show regular depletion in response to shallowing of the water level the 13 c and 18 o values of the limestones vary within an individual stromatolite cycle depleted values characterize the topmost part of the cycles the isotope pattern is explained by micritic carbonate deposition in different sub environments of the shallow marine domain having different salinity and variable duration of exposure these variations also probably caused the observed scatter in 13 c and 18 o values of supratidal microbial laminites

c o sr and pb isotope systematics of carbonate sequences of the vindhyan supergroup india age diagenesis correlations and implications for global events abstract the vindhyan supergroup of central india records a substantial portion of proterozoic time and contains some of the most disputed precambrian cambrian fossil discoveries despite the significance of these fossil finds many issues including reliable geochronology remained unresolved similarly the geochemical aspects of the sedimentary sequences and their implications to the evolution of proterozoic oceans have been studied only at a reconnaissance level here we report the results of the pb pb dating for the rohtasgarh limestone from the lower vindhyan supergroup giving an age of 1601 130 ma an estimate that is in accord with the recent u pb zircon dating of the lower vindhyan supergroup by rasmussen et al geology 30 2002 103 and ray et al geology 30 2002 131 sr isotope stratigraphy of carbonates from the upper vindhyan supergroup suggests a mid neoproterozoic 750 650 ma age for these formations this in turn argues for a long hiatus between the deposition of the lower and upper vindhyan supergroups our results conflict with the reports of cambrian age small shelly fossils and ediacaran fossils from the upper vindhyan rocks at the same time they suggest that the alleged trace fossils in the lower vindhyan rocks must be of paleoproterozoic age the most common 18 o values for calcitic components from limestone formations of the supergroup range from 10 to 7 similar to paleo neoproterozoic carbonates worldwide the modes of 13 c in the lower and upper vindhyan supergroups are 0 2 and 3 2 respectively again consistent with the worldwide dataset the secular patterns of carbon isotope trends do not support the earlier assumptions that the carbonate sequences at the southern margin correlate with those at the western or northern margins of the vindhyan basin c and sr isotope data provide new calibration points for the seawater evolutionary curves particularly for the paleoproterozoic for strontium isotopes the data from the lower vindhyan supergroup suggest that seawater departed probably less rapidly from mantle values than was previously believed

rare element enriched s type ash flow tuffs containing phenocrysts of muscovite andalusite and sillimanite southeastern peru ash flow tuffs of neogene age exposed over 2 500 km 2 in the macusani region of southeastern peru are the volcanic equivalent of s type granites the strongly peraluminous tuffs contain phenocrysts of andalusite sillimanite and muscovite and have high 87 sr 86 sr i 0 7258 and 0 7226 and 18 o 11 elevated concentrations of li cs be sn b and other minor elements compare with those in tin granites mineral phase relations and composition are indicative of low magmatic temperatures and oxygen fugacities and high a hf a h 2 o the chemical isotopic and mineralogical features and regional geologic relations are consistent with models of magma generation involving the incorporation of large amounts of pelitic rock

controls on the mineralogy of s type volcanic and plutonic rocks abstract far from being simply two mica granites s type rocks contain highly diverse mineral assemblages the most important chemical factors influencing the mineralogy of an s type rock are peraluminosity bulk feo mgo mg mg fe k na ca and the fugacities of volatile species such as h2o hf h3bo3 and o2 however it is not just initial melt chemistry that determines the mineral assemblage the pressure and temperature of crystallization exert major effects as magmas evolve the p t conditions volatile fugacities and melt chemistry change often leading to changes in stable mineralogy many s type granites and silicic volcanic rocks contain partly or completely re equilibrated mineral assemblages recognition of these may provide important information about early crystallization conditions and the nature of the magma source terrains

