geological and anthropogenic influences on sediment metal composition in the upper paracatu river basin brazil this work reports a geochemical study of sediments from the upper paracatu river basin the objective is to define the influences of au zn and pb mineral deposits and mining activities on the sediment metal sources distribution and accretion the samples were analyzed using icp oes aas and xrd techniques and were treated with principal components analysis and the geo accumulation index the main geochemical processes that control the sediment composition are pyrite oxidation muscovite weathering carbonate dissolution and the erosion of oxisols enriched with zn and pb the upper rico stream has high al fe cu cr co and mn concentrations due erosion of oxisols and pyrite oxidation and muscovite alteration present in the parental rock the artisanal alluvial gold mining increased the primary rock minerals weathering and hg sediment concentration the lower escuro river and santa catarina stream are enriched with zn and pb due the erosion of metal rich soils formed over galena sphalerite calamine and willemite mineral deposits located upstream elements such as ca mg and ba have low concentrations throughout the sampled area due the high solubility of these metals bearing minerals the dispersion of metals is limited by the basin geomorphology and their affinity to silt clayey minerals and fe and mn oxides and hydroxides in circumneutral ph waters

climate change during the early paleogene in the bogota basin colombia inferred from paleosol carbon isotope stratigraphy major oxides and environmental magnetism abstract paleosols in the bogota basin colombia record an increase in chemical weathering across the paleocene eocene p e transition rock magnetic properties and major element geochemistry exhibit an abrupt change in an interval identified as including the p e boundary as established by previously published biostratigraphy and a u pb date on volcanic zircons 56 2 1 6 ma during the stratigraphic interval that contains the p e transition magnetic susceptibility increases significantly with an order of magnitude higher concentration of magnetite maghemite and hematite the preponderance of pure stoichiometric magnetite in the paleosols of this interval indicates that the increase in magnetic susceptibility is due to changes in the rate of pedogenesis rather than an increase in the erosion of nearby volcanic rocks which would contribute titanomagnetite pedogenic structures preserved within the paleosols a lack of iron oxides as cement friability of the sandstones and previously published thermochronologic data are not consistent with burial diagenesis as an explanation for the origin of the magnetic mineral assemblage these enhanced pedogenesis and related chemical weathering is also confirmed by increases in the concentration of fe 2 o 3 and al 2 o 3 and loss on ignition values an increase in mean precipitation across this interval is inferred by the lack of carbonate nodules and a decrease in sio 2 in paleosols thus we hypothesize that there was an intensification of chemical weathering during the interval that contains the p e transition

k feldspar megacrysts in granites phenocrysts not porphyroblasts abstract k feldspar megacrysts in granitoid plutons have been interpreted as either phenocrysts or porphyroblasts most of the microstructural mineralogical and chemical evidence e g shape alignment concentration ba content zoning inclusions and twinning favours a phenocryst origin the main features that have been used to support a porphyroblast origin are occurrence of megacrysts 1 across aplite vein boundaries 2 in country rocks and 3 in or across boundaries of microgranitoid enclaves mafic inclusions however these features can be explained by the phenocryst hypothesis in particular megacrysts in microgranitoid enclaves can be explained by growth or mixing in magma before a globule of that magma or a fragment of the resulting igneous rock was incorporated as an enclave all available evidence favours or is consistent with a phenocryst origin for k feldspar megacrysts in granitoid rocks and their enclaves the large size of the megacrysts is evidently due to nucleation difficulties for k feldspar in granitic melts though k feldspar is commonly the last mineral to begin crystallizing in granitic magmas abundant melt is still present at that stage allowing sufficient space for the megacrysts to grow the reason for the common lack of megacrysts in volcanic rocks may be that the phenocrysts do not grow large enough to be called megacrysts until the magma contains such a high proportion of crystals that it cannot erupt

the yarra granite an intradeformational pluton associated with ductile thrusting lachlan fold belt southeastern australia the yarra granite is a deformed s type pluton on the eastern edge of the wyangala batholith intruded into deformed early paleozoic metasedimentary rocks detailed structural studies of the pluton include the character and geometry of the foliation and its history of development in both the pluton and wall rock the relationship of the foliation to the pluton wall rock boundary and microstructural relationships in the contact aureole evaluation of the above and regional data indicate that pluton emplacement is broadly synorogenic in detail however it is post tectonic relative to an early phase and pretectonic relative to later phases of deformation and it may be more accurately classified as intradeformational structures in the wall rock have a complex history which indicates that at least three cycles of transposition of planar elements have occurred giving rise to composite structures often exhibiting largely parallel orientation and comparable morphology these multiple composite structures show a gentle to moderate westerly dip a down dip orientation of stretching lineations and rotated fold axes and consistent west over east asymmetry of various kinematic markers field and laboratory data indicate that transposition occurred in a gentle to moderately dipping zone of ductile thrusting under upper greenschist facies conditions the yarra granite was deformed while it still contained considerable magmatic heat which promoted and facilitated ductile deformation of the wall rocks and pluton during the thrusting event we speculate that movement along the zone of ductile thrusting may have attenuated the pluton at depth and possibly detached it from its source if this is so it implies that the yarra granite and possibly the eastern part and or all of the wyangala batholith may have been brought up from some depth and displaced eastward along the zone of ductile thrusting

