forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

mesoproterozoic arc magmatism in se india petrology zircon u pb geochronology and hf isotopes of the bopudi felsic suite from eastern ghats belt abstract the southern segment of the eastern ghats mobile belt egmb in india was an active convergent margin during mesoproterozoic prior to the final collision in neoproterozoic during the assembly of the rodinia supercontinent here we present mineralogical whole rock geochemical zircon u pb and hf isotopic data from a granitoid suite in the bopudi region in the eggb the granitoid complex comprises quartz monzodiorite with small stocks of rapakivi granites the monzodiorite locally porphyritic contains k feldspar megacrysts plagioclase quartz biotite and ortho amphibole the presence of mantled ovoid megacrysts of alkali feldspar embaying early formed quartz and the presence of two generations of the phenocrystic phases in the rapakivi granites indicate features typical of rapakivi granites the k feldspar phenocrysts in the rapakivi granite are mantled by medium grained aggregates of microcline ab7 or93 which is compositionally equivalent to the rim of kfs phenocryst and pl an23 24 ab75 the geochemistry of both the granitoids shows arc like features for ree and trace elements la icp ms zircon analyses reveal 207pb 206pb ages of 1582 mswd 1 4 for the rapakivi granite 1605 3 ma mswd 3 9 for the monzodiorite the zircons from all the granitoid samples show high ree contents prominent hree enrichment and a conspicuous negative eu anomaly suggesting a common melt source the zircons from the monzodiorite have a limited variation in initial 176hf 177hf ratios of 0 28171 0 28188 with ehf t values of 2 2 to 2 8 correspondingly their two stage hf isotope model ages tdm2 ranging from 2 15 to 2 47 ga probably suggest a mixed source for the magma involving melting of the paleoproterozoic basement and injection of subduction related juvenile magmas the prominent mesoproterozoic ages of these granitoids suggest subduction related arc magmatism in a convergent margin setting associated with the amalgamation of the columbia derived fragments within the neoproterozoic rodinia assembly

the proterozoic of eastern bolivia and its relationship to the andean mobile belt abstract the previously unmapped bolivian sector of the central brazil shield 220 000 km in area has been studied during an anglo bolivian technical co operation programme the metamorphic basement comprises the lomas maneches granulitic complex the chiquitania gneissic complex and the san ignacio schist supergroup which crops out as metasedimentary belts rb sr dates on some of the granulites suggest a protolith age of 2000 ma but the outcrop pattern of the metamorphic rocks as a whole is due to the 1300 ma san ignacio orogeny which was accompanied by widespread granite emplacement and metamorphism up to granulite facies the sunsas cycle began with the deposition of the supracrustal sunsas vibosi group and closed with the development of the sunsas and aguapei mobile belts at 1000 ma the upper proterozoic brasiliano cycle is represented by a bifurcating sedimentary basin in the se of the area with a closing tectonic event of 520 ma correlation is suggested with units in the central brazil shield the bolivian belts are extrapolated east into brazil and their limits indicated to the west the belts show trends parallel to the phanerozoic andean chain and they reflect the major deflection in the chain the arica elbow furthermore when the belts are traced towards the andes their ages broadly correspond to those of the sub andean precambrian basement this leads to the proposal of an enlarged andean orogenic belt system 1000 km wide comprising a 1300 ma old proto andean orogen which has controlled the trends of subsequent mobile belts