slope controlled seismic deformation and tectonic framework of deposition koldaha shale india abstract penecontemporaneous deformations with remarkable lateral continuity at selected stratigraphic levels record frequent seismicity in the meso proterozoic koldaha shale vindhyan supergroup central india their consistent bipolar orientation reflects the master control of basinal tectonics boudinage small scale conjugate faults and bidirectional joint planes indicate east northeast west southwest basinal extension detailed study of a 27 km strike parallel stretch between chorhat and shikarganj against a broader perspective reveals existence of multiple nw se elongated subbasins stratigraphic and sedimentologic criteria point to inheritance and subsequent sustenance of steep western and gentler eastern flanks of the sub basins it is also indicated that the perennial supply of sediment took place from the east while the western slope contributed only during tectonic seiches the relatively large scale faults are correlatable with the steeper and the smaller deformation structures to the gentler flank the nnw oriented half graben structures that were reactivated time and again is elicited previous geophysical studies inferred roughly e w elongated rift valley in the granitic basement and the present observations indicate that the rifting continued at least up to the time of koldaha shale deposition the half graben could then be guided by cross faults resulted from dextral shear it is imperative then the basin interior sedimentation and deformation patterns were more closely controlled by the cross faults rather than by the basin margin faults only a set of slump folds in a 10 m thick interval indicates that the northern margin of the main basin was relatively steeper this is further corroborated by concentration of large fans in mineralogically immature sediments and southerly paleocurrent at the northern fringe of the basin in contrast the vindhyan sediments commonly belong to orthoquartzite carbonate association a regional northerly slope of the vindhyan basin floor towards the relatively steeper flank provides an explanation for the general northwesterly paleocurrent even though the eastern flank of the sub basins sloped wsw this integrated study of the physical aspects of the sedimentary pile against the background information of basement tectonics recognize rifting with a dextral shear component as the overriding factor controlling the early part of evolution of the vindhyan basin

glass rich cordierite biotite rhyodacite valle ninahuisa puno se peru petrological evidence for hybridization of lachlan s type and potassic mafic magmas

geochemistry of precambrian carbonates vii belt supergroup montana and idaho usa carbonates from the 1100 1450 ma old proterozoic belt supergroup were collected from stratigraphic sections throughout montana and idaho usa the sampled sequences in ascending stratigraphic order include the newland altyn spokane greyson transition empire wallace helena siyeh snowslip shepard and libby formations an increase in the degree of postdepositional alteration of belt limestones is reflected in a diminution of sr and mg contents an increase in mn and depletion in 13c and 18o two diagenetic trends can be resolved for the limestones one affecting the presumed originally aragonite rich sediments includes carbonates from the lower belt newland formation in contrast the middle belt carbonate wallace helena siyeh formations may have been originally of high mg calcitic mineralogy projection of the alteration trends for the lower and middle belt limestones suggest 21 smow and 2 5 to 1 0 pdb as the best preserved values for the 18o and 13c of seawater respectively both comparable to results from other mesoproterozoic carbonate sequences the oxygen isotope data for limestones show a regional westward depletion of 8 in 18o possibly reflecting a higher temperature of postdepositional alteration in the western belt basin this depletion in 18o is accompanied by a comparable decrease in 13c values most likely because a higher proportion of carbon was incorporated from co2 generated by thermal cracking of hydrocarbons at depth dolostones in the belt basin are dominantly micritic with good preservation of depositional textures chemically and isotopically their alteration trends mimic those of limestones leading to comparable projected best values for 13c 87sr 86sr values of belt carbonates range between 0 70484 and 0 74991 progressive diagenesis as indexed by decreasing concentrations of sr and depletions in 18o and 13c results in an increase in 87sr 86sr values the least radiogenic measurement from a lower belt newland limestone sample appears to fit reasonably well into the general trend for proterozoic seawater as discussed in mirota and veizer 1994 this and the consistency of 18o and 13c in limestones with other coeval sequences suggests that the lower and middle belt carbonate sections are marine deposited in an environment that was not continuously separated from the open ocean

subaerial exposure and deposition of shallowing upward sequences evidence from stable isotopes of purbeckian peritidal carbonates basal cretaceous swiss and french jura mountains purbeckian lowermost cretaceous peritidal carbonates are characterized by open marine lagoonal intertidal and lacustrine facies arranged in milankovitch type shallowing upward sequences shallowing upward sequences typically consist of 2 6 individual beds the sequences may be i complete ii incomplete or ii pedogenetically overprinted reflecting the duration of subaerial exposure and or the extent of erosion and pedogenetic modification at the cycle tops the stable isotopic composition of the peritidal micrites reveals homogenous 6 o values attributed to diagenetic stabilization in a meteoric water buffered system carbon isotopes show three distinctly different carbon isotope patterns dependent on the completeness of the shallowing upward sequences complete shallowing upward sequences consist of l6 individual carbonate beds the carbon isotope values show a facies dependent pattern open marine carbonate muds record enriched 6i3c values of 0 28 while lagoonal 0 82 intertidal 2 46 10 and lacustrine micrites 2 96 are increasingly depleted this distinct pattern is explained by carbonate mud deposition in environments of differing salinity and marine influence incomplete sequences 2 5 carbonate beds are characterized by depleted 6 c values below subaerial exposure surfaces that become progressively enriched in c with increasing depth pedogenetically overprinted sequences 1 3 carbonate beds show strong i3c depletion throughout the sequence with little variation in the carbon isotopic composition the depleted values 4 51 10 of the pedogenetically altered micrites suggest that modification during subaerial exposure was associated with equilibration with meteoric solutions enriched in isotopically light soil gas coz the duration of subaerial exposure is the most crucial factor determining the extent of pedogenetic alterations the completeness of the shallowing upward sequences and the carbon isotope pattern the recorded patterns clearly illustrate that micrites have a good potential for the preservation of their primary carbon isotopic composition if the duration of subaerial exposure is rather brief otherwise the recorded carbon isotope patterns may support sequence stratigraphic analysis by providing a refinement of the time stratigraphic interpretation