mobilization of trace metals from caprock and formation rocks at the illinois basin decatur project demonstration site under geological carbon dioxide sequestration conditions abstract one concern for geologic co2 sequestration is the potential leakage of co2 or co2 saturated brine containing trace metals into overlying aquifers which poses the risk of adversely affecting underground sources of drinking water in this work rock and brine samples collected in the illinois basin decatur project ibdp a large scale geological co2 sequestration demonstration project in decatur illinois usa as well as synthetic brine samples were used to understand the mobilization of trace metals as a result of co2 rock brine interactions under ibdp specific conditions 50 c and 20 6 mpa rock sample characterization indicated that at the ibdp site trace metal concentrations were greatest in cap rock samples from the eau claire formation compared with those from formations above and below this formation the natural brine samples collected from the mt simon sandstone reservoir were highly reducing and saline with trace metal concentrations up to 680 times greater than the u s environmental protection agency prescribed drinking water standards batch leaching experiments indicated that both trace metal mobilization from rock and immobilization from the brine occurred when high pressure co2 was introduced into the rock brine system the amounts of metals mobilized from the rock generally accounted for

geology petrography and mineral chemistry of the vazante non sulfide and ambrosia and fagundes sulfide rich carbonate hosted zn pb deposits minas gerais brazil the vazante paracatu region represents the most important zn district known in brazil and includes the vazante hypogene non sulfide zn deposit composed mainly of willemite zn2sio4 and sphalerite rich carbonate hosted zn pb deposits fagundes is a stratabound deposit characterized by strong silicification dolomitization and a fe rich carbonate alteration halo the main ore is represented by rhythmically banded colloform and zoned pyrite sphalerite and galena related to wall rock dissolution and sulfide infilling which probably occurs late during the burial diagenesis later veins and breccia ore types reflect epigenetic mobilization related to brittle ductile structures the ambrosia mineralization is mainly fault controlled and related to brecciated dolomite which is tectonically imbricated with black shales and slates typical features include host rock recrystallization minor silicification baroque dolomite and ankerite formation pyrite marcasite sphalerite and minor galena occur in brecciated comb veins and veinlets which overprint stylolites and tectonic fractures suggesting an epigenetic origin for the ore the vazante deposit differs from all other deposits of the district due to the presence of willemitic ore which is composed of willemite dolomite siderite quartz hematite zn rich chlorite barite franklinite and zincite the willemitic ore occurs tectonically imbricate to small sulfide ore bodies which comprises sphalerite and galena metabasites and hydrothermalized dolomites within the vazante shear zone the relationships between willemite formation and the development of mylonitic structures suggest that willemitic mineralization and deformation are synchronous episodes closely related to the vazante shear zone low zn cd ratios in sphalerite from vazante 64 to 98 and fagundes colloform 96 to 244 and zoned sphalerite 89 to 305 could reflect the regional role of mineralizing fluids with similar low zn cd ratios and low contents of reduced sulfur sred high ge up to 2200 ppm and the low fe cu mn and ag contents in late diagenetic fagundes sphalerite might suggest that this metal bearing fluid could be derived from the underlying basin fill which comprise clastic sediments and organic matter rich pelitic sequences systematic relationships among sphalerite composition and paragenetic evolution of the fagundes and ambrosia deposits suggest that progressive fluid mixing processes were important for the genesis of the sulfide rich deposits in the district these mixing processes possibly involved the hot metal bearing fluid with low contents of reduced sulfur and moderate temperature highly saline fluids which could represent an important sulfur supply the predominance of the highly saline brines in later epigenetic mineralization episodes might be related to episodic migration of hydrothermal fluids mainly derived from reduced shale sequences during the brasiliano compressive events the small variation in chemical and sulfur isotopic composition of the vazante sphalerite could imply that the high temperature metal bearing fluid with low zn cd ratios could represent a minor reservoir of reduced sulfur which permitted only subordinate sphalerite precipitation in the vazante deposit the lack of reduced shale sequences above the vazante deposit could represent a limiting factor for h2s supply additionally overall mixture between this hot sulfur deficient metal bearing fluid and meteoric fluids channeled to the vazante shear zone favor the high fo2 s2 conditions responsible for the stability of the vazante willemitic assemblage