tectonic evolution of south america during the late proterozoic abstract this paper discusses the evolution and the late proterozoic assembly of the south american continent i e the west gondwana under the light of global tectonics the northern portion of the continent the amazonian craton is a pre brasiliano domain it may be a fragment derived from laurentia this domain behaved as a large continental plate which was accreted to the central and southeastern portions of the continent brasiliano domain the brasiliano domain had a more complex history and composition during the brasiliano cycle middle proterozoic to early paleozoic in the brasiliano domain several late proterozoic continental plates have been identified sao luiz west africa congo kasai sao francisco rio de la plata arequipa antofalla which acted as forelands to the brasiliano orogenic belts that surround and amalgamate these plates smaller fragments of pre brasiliano continental lithosphere are common in the interior of the brasiliano orogenic belts and played diversified roles microplates microcontinents structural highs some of them have acted as true backlands hinterlands flanked by arc magmatism two principal types of brasiliano orogenic belts are recognized between the continental plates a marginal orogenies to the late proterozoic plates with pelitic carbonatic rocks and discrete volcanism which were thrust by collision onto the margin of the plates and which seem to represent final products of previous passive margin sequences b distal orogenics internal belts within the continental masses sometimes a branching system of orogenics that contain varied clastic and minor carbonate sedimentary rocks accompanied by important bimodal volcanism and calc alkaline plutonism for these orogenies a greater variety of basinal scenarios and tectonic settings may be visualized the amalgamation of the continent during the late proterozoic i e the formation of the brasiliano structures indicates the existence of practically synchronous multilateral stresses within lithospheric plates this can be produced by complementary adjustment of the continental plates as a response to major global events elsewhere analogous to modern continental collisions two large and important tectonic zones may be candidates to such a scenario the hijaz mozambique zone site of connection of east and west gondwana and the transbrasiliano lineament which cuts across south america and extends to north africa following the zone of connection between pre brasiliano and brasiliano domains other small ocean openings and closings inside the various brasiliano areas dom feliciano gariep for instance were minor events and complementary to the other two major tectonic zones mentioned above

evolution of the archean palaeoproterozoic bundelkhand massif central india evidence from granitoid geochemistry the bundelkhand massif of archean palaeoproterozoic age is primarily a granite gneiss complex three distinct granitoid suites have been identified within the massif hornblende granitoids biotite granitoids and leucogranitoids in order of decreasing age these granitoids were emplaced in previously deformed basement consisting of gneisses banded iron formations and other metasediments mafic to felsic volcanics the granitoids exhibit a large compositional range from quartz diorite to syenogranite and show a calc alkaline trend they are metaluminous to peraluminous and have i type characteristics the sio2 content ranges from 49 to 77 wt low k2o na2o characterizes the granitoids the oldest hornblende granitoids have low rb and yb contents compared to the younger biotite granitoids and leucogranitoids rb sr values for most of the granitoids are low 1 2 which is a characteristic feature of magmas derived from sources chemically similar to island arc or continental margin basalts the features mentioned above coupled with concentrations of rb y nb yb ta and th indicate a volcanic arc tectonic setting for the granitoids it is proposed that the massif represents subduction related magmatism of an ocean in the southern part of the massif an andean plate margin

geology geochemistry and pb pb zircon geochronology of the paleoproterozoic magmatism of vila riozinho tapajos gold province amazonian craton brazil abstract pb pb zircon geochronologic data define two different periods of intense igneous activity in the vila riozinho region tapajos gold province tgp south central amazonian craton at 2 00 1 97 ga the vila riozinho volcanic sequence 2000 4 ma 1998 3 ma and the older sao jorge granite 1981 2 ma 1983 8 ma were formed at 1 89 1 87 ga the younger sao jorge 1891 3 ma jardim do ouro 1880 3 ma and maloquinha 1880 9 ma granites and the moraes almeida volcanic sequence 1890 6 ma 1881 4 ma 1875 4 ma were emplaced similar age intervals are registered throughout the tgp that is thus a little younger than the paleoproterozoic terranes of the maroni itacaiunas province in the guiana shield geochemical and geochronologic data demonstrate that the sao jorge pluton is composed of two different granitoids the older and younger sao jorge granites these as well as the jardim do ouro granite are i type and magnetite series the sao jorge granites are high k calc alkaline and show significant geochemical variation the jardim do ouro granite is more iron rich and less oxidized the vila riozinho formation is calc alkaline to shoshonitic and geochemically similar to the sao jorge granites the maloquinha granite and the moraes almeida formation differ from the other studied rocks in petrographic and geochemical characteristics have aluminous a type affinities and were probably derived by low temperature crustal melting the volcanic sequences of the studied area formerly included into the iriri group of the uatuma supergroup are divided into the vila riozinho and moraes almeida sequences the presence of these two sequences in the same tectonic domain demonstrates the heterogeneity of the uatuma supergroup the tgp registers accretionary processes related to the formation of the atlantica supercontinent at 2 00 ga this was followed at 1 88 ga by an intracontinental taphrogenic event that lasted throughout the mesoproterozoic the tectonic setting of the tgp was thus transitional between a subduction related magmatic arc and a stable continental block undergoing extension