geological map of central and western sri lanka provisional series 1997 scale 1 250 000 and seven 1 100 000 map sheets geological survey and mines bureau sri lanka and the british geological survey us 10 00 postage

distribution and geometry of facies and early diagenesis the key to accommodation space variation and sequence stratigraphy upper cretaceous congost carbonate platform spanish pyrenees abstract through only moderate structural deformation the upper cretaceous carbonates of the tremp region in the spanish pyrenees can be studied in the form of a platform to basin cross section comparable in scale to that of a seismic section this study examines the upper turonian to mid coniacian congost platform and its associated basinal succession the sedimentology facies geometries and cycle stacking patterns along with evidence of exposure of the platform are interpreted in terms of fluctuations in relative sea level and variations in accommodation space differences in the facies and early diagenesis of carbonates from different parts of the congost platform suggest two separate phases of platform development an earlier more land ward platform was dominated by clinoforms prograding on to a sloping basin floor with a thin lagoonal sand bank facies upon the platform itself supplying abundant excess sediment to the clinoforms a later more basin ward platform has a more complex internal geometry of depositional units and marginal rudist coral buildups the interpretations are used to propose a new sequence stratigraphic architecture for the congost platform involving two sequences the second produced through dynamic forced regression sequence boundaries are characterised by subaerial exposure on the platform top that can be correlated with hardgrounds and or glauconitic horizons within the more basinal locations overlain by deeper water facies the presence of a submarine onlap surface and down slope slide deposits immediately above the sequence boundary suggests that the major transgressive event that followed sequence boundary development was brought about by local extensional tectonic activity the small scale cyclicity within the sequences and the major falls in sea level that produced the sequence boundaries are interpreted to have resulted largely from eustatic processes probably with tectonic enhancement

sedimentation rates basin analysis and regional correlations of three neoarchaean and palaeoproterozoic sub basins of the kaapvaal craton as inferred from precise u pb zircon ages from volcaniclastic sediments abstract calculation of sedimentation rates of neoarchaean and palaeoproterozoic siliciclastic and chemical sediments covering the kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents zircons from tuff beds in carbonate facies of the campbellrand subgroup in the ghaap plateau region of the griqualand west basin transvaal supergroup south africa were dated using the perth consortium sensitive high resolution ion microprobe ii shrimp ii dates of 2588 6 ma and 2549 7 ma for the middle and the upper part of the nauga formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below wave base stratifera and clastic carbonate facies southwest of the ghaap plateau at prieska was of up to 10 m ma when not corrected for times of erosion and non deposition dates of 2516 4 ma for the upper gamohaan formation and 2555 19 for the upper monteville formation indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 ma to 62 ma on the ghaap plateau for these predominantly peritidal stromatolitic carbonates decompacted sedimentation rates were of 40 m ma to over 150 m ma bubnoff units the mixed siliciclastic and carbonate shelf facies of the schmidtsdrif subgroup and monteville formation accumulated with decompacted sedimentation rates of around 20 b for the kuruman banded iron formation a decompacted sedimentation rate of up to 60 b can be calculated thus for the entire examined deep shelf to tidal facies range archaean and phanerozoic chemical and clastic sedimentation rates are comparable four major transgressive phases over the kaapvaal craton followed by shallowing upward sedimentation can be recognized in the prieska and ghaap plateau sub basins in griqualand west and partly also in the transvaal basin and are attributed to second order cycles of crustal evolution first order cycles of duration longer than 50 ma can also be identified the calculated sedimentation rates reflect the rate of subsidence of a rift related basin and can be ascribed to tectonic and thermal subsidence comparison of the calculated sedimentation rates to published data from other archaean and proterozoic basins allows discussion of general precambrian basin development siliciclastic and carbonate sedimentation rates of archaean and palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical chemical and biological processes were active in the precambrian as found for the phanerozoic particularly for stromatolitic carbonates matching modern and neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite building microbiota the new data also allow for improved regional correlations across the griqualand west basin and with the malmani subgroup carbonates in the transvaal basin the nauga formation carbonates in the southwest of the griqualand west basin are significantly older than the gamohaan formation in the ghaap plateau region of this basin but are in part correlatives of the oaktree formation in the transvaal and of parts of the monteville formation on the ghaap plateau