geochemistry and spatial distribution of heavy metals in oxisols in a mineralized region of the brazilian central plateau abstract the majority of the soils of the central plateau of brazil are product of long time development over stable surfaces usually associated with erosion and re deposition cycles in this context to successfully study the geochemistry and spatial distribution of heavy metals it is essential to understand the geology as well as the geomorphology of the landscape and the properties of the metals related to their mobility the main objective of this work was to evaluate contents of naturally occurring heavy metals co cu mn ni pb and zn in some highly weathered tropical soils as related to their geochemistry geology and geomorphology additionally it was aimed to interpret the spatial distribution patterns of these metals and of major elements al fe ti and mg evaluating the association and taking the influence of the geology and geomorphology into consideration in geo referenced sites ranging from 480 to 1040 m in altitude samples were collected at two depths 0 20 and 60 80 cm ground sieved and prepared for analysis heavy metals and major element contents were determined by extraction with aqua regia descriptive statistics pearson correlation coefficients principal components analysis pca and spatial variability analyses were conducted and soil metal concentrations for the whole area were predicted by kriging interpolation a close association between pb zn and mn was observed using pca and pearson s correlation highest concentrations of pb zn and mn were determined in soils between 650 and 550 m altitude surrounding the dolomite massif hills and ambrosia and fagundes mineral deposits copper and fe were found to be geochemically associated with highest concentrations observed in soils formed from carbonaceous phyllite with quartzite layers cobalt and ni concentrations were associated with areas characterized geomorphologically as alluvial colluvial deposits at the lowest altitudes within the region although there is a marked dispersive action of tropical weathering on metal distribution in the region cu pb and zn were found spatially associated with their geological source this probably happens because of greater affinity of those metals to fe and mn oxides formed predominantly under tropical conditions and present in higher concentrations near to the sources of these heavy metals on the other hand both the low affinity of ni and co for these oxide minerals and the region s geomorphology allowed for their migration towards the drainage network to the regions of sediment accumulation at lower altitudes during the landscape s evolution

structural relationships and srnd isotope systematics of polymetamorphic granitic gneisses and granitic rocks from central rajasthan india implications for the evolution of the aravalli craton this paper reviews the state of structural and geochronological knowledge of precambrian cratonic rocks in part of central rajasthan india and contributes new observations and isotopic data on the structural features and age of granitic rocks in the area the granitic rocks comprise plutons of two suites bgc and sendra suites which intrude ortho and paragneisses the archean banded gneiss complex bgc and its inliers as well as the proterozoic delhi supergroup nearly all the plutons show a foliation and a lineation which are mostly the result of solid state deformation although a magmatic component of strain is apparent in some of them the intensity of these structural fabrics varies from weak to extremely strong samples of granitic rocks from the bgc suite yield ages from sm nd isotopic data which scatter around a 2 83 ga reference isochron nd t of 4 1 with depleted mantle model ages ranging from 2 6 to 3 0 ga scatter of the data suggests that the bgc is a composite group of gneisses and confirms the antiquity of the bgc within this part of the aravalli delhi orogenic belt rb sr isotopic data of the bgc suite lie on a 1 7 ga reference isochron indicating a thermal event at that time data from plutons of the sendra suite on the other hand show some scatter but define a model ii isochron with slope equivalent to an age of 0 966 250 ga initial 87sr86sr of 0 70390 0 01889 mswd 311 the scatter of data could reflect open system behavior during a subsequent thermal event malani volcanics although independent evidence for such is lacking in the study area ages calculated from our rb sr data are within the error of published results from previous workers structural observations on granitic rocks within the bgc inlier at ajmer suggest that the inlier could represent a remobilized diapiric ridge emplaced into the delhi supergroup during the 1 7 ga thermotectonic event contemporaneously with formation of d1 structures in the wall rock although the radiometric age of the latter structures is not well constrained microtexture mineralogy and field relations of granitic rocks of the sendra suite point to those plutons being emplaced into the delhi supergroup during the 0 966 ga thermotectonic event late syntectonic relative to d2 indicating that d2 is 0 96 1 0 ga d3 represents retrograde deformation synchronous with cooling of the regional thermal framework and local pluton cooling probably between 0 96 and 0 90 ga further geochronological geochemical and structural work on the plutons and wall rock of both suites is necessary to establish a rigorous thermotectonic chronology by which to accurately model the crustal evolution of this part of central rajasthan

sm nd evidence for 3 3 ga old rocks in rajasthan northwestern india abstract tonalitic to granodioritic gneisses from the basement gneiss of the aravalli region northwestern india define a sm nd isochron corresponding to an age of 3 31 0 07 ga and an initial nd isotopic ratio that is close to chondritic lenticular bodies of mafic amphibolites within the gneisses however define an isochron corresponding to a younger age of 2 83 0 05 ga and small positive initial nd this age is similar to that reported for a large intrusive granite the untala granite in this area and combined with field evidence suggests that mafic volcanism may have been associated with formation of the granite a highly hree enriched garnet separate from one of the tonalitic gneisses defines a two point garnet wholerock isochron age of 2 45 0 02 ga which also is similar to the age of a known thermal event in the region intrusion of the berach and bundelkhand granites at approximately 2 5 ga these results document in broad outline archean crustal formation events in the northwestern segment of the indian subcontinent and demonstrate the need for more detailed studies of the complex terranes mapped by heron 1953 as a single unit the banded gneiss complex