rb sr and k ar evidence for the presence of a 1 6 ga basement underlying the 1 2 ga garzon santa marta granulite belt in the colombian andes abstract rb sr and k ar investigations of rocks from the garzon massif in the northern andes indicate the presence of three chronological units 1 an 1 6 ga old basement of augen gneisses showing ages and lithology similar to the adjacent parts of the guiana shield 2 a sequence of supracrustals metamorphosed to the granulite facies around 1 2 ga ago and 3 a set of pegmatite dykes intruded into the other units 850 ma ago it is suggested that the supracrustal protoliths of the granulites were deposited in a cordillera type continental margin of the guiana shield cratonized in the parguazan tectonomagmatic event and that the 1 2 ga orogenic event in the andes was of the continental collision type the latter event is tentatively correlated with the grenville orogeny in north america

grenvillian granulite facies metamorphism in the arequipa massif peru a laurentia gondwana link abstract the arequipa massif is a major component of the central andean orogenic basement extensively exposed along the coast of southern peru in the vicinity of the arica bight and bolivian orocline the southern part of the massif is dominated by granulitic gneisses for which a ca 1900 ma age for metamorphism of a slightly older protolith has been accepted on the basis of rb sr whole rock isochrons and bulk u pb zircon dating new u pb single grain zircon geochronology however reveals two mesoproterozoic granulite facies domains 1200 ma to the ne of quilca 16 44 s 72 24 w and 970 ma in the vicinity of mollendo 16 58 s 72 03 w these ages correspond to the major metamorphic events represented in the grenville province of north america and lend credence to proposed reconstructions based on sparse isotopic data that grenvillian metamorphic belts underlie the western margins of south american cratons and constituted the conjugate rifted margin of eastern laurentia the protolith ages of the samples further substantiates the proposed neoproteozoic reconstruction in both domains of the massif the orthogneisses have zircon upper intercept ages of about 1950 ma and the paragneisses contain younger detrital zircons suggesting sedimentation in the early mesoproterozoic correlative zircon ages are found in the makkovik ketilidian belt and the adjacent trans labrador batholith of eastern laurentia the arequipa massif is therefore envisaged to represent an exotic terrane that originated as the tip of a pre grenville laurentian promontory comprising labrador greenland and scotland that was embedded into the proto andean margin during the grenville orogeny

rock physics analysis and time lapse rock imaging of geochemical effects due to the injection of co2 into reservoir rocks the fundamental concept of time lapse seismic monitoring is that changes in physical parameters such as saturation pore fluid pressure temperature and stress affect rock and fluid properties which in turn alter the seismic velocity and density increasingly however time lapse seismic monitoring is called upon to quantify subsurface changes due in part to chemical reactions between injected fluids and the host rocks this study springs from a series of laboratory experiments and high resolution images assessing the changes in microstructure transport and seismic properties of fluid saturated sandstones and carbonates injected with co2 results show that injecting co2 into a brine rock system induces chemo mechanical mechanisms that permanently change the rock frame injecting co2 into brine saturated sandstones induces salt precipitation primarily at grain contacts and within small pore throats in rocks with porosity lower than 10 salt precipitation reduces permeability and increases p and s wa

observed relation of soil magnetic susceptibility and soil gas hydrocarbon analyses to subsurface hydrocarbon accumulations magnetic susceptibility ms studies on soils over 19 oil and gas fields showed anomalously large amounts of diagenetic magnetic minerals in shallow depth samples in about 89 of the cases tested soil ms anomalies were compared with soil gas hydrocarbon ghc anomalies over 12 oil and gas fields including several stratigraphic traps and one gas storage reservoir samples were collected along the same profiles six examples are presented with detailed data listings our goal is to test the utility of soil magnetic susceptibility and soil gas hydrocarbons as petroleum prospecting methods we emphasize statistical evaluation of anomalies and their empirical correlation with producing areas and leads identified by other geochemical geological or geophysical data for direct comparison of anomaly strengths data are expressed statistically in terms of the appropriate background mean and standard deviation soil ms and soil ghc data complement each other providing better guidance to productive areas than either data set alone when used in an integrated exploration program their combined positive evidence of hydrocarbon presence under prospects could result in fewer dry holes than when drilling with structural information only