experimental phase equilibria in the system lialsio4 sio2 h2o a petrogenetic grid for lithiumx rich pegmatites phase relations for the bulk composition lia1sis01z system lia1si04 sioz hzo have been investigated experimentally over the range 340 950 c and 0 5 6 0 kbar p hzo stability relations among eucryptite spodumene petalite b spodumene tetragonal and virgilite hexagonal have been determined for quartz saturated subsolidus conditions the result is a quantitative p t phase diagram that is applicable to quartz saturated lithium aluminosilicate assemblages in pegmatites and some volcanic rocks thermochemical data derived from these experimental results are internally consistent and in close agreement with values obtained from calorimetry under the quartz saturated conditions that prevail in pegmatites stability relations among the lithium aluminosilicates are a function of p and t and are largely independent of the nature and proportions of other phases in the chemically complex pegmatite system thus the lithium aluminosilicate phase diagram constitutes a petrogenetic grid from which p t conditions of emplacement and crystallization can be ascertained for many lithium alumino silicate bearing pegmatites

the role of collisional tectonics in the metallogeny of the central andean tin belt abstract the inner arc of the central andes broadly corresponding to the eastern cordillera is the location of a rich tertiary and triassic sn w ag base metal metallogenic province commonly referred to as the bolivian tin belt we propose that the tertiary metallogeny which generated most of the tin ores was a direct consequence of discrete collisions between the south american plate and the nazca slab and sub slab mantle during the ongoing andean orogeny evidence supporting this proposal include 1 the coincidence of the tin province and the inner arc in a marked hump in the andean orogen which may represent tectonic indentation 2 the symmetry of the tin province with respect to the bolivian orocline the axis of which corresponds to the direction of highest compression 3 the relative symmetry of the magmatism and tin mineralization with respect to this axis 4 the concurrent timing of mineralization and compressional pulses 5 the similar host rock geochemistry and ore lead isotope data testifying to a common crustal reservoir and 6 the striking similarity of the igneous suites associated with the ore deposits to those from typical collisional orogens a number of studies have called upon a persistent tin anomaly to explain the metallogeny of the region we propose instead that the latter is better explained by periodic compressional interaction between the farallon nazca oceanic plate and the south american continent this led to the generation of peraluminous magmas which during fractional crystallization exsolved the fluids responsible for the voluminous sn w mineralization

beryllium in silicic magmas and the origin of beryl bearing pegmatites this chapter addresses the portion of the geochemical cycle of beryllium that entails its transfer through the continental crust by the generation and migration of silicic magmas which will be referred to loosely as granitic but will include all of the textural variants of rocks derived from the granitic bulk compositions within the granite system we will examine the distribution of be between anatectic melts and residual crystals and we will track the accumulation of be in melt via crystal fractionation culminating in the saturation of pegmatite forming melts in the mineral beryl beryl is unquestionably the most common manifestation of be enrichment and the most abundant of the phases that contain essential beryllium see cern this volume to explain the presence of beryl in pegmatites we pose four questions 1 how much be does it take to saturate a granitic pegmatite forming melt in beryl 2 how much be is likely to be incorporated in silicic partial melts at their source 3 consequently how much fractionation of a typical granitic magma must occur to achieve beryl saturation in pegmatitic derivatives or is beryl saturation possible without extended magmatic fractionation 4 and should all granitic magmas be expected to achieve beryl saturation eventually or are there special attributes of the source material or crystalline phases in the magma that predetermine the likelihood of forming beryl in pegmatitic differentiates to address these questions we start with a review of beryl saturation in pegmatites we then examine the likely abundance of be derived by anatexis at the sources of granitic magmas with the two ends of the system bracketed we can then subject be fractionation to conventional modeling methods to ascertain how much fractionation must take place to bring granitic melts to beryl saturation as a review this chapter relies as much as possible on readily available publications