geochemistry and isotopic characteristics of mafic phulad ophiolite and related rocks in the delhi supergroup rajasthan india implications for rifting in the proterozoic abstract regionally metamorphosed mafic rocks phulad ophiolite in the delhi supergroup n w india have tholeiitic compositions the data indicate that there are two independent rock suites phulad barr jetgarh pbj meta basalts with element abundances and ratios resembling midocean ridge basalts morb and ranakpur desuri r d meta basalts and meta gabbros with relatively high al2o3 and low tio2 for similar mgo and high lil element rb ba sr coupled with low hfs zr nb and ree sm nd abundances the compositions of these rocks are characteristic of ocean arc basalts dioritic rocks at ranakpur exhibit low k tholeiitic differentiation trends and have close chemical affinity with the spatially associated meta basalts ranakpur diorites define a sm nd whole rock isochron corresponding to an age of 1012 78 ma with an initial nd isotopic composition that is similar to bulk earth sm nd mineral isochrons for a meta gabbro diorite and quartz diorite at ranakpur correspond to ages of 838 36 835 43 and 791 43 ma respectively these younger ages reflect metamorphic re equilibration of isotope compositions during widespread granite and granodiorite emplacement in this region during the late proterozoic model sm nd ages for the meta basalts calculated relative to undepleted mantle vary between 1300 1700 ma the chemistry of both mafic rock suites indicates derivation from source regions which had been depleted during previous melting episodes so the model ages are a maximum age constraint ranakpur desuri mafic rocks have negative initial nd 1 0 to 4 5 and low sm nd ratios suggesting crustal contamination or reworking with a few exceptions the pbj meta basalts have positive initial nd 1 5 to 4 8 indicating that they were derived from heterogeneously enriched although longterm depleted mantle the chemical data support the hypothesis that the pbj meta basalts are fragments of proterozoic ocean crust

sediment hosted lead zinc deposits of the neoproterozoic bambui group and correlative sequences sao francisco craton brazil a review and a possible metallogenic evolution model abstract the proterozoic sediment hosted zn pb sulfide and non sulfide deposits of the sao francisco craton brazil are partially syn diagenetic and epigenetic and were probably formed during extensional events the majority of the deposits occur within shallow water dolomites the pb isotopic data of sulfides are relatively homogeneous for individual deposits and plot above the upper crust evolution curve of the plumbotectonic model some of the deposits are characterized by highly radiogenic lead 206 pb 204 pb 21 originating from the highly radioactive crust of the sao francisco craton pb and s isotopic data suggest the sources of metal and sulfur for the deposits to be the basement rocks and seawater sulfates in the sediments respectively the relatively high temperatures of formation 100 to 250 c and moderate salinity 3 to 20 nacl equiv of the primary fluid inclusions in the sphalerite crystals suggest the participation of basinal mineralizing fluids in ore formation the steep paleo geothermal gradient generated by the radioactively enriched basement rocks probably assisted in heating up the circulating mineralizing fluids

response of basement and cover rocks to multiple deformations a study from the precambrian of rajasthan western india abstract metasedimentary rocks of the aravalli group of early proterozoic age 2000 2500 ma are underlain by migmatitic gneisses forming the banded gneissic complex bgc with radiometric ages varying widely from 3500 ma to much younger both the basement and the cover rocks are however involved in superposed folding of identical style and sequence in the metasedimentary cover rocks isoclinal folds of the first set f 1 with an axial planar cleavage have been affected by coaxial folds f 14 in some places and have been overprinted by a set of open upright folds f 2 with n s striking axial planes these structures are affected by small scale conjugate folds and kink bands f 3 with subhorizontal axial planes caused by a vertical compression and by upright kinks and conjugate folds f 4 developed by longitudinal shortening the f 1 f 1a and f 2 structures match in their entirety with those in the migmatites forming the basement gneisses the migmatization being synkinematic with the first folding this suggests that the bgc in its present state represents a largely mobilized basement

present status of the geochronology of the precambrian rocks of rajasthan abstract age determinations mostly by rb sr analyses of the precambrian rocks of rajasthan by us and by others are summarized and discussed broad periods of acid magmatism at 1 3000 2900 m y 2 2600 2500 m y 3 2000 1900 m y 4 1700 1500 m y and 5 850 750 m y were identified the oldest rocks in the area are the yet undated banded gneisses bgc east of udaipur intruded by the untala granite dated at 2950 m y and hence of mid archean age the basal status hitherto attributed to the berach granite dated at 2600 m y is no longer tenable the radiometric control on the beginning and duration of the overlying aravalli supergroup is not yet satisfactory though a lower limit at 2000 m y is indicated heron s original delhi rocks have recorded two magmatic events widely separate in space and time while the earliest granitic activity at 1600 m y is recorded only in the alwar basin in the northeast the younger activity between 850 750 m y is widespread as shown by the nearly concordant ages of erinpura type granites along the aravalli mountain range and the malani rhyolites in the western plains of the aravalli range