recent advances in time lapse laboratory rock physics for the characterization and monitoring of fluid rock interactions abstractmonitoring thermo chemo mechanical processes geophysically e g fluid disposal or storage thermal and chemical stimulation of reservoirs or natural fluids simply entering a new system raises numerous concerns because of the likelihood of fluid rock chemical interactions and our limited ability to decipher the geophysical signature of coupled processes one of the missing links is understanding the evolution of seismic properties together with reactive transport because rock properties evolve as a result of chemical reactions and vice versa capturing this coupling experimentally is one of the missing elements in the existing literature this paper describes recent advances in rock physics experiments to understand the effects of dissolution induced compaction on acoustic velocity porosity and permeability this paper has a dual aim understanding the mechanisms underlying permanent modifications to the rock microstructure and providing a richer set of experimental information to inform th

origin of carbonate cemented beds on the naglfar dome voring basin norwegian sea abstract lower eocene sediments cored north in the voring basin include calcite cemented beds with unusual microstructures the origin of these beds has been interpreted on the basis of optical microscopy sem microprobe analyses and stable isotope analyses the calcite cement includes perfect pseudomorphs of vesicular ash and lapilli and a few micro sized 0 1 mm remains of unaltered sideromelane were discovered the sideromelane has the composition of relatively alkali rich tholeiitic basalt similar to lower eocene plagioclase basalt from dsdp site 342 on the voring marginal high the earliest phase of diagenesis in the marine environments was associated with palagonitisation and glauconitisation of volcanic glass and growth of smectite and zeolites an early diagenetic origin is also inferred for the more extensive growth of calcite and pyrite cement stable isotope analyses of the calcite cement show very light 13 c values between 55 and 28 4 and with the largest negative values in beds with pseudomorphic textures after ash lapilli such low 13 c values are consistent with a carbon source associated with oxidation of methane within the upper levels of the sediment body close to the sediment water interface the metastable basalt glass acted as a source for ca 2 and fe 2 ions required for calcite and pyrite growth whereas sulphur for pyrite formation may have been supplied from accompanying sulphate reduction the combined occurrence of volcaniclastic deposits and methane seepage as documented in the voring basin may have implications for more widespread formation of calcite cemented beds in the offshore tertiary sediments

andean trending mobile belts in the brazilian shield much of the phanerozoic andean mountain chain is floored by precambrian basement1 3 in places as old as 2 000 myr4 5 but relations between these rocks and those of the brazilian shield have long been obscure mainly because of the intervening cenozoic sedimentary cover we describe here recent work over the bolivian sector of the brazilian shield fig 1 which shows a pattern of proterozoic mobile belts approximately parallel with the andean chain to the west when the trends of these bolivian belts are projected towards the andes under the cenozoic cover their ages broadly accord with those of the andean basement our interpretation is that the two domains are linked in the form of a chain of five andean trending mobile belts with a combined width of 1 000 km becoming progressively younger to the west with ages ranging from 1 300 myr to the present

organic geochemical study and genetic correlation of natural gas oil and menilite source rocks in the area between san and stryi rivers polish and ukrainian carpathians abstract we studied the source rocks and petroleum accumulations in the carpathian mountains of the polish ukrainian border area between the san and stryi rivers a total of 108 oligocene menilite formation rock samples 12 oil and 15 gas samples accumulated in the oligocene reservoirs and one oil and gas sample from an upper cretaceous reservoir were analysed the aim of this study was to evaluate the petroleum potential of the menilite formation in this area and to evaluate potential genetic correlations between source rock and petroleum the menilite formation represents a prolific source rock with toc content ranging from 2 to 7 wt locally exceeding 20 wt algal type ii and mixed type ii iii kerogens dominate but sometimes type iii kerogen and rarely type i kerogen occur thermal maturity of the oligocene menilite formation increases from outer to inner tectonic units from the immature in the boryslav pokuttya and skole units to the overmature in the southern parts of silesian and dukla units as a consequence petroleum potential decreases in the same order geochemical analyses reveal that oil and gas generation and migration processes within the carpathian flysch succession are complicated all analysed oils were generated from the menilite formation during early and peak stages of the oil window associated gas is of low temperature thermogenic origin sometimes containing an admixture of microbial gas high temperature thermogenic gas occurs only in the polish part of the silesian unit