microbially originated wrinkle structures on sandstone and their stratigraphic context palaeoproterozoic koldaha shale central india abstract wrinkle structures on sandstone beds are abundant and present in diverse forms in the palaeoproterozoic koldaha shale vindhyan basin central india the inferred palaeogeography of sedimentation ranges from offshore marine to terrestrial the wrinkled sandstones belonging to the relatively deeper marine part good preservation of ultramicroscopic microbial elements within these sandstones strongly supports a microbial mat origin of the wrinkles which was facilitated by development of an oxygen depleted condition immediately beneath the sediment water interface as depicted in early pyrite growth later impermeable shale encasing the sandstone beds arrested further scope for destruction of the microbial elements trapping of fine detritus by microbial filaments is documented by the invariable occurrence of a silty clay veneer on the wrinkled surfaces and by its characteristic texture wrinkle morphology has been classified and an attempt has been made to understand the origin of each of the varieties and to visualize the processes that acted upon the microbial mats to produce the wrinkle structures in their specific palaeogeographic context the general palaeogeographic bias in occurrence of the sandy wrinkles is well reflected in their concentration in the lower part of the highstand systems tracts hst s one superposed above the other comprising the koldaha shale the smaller scale palaeogeographic control is indicated by compartmentalization of two wrinkle varieties in the upper and the basal intervals of the hst s and overlapping occurrence of other varieties in the medial interval with apparent preference for either the upper or the lower levels

commingling and mixing of s type peraluminous ultrapotassic and basaltic magmas in the cayconi volcanic field cordillera de carabaya se peru abstract the cayconi district of the cordillera de carabaya se peru exposes a remnant of an upper oligocene lower miocene 22 2 24 4 ma volcanic field comprising a diverse assemblage of s type silicic and calc alkaline basaltic to andesitic flows members of the picotani group of the central andean inner arc basaltic flows containing olivine plagioclase clinopyroxene ilmenite and glass and glassy rhyolitic agglutinates with phenocrystic quartz cordierite plagioclase sanidine ilmenite and apatite respectively exhibit mineralogical and geochemical features characteristic of medium k mafic and lachlan s type silicic lavas cordierite bearing dacitic agglomerates and lavas however are characterized by dispersed melanocratic micro enclaves and phenocrysts set in a fine grained quartzo feldspathic matrix they contain a bimodal mica population comprising phlogopite and biotite as well as complexly zoned sieve textured plagioclase grains sector zoned cordierite sanidine quartz irregular patches of replaced olivine clinopyroxene and orthopyroxene and accessory phases including zircon monazite ilmenite and chromite the coexistence of minerals not in mutual equilibrium and the growth dissolution textures exhibited by plagioclase are features indicative of magmatic commingling and mixing trachytic textured andesite flows interlayered with olivine plagioclase glomerophyric calc alkaline basalts have a phenocrystic assemblage of resorbed orthopyroxene and plagioclase and exhibit melanocratic groundmass patches of microphenocrystic phlogopite ca rich sanidine ilmenite and aluminous spinel the mineralogical and mineral chemical relationships in both the dacites and the trachytic textured andesites imply subvolcanic mixing between distinct ultrapotassic mafic melts not represented by exposed rock types and both the s type silicic and calc alkaline mafic magmas such mixing relationships are commonly observed in the oligo miocene rocks of the cordillera de carabaya suggesting that the s type rocks in this area and by extension elsewhere derive their unusually high k 2 o ba sr cr and ni concentrations from commingling and mixing with diverse mantle derived potassic mafic magmas