the problem of the precambrian basement in rajasthan western india abstract the banded gneissic complex bgc of rajasthan considered to form the basement underlying the precambrian proterozoic aravalli metasediments shows an erosion surface marked by a conglomerate and an angular unconformity with gneissic foliation crossing the metasedimentary bands at only a few places the bgc is a composite gneiss evolved by extensive migmatization of metasedimentary rocks of diverse composition and possibly metaigneous rocks the contact between the bgc and the aravalli rocks is a gently curved surface whereas the gneissic foliation as well as the large scale metasedimentary enclaves within the gneissic complex show all the intricate patterns of super imposed folding traceable in the aravalli rocks this implies that the basement gneisses have been involved in ductile deformation with the aravalli rocks the migmatization being synkinematic with the first deformation in the latter all these apparently conflicting lines of evidence can be resolved if the gneisses as we see them now represent not the original but the mobilized basement with the bgc aravalli boundary representing for a large part a migmatite front rather than the original basement cover interface only at a few places was there a chance of the original basement escaping mobilization and thus little chance of identifying this original interface

proterozoic tectonic evolution and metallogenesis in the aravalli delhi orogenic complex northwestern india abstract the proterozoic aravalli delhi orogenic complex hosts a large number of economically important stratabound base metal sulphide deposits a few w sn deposits associated with granites are also known in the complex based on tectono lithological features the following domains are distinguished the archaean basement consisting of the banded gneissic complex and granites the early proterozoic bhilwara aravalli jharol and north delhi belts and the middle to late proterozoic south delhi belt and the vindhyan basin intra cratonic rifting of the archaean basement commencing 2 2 ga ago resulted in the rock association of the bhilwara belt a thick pile of detritus derived largely from felsic sources and to a minor extent from tholeiitic sills with the geochemical characteristics of ocean floor basalts stratiform zn pb cu sulphide deposits at rajpura dariba rampura agucha and also possibly those at pur banera and jahazpur formed 1 8 0 04 ga ago by convective seawater circulation in zones of crustal extension the metal content of exhalative brines was precipitated in troughs where biologic activity was prolific the shelf sediments of the aravalli belt characterized by dolomites with stromatolitic phosphorites were deposited on a passive continental margin at the rifted western edge of the archaean basement complex the monotonous pelitic pile of the jharol belt represents deep pelagic continental rise sediments the rakhabdev lineament delineates the shelf rise boundary the mafic ultramafic bodies along this lineament represent aravalli oceanic crust which subducted westward and eventually obducted as a consequence of an initial collision of the aravalli continental margin with an incipient arc to the west around 1 5 ga ago in the aravalli belt at zawar strongly radiogenic stratabound pb zn deposits were formed close to the basement in second order basins with biologic activity by hydrothermal solutions convecting through a heterogeneous source the north delhi belt comprises three sedimentation domains the khetri sub basin the alwar sub basin and the lalsot bayana sub basin sedimentation in this belt commenced with shelf carbonates followed by coarse clastic sediments and volcanites finally pelites and semi pelites were deposited in a multi lagoonal shallow water locally evaporitic environment synkinematic granitic intrusives in the north delhi rocks are in the age range of 1 7 1 5 ga the stratabound cu sulphide deposits of the khetri belt and the pyrite pyrrhotite deposits at saladipura with a pb pb model age of 1 8 ga were formed from hydrothermal seawater and partly bacteriogenic sulphur in small shallow rift related basins the south delhi belt with extensive mafic volcanism localized felsic volcanism argillaceous arenaceous carbonate accumulations in successive elongated basins and conspicuous felsic plutonism is interpreted as an island arc the geochemistry of pillowed and massive metavolcanites and an ophiolite assemblage indicate subduction in the western side of the arc part of the rocks of the delhi belt may also have formed in rift related back arc basins as suggested by the ocean floor characteristics of metavolcanic rocks subduction on the western side of the arc ultimately led to its terminal collision with the aravalli continental margin around 1 0 ga involving intense progressive strain and oblique convergence small cu zn deposits were formed within sedimentary intercalations in calc alkaline basalts close to the hypothetical subduction zone extensional tectonics in the back arc basins produced stratiform zn pb cu deposits ambaji deri around 1 1 ga ago by high temperature reduction of seawater convecting through multiple sources sn w mineralizations are apparently related to the malani felsic igneous phase of 735 ma rb sr age