emergence of anomalous transport in stressed rough fractures abstract we report the emergence of anomalous non fickian transport through a rough walled fracture as a result of increasing normal stress on the fracture we show that the origin of this anomalous transport behavior can be traced to the emergence of a heterogeneous flow field dominated by preferential channels and stagnation zones as a result of the larger number of contacts in a highly stressed fracture we show that the velocity distribution determines the late time scaling of particle spreading and velocity correlation determines the magnitude of spreading and the transition time from the initial ballistic regime to the asymptotic anomalous behavior we also propose a spatial markov model that reproduces the transport behavior at the scale of the entire fracture with only three physical parameters our results point to a heretofore unrecognized link between geomechanics and particle transport in fractured media

a review of the geochronology of the amazonian craton tectonic implications abstract the available geochronological data for the amazonian craton permit delineation of its main age provinces and their respective tectonic development demarcation of the boundaries of each age province is primarily based on rb sr whole rock isochrons supported by k ar determinations on minerals some u pb results on zircons pb pb whole rock isochrons and sm nd model ages complement these data in key areas at the center of the amazonian craton lies the archean age central amazonian province bounded in its northern part by the maroni itacaiunas mobile belt which developed during early proterozoic times the western and southern margins of the archean core are bordered by three tectonic provinces 1 the rio negro juruena magmatic arc 2 the rondonian mobile belt and 3 the sunsas mobile belt each belt borders the western margin of the preceding belt and each records a successively younger middle proterozoic orogeny within the central amazonian province 2 7 ga age determinations have been obtained from the serra dos carajas area although the maroni itacaiunas belt includes several reworked archean nuclei rock units formed during the 2 2 1 9 ga trans amazonian orogeny predominate the isotopic evidence suggests that a mantle derived component constituted a significant portion of the crust accreted at that time the rio negro juruena province is of mid proterozoic age 1 75 1 50 ga and includes almost entirely mantle derived material juxtaposed with the pre existent continental mass along a series of successive magmatic arcs the rondonian and sunsas provinces both exhibit older sialic basements 1 5 ga the former is characterized by important tectonomagmatic events in the 1 45 1 25 ga interval whereas the latter records tectonic reworking at 1 1 0 9 ga finally the amazonian craton is bordered to the east by the late proterozoic paraguai araguaia or araguaia tocantins fold belt

major and trace element geochemistry and crustal evolution of archaean granodioritic rocks from eastern finland in the eastern part of central finland vast areas of granodioritic gneisses surround the ca 2 65 ga greenstone belts these granodioritic gneisses can be divided into two chronological and lithological units 1 grey gneisses 2 86 ga old with a typical archaean ttg tonalite trondhjemite granodiorite composition 2 augen gneisses 2 51 ga old with a granodioritic to granitic composition the sr and nd isotopic data preclude derivation of the grey gneisses from very much older continental material the highly fractionated ree distribution patterns la yb n 37 103 with few or no eu anomalies militate against a direct mantle origin the data suggest that the grey gneisses have most likely been derived by partial melting of short lived basaltic crustal material garnet bearing amphibolite it is estimated that the basaltic protoliths were separated from a mantle source no more than 150 ma before the melting such an early basaltic crust might have been completely destroyed or might have left some remnants which are now represented by mafic inclusions in the grey gneisses the augen gneisses have a relatively high isr value 0 705 which suggests an origin by recycling of pre existing crustal material grey gneisses their ree patterns are less fractionated than those of the grey gneisses and sometimes have negative eu anomalies using the sr isotopic and trace element data a model calculation suggests that the augen gneisses could have been derived by partial melting 50 of mixed protoliths with ttg greenstone belt volcanics in a proportion of 70 30 by volume followed by fractional crystallisation