carbon isotopes as biogeochemical recorders of life over 3 8 ga of earth history evolution of a concept ever since pioneering studies in the late 1930s had shown that the conversion of inorganic carbon into biogenic substances entails sizeable redistributions of the stable carbon isotopes biologically mediated 13c 12c fractionations have come to be recognized as a common corollary of biochemical reactions meanwhile it is firmly established that the universal bias in favour of 12c characterizing biological materials primarily derives from a kinetic isotope effect that is imposed on the first carbon fixing enzymatic carboxylation reaction in the primary metabolism of co2 fixing autotrophic organisms this preference for 12c has turned out to be one of the most enduring relics of the ordered state of the biological precursor substances that may be preserved in fossil organics over billions of years with the currently known sedimentary record at hand it can be stated with confidence that biological carbon isotope fractionations have persisted throughout 3 8 ga of recorded earth history indicating that microbial prokaryotic and archaeoprokaryotic ecosystems had been prolific already on the archean earth while for the time span 3 5 ga the isotopic evidence is unequivocal the information encoded in the preceding record is commonly blurred by a metamorphic overprint this holds particularly for the metasediments of the 3 8 ga old isua supracrustal belt of west greenland which apart from widespread metasomatism have suffered amphibolite grade metamorphism it is known that 13c 12c exchange occurs in organic kerogenous rock constituents during both amphibolite and granulite facies metamorphism if a second carbon partner is available as either fluids or carbonate with isotopic re equilibration often only partially achieved due to the sluggish kinetics of the exchange reaction thermodynamic equilibria predict however that 13c 12c ratios in kerogen and graphite increase during this process hence high t exchange equilibria are always bound to drive 13c values in positive direction the lowermost values encountered being consequently the least exchanged and most pristine with the lowest values of reduced graphitic carbon obtained in early isua studies falling into the range 22 to 28 pdb we had straightforward evidence since the late 1970s that carbon constituents with the isotopic composition of biogenic matter were indeed present in the pre metamorphic isua suite it was therefore by no means surprising that the results of recent isotope work performed on apatite hosted carbonaceous microdomains in isua banded iron formation utilizing advanced techniques of instrumental microanalysis had prompted similar conclusions hence the mainstream of the sedimentary carbon isotope record can be best interpreted as the geochemical manifestation of the isotope discriminating properties of the principal co2 fixing reactions s in biological carbon assimilation suggesting an extreme degree of evolutionary conservatism in the biochemistry of autotrophic carbon fixation as a consequence biological modulation of the geochemical carbon cycle had been established at least 3 8 ga ago having been fully operative by the time of formation of the earth s oldest sediments

geochemistry of precambrian carbonates v late paleoproterozoic seawater in an effort to obtain better constrained data for the first and second order variations in the isotopic composition of late paleoproterozoic 1 8 0 2 ga seawater we report a study of mineralogy chemistry and isotopic composition of the coronation supergroup 1 9 ga nwt canada and the mcarthur group 1 65 ga nt australia petrologically both carbonate sequences are mostly dolostones the mcarthur population however contains more abundant textural features that attest to the former presence of sulfates and halite and the facies investigated herein probably represent ancient equivalents of modern evaporitic sabkhas and lacustrine playa lakes this is reflected in 18 o enrichment up to 2 9 pdb and high mn fe and sr concentrations nonetheless the available 18 o data indicate that the best estimate for the archean to late paleoproterozoic limestones is about 7 pdb with dolostones some 3 2 heavier the observed 18 o doiomitc calcite is thus within the range of the proposed fractionation factor for a cogenetic mineral pair this together with the consistency of isotopic patterns for sedimentological facies over almost 2 billion years from the late paleoproterozoic to the present suggest that dolomitization was an early diagenetic event and that the 18 o depletion of the archean to late paleoproterozoic carbonates is not an artefact ofpost depositional alteration the 13 c although within the range of other precambrian and phanerozoic carbonates indicates a possible secular evolution from 0 2 pdb at 1 9 ga ago to 1 1 5 at 1 65 ga ago the secular evolution for reflects the 87 sr increase from 0 7031 0 0008 at 3 5 0 1 ga ago through 0 7025 0 0015 at 2 8 0 2 ga 0 7033 at 2 5 ga 0 7050 at 2 25 0 25 ga 0 7047 at 1 91 to 0 706 at 1 65 ga ago

the geometry and petrogenesis of dolomite hydrocarbon reservoirs the wide distribution of dolomite rocks in north american middle and far eastern hydrocarbon reservoirs is reason enough for their intensive study in this volume dolomite enthusiasts review progress and define the current boundaries of dolomite research related particularly to the importance of these rocks as reservoirs as might be expected reviews provide critical analyses of past ideas on origins and why many are no longer tenable discussions range between conceptual models and numerical simulations of the flow paths of dolomitizing fluids geochemical modelling and microbial mediation case studies from areas as diverse as north america the middle east se asia and australia provide a solid background but illustrate important differences in views on the origins of these enigmatic rocks