evolution of lower proterozoic epicontinental deposits stromatolite bearing aravalli rocks of udaipur rajasthan india abstract the aravalli rocks 2060 ma old which crop out around udaipur western india comprise a thick sequence of metasediments with stromatolites and basal volcanics resting unconformably over a peneplained basement known as the banded gneissic complex ca 2585 ma old the rocks have undergone a very low grade of metamorphism and display a complex structure resulting from two major and several minor episodes of folding there are two distinctly different facies sequences in the aravalli rocks indicating deep sea and nearshore shelf environments the stratigraphic sequence of the rocks deposited under the shelf environment starts with basic volcanics and tuffs altered to greenschists and quartizites with arkosic conglomerate in the next sequence carbonates predominate in association with orthoquartzites carbonaceous phyllites phyllites and stromatolitic rock phosphate the carbonate sequence passes upward into greywacke phyllite lithic arenite in the distal parts and conglomerate arkose orthoquartzite in proximal areas the youngest sequence comprises orthoquartzite with silty arenite the distribution of different facies particularly that of dolomite with stromatolitic rock phosphate is controlled by sea floor topography suggesting the presence of an epicontinental sea bounded by a landmass to the west and a series of islands and shoals sedimentation in the shelf and epicontinental sea was presumably triggered by development of fault controlled troughs along craton margins terrigenous debris was deposited in newly developed troughs with contemporaneous volcanicity along trough margins with the erasing of the ephemeral relief in the provenance carbonate deposition was initiated the environment encouraged algal growth and formation of stromatolitic rock phosphate carbonaceous phyllites developed in areas of restricted circulation rapid influx of terrigenous detritus with renewed tectonism in the next phase resulted in the deposition of a turbidite sequence of greywacke phyllite and lithic arenite in the deeper parts of the epicontinental sea and conglomerate arkose orthoquartizite in the marginal areas the final phase of sedimentation was presumably under fluvial conditions which marked the completion of epicontinental trough filling the nature of the terrigenous clasts indicates a predominantly granitic source of sediments supply of sediment was mainly from the continent to the east and partly from a landmass to the west the cycle of sedimentation noted in the epicontinental aravalli sea is broadly similar to the model of tectonic stages suggested by krynine 1942

age source and stratigraphic implications of pb isotope data for conformable sediment hosted base metal deposits in the proterozoic aravalli delhi orogenic belt northwestern india abstract stratiform sediment hosted base metal deposits in the aravalli delhi orogenic belt of northwestern india are hosted by the sediment dominated bhilwara aravalli and delhi sequences of proterozoic age pb isotope data for 36 samples from five ore districts provide useful stratigraphic and approximate age information model pb ages using a newly developed proterozoic model are 1800 ma for the rampura agucha rajpura dariba and saladipura deposits 1700 ma for the zawar deposits and 1100 ma for the ambaji and deri deposits mineralization in this orogenic belt thus occurred at three stages in middle to upper proterozoic times the oldest of these reflected by deposits in the bhilwara sequence was of broad regional extent the rampura agucha deposit is thus hosted by highly metamorphosed rocks that belong to the bhilwara belt and not to an older supracrustal component of the archean banded gneissic complex the host rocks to the saladipura deposit are bhilwara equivalent and are not part of the delhi supergroup as previously mapped and therefore stratigraphic assignments in the area must be re examined model ages of 1100 ma for the ambaji deri deposits are the only evidence obtained in this study regarding the age of the delhi supergroup and are in apparent conflict with much older ages indicated by previous studies rocks of such late upper proterozoic age may be restricted to the southwestern segment of the orogenic belt or alternatively thrust stacking and or interfolding of lithologically similar sequences of a wide range of ages throughout the belt may not yet be evident because of the limited age and isotopic data presently available all the data can be readily explained by the extraction of pb from sediments derived from an upper crustal basement the isotopic compositions of the zawar deposits provide the clearest evidence for an ancient u enriched upper crustal source the ambaji deri pb data suggest a mature status of the delhi are when these deposits were formed

significance of contrast in the early stages of the structural history of the delhi and the pre delhi rock groups in the proterozoic of rajasthan western india abstract structures of four generations are decipherable both in the pre delhi rocks of central rajasthan and in the delhi rocks of khetri in northeastern rajasthan and around todgarh in central rajasthan there is a remarkable identity in the later phases of the deformational history of the two groups with gravity induced structures followed by conjugate folds due to longitudinal shortening n s in northeastern rajasthan and ne sw in central rajasthan the earlier stages of the structural history of the two groups are however significantly different the e w trending reclined folds of the first generation in the pre delhi rocks are absent in the delhi rocks throughout rajasthan the nne to ne trending folds of the second generation in the pre delhi groups are upright whereas these structures in the delhi rocks are of two phases recumbent folds followed by coaxial upright folds the folds of the first and the second phases in the delhi rocks plunge gently ne or sw where they are not affected by subsequent deformations but the ne trending folds in the pre delhi rocks show an extreme variation in axial plunge from horizontal to vertical even where they are unaffected by later movements evidence has been adduced to suggest that these differences in the earlier phases of the structural evolution of the two groups are due to an angular unconformity between the delhi and the pre delhi rocks

sr and nd isotope systematics of polymetamorphic archean gneisses from southern west greenland and northern labrador sr and nd isotopic data for middle to late archean polymetamorphic felsic gneisses from localities in the nuuk area west greenland are compared and contrasted with new isotopic results for early archean amitsoq gneisses and with data for isotopically reworked kiyuktok gneisses from the saglek area labrador sr isotopic data for individual suites of felsic gneisses record the time integrated effect of variable rb sr fractionation during prograde and retrograde events as well as the effect of source inhomogeneity contrasting petrologic and sr nd isotopic characteristics are the result of differences in level of exposure caused partially by juxtaposition of terranes of different metamorphic character by movement on ductile shear zones and post shearing folding deformation sm nd systematics of felsic gneisses from nordafar ikerasakitsup akornga tinissaq and kangimut sammisoq qasigianguit define a geologically meaningless ca 3280 ma nd isochron which is the result of mixing of samples from unrela