proterozoic geologic evolution of the sw part of the amazonian craton in mato grosso state brazil abstract this paper presents new geochronologic isotopic and geochemical data bearing on the evolution of proterozoic crust in sw mato grosso state brazil which is at the southern end of the 1 6 1 8 ga rio negro juruena orogenic belt of the amazonian craton amazonia our data define three major crustal events i the alto jauru terrane occurs in the eastern part of the region and is comprised of island arc related rocks with u pb ages from 1 79 to 1 74 ga these rocks have e nd t values range from 2 8 to 2 0 with crustal residence ages t dm from 1 93 to 1 78 ga indicating a mainly juvenile signature at 1 8 ga ii the cachoeirinha suite occurs in the central part of the region and consists of calc alkaline plutons emplaced into alto jauru terrane host rocks these plutons have u pb ages from 1 56 to 1 54 ga and e nd t values ranging from 1 0 to 0 8 with crustal residence ages t dm from 1 88 to 1 75 ga we interpret this orogen as the roots of a continental margin arc built upon basement comprised of the alto jauru terrane iii the santa helena batholith occurs in the western part of the region and is a large elongate body of calc alkaline rocks ranging from granodiorite to highly evolved granite these units yield u pb ages from 1 45 ma to 1 42 ga with e nd t values ranging from 4 1 to 2 6 and crustal residence ages t dm from 1 70 to 1 50 ga the rio alegre domain occurs west of the santa helena batholith and includes juvenile 1 52 to 1 47 ga volcanic and mafic plutonic rocks regional geologic relationships suggest that these rocks are part of the crust into which the santa helena batholith was emplaced we interpret this batholith as the magmatic core of a juvenile arc accreted to the edge of the alto jauru terrane with incorporation of some older crust alto jauru terrane in the east and derivation from mainly juvenile crust rio alegre domain in the west the rio branco suite occurs to the east of exposed alto jauru terrane rocks as large hills protruding through younger aquapei group sedimentary rocks it consists of gabbro and granophyric rocks with u pb ages of 1 47 ga gabbro and 1 43 ga granophyre and inherited older nd suggesting hinterland derivation from alto jauru terrane basement during development of the santa helena batholith several undeformed 1 5 1 4 ga granitic plutons occur within the alto jauru terrane these are also regarded as inboard manifestations of subduction related magmatism associated with accretion of the rio alegre domain and formation of the santa helena batholith the siliciclastic aguapei group was deposited sometime between 1 0 and 1 4 ga it overlies jauru terrane basement rocks of the santa helena batholith and the rio alegre domain in the east it is flat lying and undeformed but in the west it is deformed and metamorphosed in the nnw trending aguapei thrust belt farther west the aguapei group is horizontal undeformed and overlies the paleoproterozoic paragua block in bolivia where it has been correlated with the sunsas group the aguapei thrust belt has k ar cooling ages of about 930 ma and is apparently a foreland fold and thrust belt formed by reactivation of an older rift basin during the 1 0 ga sunsas orogeny which occurs west of the paragua block in bolivia

model for hydrocarbon microseepage and related near surface alterations despite recent advances in surface and near surface geochemical methods there continues to be a need for improved understanding of the scientific basis relating these methods to petroleum deposits we have reviewed the literature pertinent to the several methods we have used and integrated that information with some 100 man years of our experience to contribute to that end evidence indicates that hydrocarbon micro seepage from reservoirs most probably involves buoyant colloidal size microbubbles of light hydrocarbons principally methane through the butanes ascending relatively rapidly through a water filled network of fractures joints and bedding planes bacterial consumption of these hydrocarbons creates carbon dioxide and hydrogen sulfide which may alter sediments producing detectable modifications in geomorphic seismic magnetic and radiometric properties we have used combinations of 1 airborne microwave sensing and laboratory analyses of soil gas hydrocarbons 2 shallow source aeromagnetic and soil magnetic susceptibility measurements 3 aerial and surface gamma ray measurements and 4 geomorphology to find productive oil and gas deposits results suggest that using these methods can substantially increase the probability of wildcat success and reduce finding costs in selected geologic settings however these surface methods cannot reveal depth size or quality of reservoirs or even if producible hydrocarbons will be found