direct dating of weathering phenomena by 40ar39ar and k ar analysis of supergene k mn oxides abstract potassium bearing manganese oxides cryptomelane k1 2 mn3 mn4 8 o16 xh2oand hollandite k ba 1 2 mn3 mn4 8 o16 xh2o are often authigenically precipitated in weathering profiles the presence of structural potassium in manganese oxides makes these minerals datable by the k ar and 40 ar 39 ar methods dating of these phases allows us to time the progression of oxidation fronts during weathering and pedogenic processes detailed age resolution of the weathering history allows us to use deep weathering profiles as land based stratigraphic records of past climatic and geomorphic events complementing evidence from the marine sedimentary and paleontological record potential problems in manganese oxide dating such as ar and or k losses excess argon 39ar loss by recoil during neutron irradiation contamination by primary minerals resistant to weathering the presence of multiple generations of manganese oxides and proportionately low radiogenic ar yield due to adsorbed atmospheric gases are addressed the k ar and 40 ar 39 ar analytical results indicate that ar and or k losses excess 40ar and 39ar recoil seem not to pose problems in manganese oxide dating this investigation suggests that fine scale laser probe 40 ar 39 ar technique is most appropriate for dating of weathering phenomena because this technique permits us to identify contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles k ar and 40 ar 39 ar dating of supergene k bearing manganese oxides formed during lateritization of archean and proterozoic bedrocks in the caraj s region amazonia brazil indicates that weathering started before 72 6 ma petrographic electron microscope and electron microprobe investigation reveal multiple generations of manganese oxide precipitation k ar and 40 ar 39 ar laser probe step heating results independently confirm the presence of these various generations of manganese oxides age clusters at 65 69 51 56 40 43 33 35 20 24 12 17 ma and zero age 0 2 0 2 ma suggest episodic precipitation of k mn oxides resulting from changing weathering conditions in the amazon throughout the cenozoic k ar and 40 ar 39 ar dating of supergene cryptomelane from weathering profiles in eastern minas gerais southeastern brazil suggests continuous weathering from 10 to 5 6 ma ago possibly reflecting local climatic conditions due to the proximity with the atlantic ocean

evidence for isotopic equilibration of smnd whole rock systems in early archaean crust of enderby land antarctica smnd isotopic data indicate that differential ree mobility occurred on a whole rock scale during transitional amphibolite to granulite facies regional metamorphism 700 c 7 kbar in early archaean rocks 3930 ma of the napier complex of enderby land antarctica the degree of mobility is independent of metamorphic grade but correlates directly with development of tectonic fabric whole rock samples with d3 m3 internal fabrics lie along an array corresponding to an age of 2410 100ma whereas samples preserving only earlier fabrics preserve an older albeit imprecisely defined isochron age in contrast to a widely held belief such changes did not require the presence of a large hydrous fluid flux if the mechanism responsible for smnd resetting at this locality where tchur ages range from 1990 ma to 6090 ma is more widespread than is currently recognised isolated smnd model ages particularly in complex terrains should be treated with caution

smnd isotopic systematics of enderby land granulites and evidence for the redistribution of sm and nd during metamorphism measurements of143nd 144nd and147sm 144nd are reported for whole rocks and mineral separates from granulites of the napier complex at fyfe hills charnockites leuconorites and gabbros yield a whole rock smnd isochron age of3060 160m y and an initial143nd 144nd ratio of0 50776 10 nd 3060m y 2 0 1 8 the negative nd value and the presence of geologically induced dispersion in the data suggest that the isochron age does not represent the time of primary crystallization of the complex but instead indicates a time of later redistribution of sm and nd and partial re equilibration of143nd 144nd ratios this probably occurred during the upper granulite facies metamorphism which has also been dated at 3100m y by rbsr and upb zircon studies 1 coexisting clinopyroxene apatite and total rock fractions in two adjacent samples define an approximately linear array corresponding to an age of 2300 300 m y this array indicates that redistribution of sm and nd and re equilibration of143nd 144nd ratios occurred on an intermineral scale during the upper amphibolite to lower granulite facies metamorphism at 2450m y due to the resetting of the smnd system on both whole rock and mineral scales the primary crystallization age of the igneous protolith is not well constrained by the present data although it is clearly3100m y if it is assumed that the complex was derived initially from a depleted mantle reservoir nd t 2 evolution of the negative nd value of 2 0 with the observed sm nd ratios requires a prehistory of 380m y this implies a primary age of 3480m y however substantially older primary ages can be inferred if the source reservoirs had nd t 2 and or substantial reductions in the sm nd ratio occurred in whole rocks during the granulite facies metamorphism at 3100 m y such an inferred reduction in the sm nd ratio may have been the result of preferential loss of sm relative to nd or introduction of a low sm nd fluid with nd 0 during granulite facies metamorphism

trace element geochemistry in brazilian cerrado soils the cerrado region on the central plateau of brazil contains some of the oldest soils on earth with some soils weathering in place since the late cretaceous the region is currently undergoing rapid agriculturalization and industrialization the objectives of this paper are 1 to inventory the native trace element contents of pristine cerrado soils and 2 to better understand which trace elements accumulate and which are depleted in well drained soils over extremely long periods of pedogenic weathering we sampled the 0 20 and 80 100 cm layers of 45 pedons located in three widely separated areas on three different geomorphic surfaces and determined total elemental contents by x ray fluorescence analysis in comparing the contents of cerrado soils to soils worldwide the contents of sc v cr ga zr nb ce and th are 1 2 times the world average y la and u are about the same and mn co ni cu zn rb sr ba and pb are 0 4 times the world average trace elements that accumulated during weathering are either pentavalent nb5 tetravalent ti4 zr4 u4 th4 or trivalent sc3 v3 cr3 ga3 y3 la3 ce3 at the acid ph of these soils while trace elements that were depleted are either divalent or monovalent rb mn2 co2 ni2 cu2 zn2 sr2 ba2 and pb2 at acid ph thus trace element geochemistry over long periods of pedogenic weathering at acid ph mirrors major element geochemistry in that mono and divalent cations are leached and depleted while tri tetra and pentavalent elements accumulate

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

erosion sedimentation and pedogenesis in a polygenetic oxisol sequence in minas gerais brazil the geomorphic evolution of the south eastern brazilian landscape is attributed to climatic changes coupled with tectonic activity soils developed on the resulting surfaces are mainly deep polygenetic oxisols the combination of stable landscapes with continuously exposed soils and neo cenozoic graben zones episodically filled with sediments that may have undergone soil formation offers the possibility to unravel the history of the soils that are found at the present day surface soil sediment sequences in the southern part of the state of minas gerais brazil were investigated by micromorphology and mineralogy in order to understand the genesis of the soils erosion profile thinning weathering biological activity clay accumulation and iron translocation are the main soil forming processes imprinted in the various pedosedimentary layers strong weathering is shown by dissolution of quartz and weathering of ilmenite bioturbation is ubiquitous even in older and deeper buried layers clay accumulation is observed as illuviation and precipitation features which were micromorphologically differentiated accumulation and movement of iron are represented by gley and pseudogley features related to groundwater and surface water saturation processes these features and the layering of sediments and soils play key roles in understanding the evolutionary phases undergone by the soils and sediments as main soil formation episodes the graben fillings show two phases of clay formation and illuviation separated by a phase of ferralitization interrupted by various episodes of erosion the whole material was eventually overprinted by a phase of ferralitization and plinthite formation apart from water saturation these processes also appear to have acted on the soils of the stable areas not affected by tectonic activity but overprinting and erosion prevents recognition of the separate evolutionary phases

nonsulfide and sulfide rich zinc mineralizations in the vazante ambrosia and fagundes deposits minas gerais brazil mass balance and stable isotope characteristics of the hydrothermal alterations the vazante group hosts the vazante nonsulfide zinc deposit which comprises high grade zinc silicate ore znsio4 and late diagenetic to epigenetic carbonate hosted sulfide rich zinc deposits e g morro agudo fagundes and ambrosia in the sulfide rich deposits hydrothermal alteration involving silicification and dolomitization was related with ground preparation of favorable zones for fluid migration e g fagundes or with direct interaction with the metalliferous fluid e g ambrosia at vazante hydrothermal alteration resulted in silicification and dolomite siderite jasper hematite and chlorite formation these processes were accompanied by strong relative gains of sio2 fe2o3 t rb sb v u and la which are typically associated with the nonsulfide zinc mineralization all sulfide rich zinc ores in the district display a similar geochemical signature suggesting a common metal source from the underlying sedimentary sequences oxygen and carbon isotope compositions of hydrothermally altered rocks reveal a remarkable alteration halo at the vazante deposit which is not a notable feature in the sulfide rich deposits this pattern could be attributed to fluid mixing processes involving the metalliferous fluid and channelized meteoric water which may control the precipitation of the vazante nonsulfide ore sulfide deposition resulted from fluid rock interaction processes and mixing between the ascending metalliferous fluids and sulfur rich tectonic brines derived from reduced shale units

analysis of volcanic activity patterns using modis thermal alerts we investigate eruptive activity by analysis of thermal alert data from the modis moderate resolution imaging spectrometer thermal infrared satellite instrument detected by the modvolc modis volcano alert algorithm these data are openly available on the internet and easy to use we show how such data can plug major gaps in the conventional monitoring record of volcanoes in an otherwise generally poorly documented region melanesia including characterising the mechanism of lava effusion at pago demonstrating an earlier than realised onset of lava effusion at lopevi extending the known period of lava lake activity at ambrym and confirming ongoing activity at bagana langila and tinakula we also add to the record of activity even at some generally better monitored volcanoes in indonesia but point out that care must be taken to recognise and exclude fires