western parana suture shear zone and the limits of rio apa rio tebicuary and rio de la plata cratons from gravity data abstract we present a new gravity map between 45 70 w and 5 40 s integrating open source terrestrial gravity data of argentina with the south american gravity model 2004 sagm04 a 5 min arc resolution gravity model the bouguer anomaly map reveals a 2000 km long linear gravity feature from 15 s to 30 s at longitude 55 w with a steep horizontal gradient separating two gravity domains the eastern domain is the parana basin with ne sw trending bouguer anomalies of 80 mgal in average the western domain comprises the chaco parana chaco tarija and pantanal basins with circular positive anomalies of up to 20 mgal in amplitude previous seismic studies mapped a thinner crust of less than 35 km in the western domain and the present gravity models indicate a 10 20 kg m 3 denser crust on the other hand the eastern domain has a thicker crust of more than 40 km seismic tomography models also show p and s wave velocity reduction in the western domain whereas high velocity characterises the parana basin these geophysical data indicate that the gravity gradient marks a transition between two distinct lithospheres the gravity gradient is associated with a tectonic feature referred to as the western parana suture shear zone granites of 530 570 ma ages located parallel or over the gravity gradient suggest a neoproterozoic to early cambrian age suture shear zone thus approximately synchronous and parallel to the pampean belt sediment corrected residual gravity map and its vertical derivative allow us to define the limits of the rio apa rio de la plata and rio tebicuary cratons their eastern and western limits are the western parana suture and the pampean belt respectively this study unravels precambrian tectonic elements concealed by the phanerozoic sedimentary basins adding new constraints for the amalgamation history of sw gondwana

geochemical characteristics and variations of the ryoke granitoids southwest japan petrogenetic implications for the plutonic rocks of a magmatic arc abstract geochemical characteristics and variations of the ryoke granitoids southwest japan have been elucidated based on more than 70 analyses from the mikawa and kinki districts for major and trace elements including ree the ryoke granitoids are the products of a continental margin arc magmatism of pacific asia during late cretaceous period and represent its magmatic front they are classified into the older and younger groups according to their intrusive ages and textural and petrographic characteristics the older ryoke granitoids were emplaced at ca 20 km depth during 100 85 ma and deformed and recrystallized together with the host ryoke metamorphic rocks the younger ryoke granitoids are post tectonic plutons emplaced at ca 15 km depth 85 75 ma and accompanied by the formation of contact aureoles the majority belongs to i type and ilmenite series granitoids the granitoids of both groups are geochemically quite similar calcic alkali lime index 65 very low in fe 3 fe 2 ratio and both metaluminous and peraluminous they have low rb

the southern border of the guapore shield in western brazil and bolivia an interpretation of its geologic evolution abstract the geology of the southeastern and southwestern margins of the guapore shield is described the shield rocks comprise high grade granulite and low grade greenschist sequences these rocks are poorly known oldest recorded ages include transamazonian datings 2 ma the margins of the shield show the development of a mobile belt formed by rocks of upper proterozoic to cambrian age within this mobile belt the cuiaba group of uncertain but possibly upper proterozoic age outcrops to the east and shows tectonic contacts with younger rocks that span the vendian upper proterozoic to cambrian age the rocks of vendian to cambrian age form three isolated elongated basins which show evidence of fault control the axes of these basins are equally separated by angles of 120 and meet at a triple junction near corumba the stratigraphy of the vendian to cambrian basins is correlated from bolivia to brazil based on tectonic continuity sedimentological similarity and fossil content the central location at corumba shows instability since the middle proterozoic when important igneous activity is recorded at rincon del tigre bolivia and in brazil this igneous activity is interpreted as being related to hot spot activity in an intercratonic setting from which the younger upper proterozoic basins originated

timing of continental arc type magmatism in northwest india evidence from u pb zircon geochronology charnockites and granodiorites which occur within granulite facies metasediments of the bhilwara supergroup of rajasthan northwest india are cogenetic and exhibit petrologic characteristics indicative of a magmatic derivation zircon u pb data yield a common crystallization age of 1723 14 7 ma these rocks earlier believed to represent oldest archean and deepest crust actually mark a major proterozoic event they are believed to be the products of continental arc magmatism where a basic layer underplating the lower crust led to intrusion of differentiated melts in crustal domains undergoing dry metamorphism

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

laboratory measurements of elastic properties of carbonate rocks during injection of reactive co2 saturated water 1 to investigate the coupled physical and chemical effects of injecting co2 rich water into carbonate rock samples we monitor the elastic and transport properties of six carbonate rocks along with the calcium content and ph of the pore fluid under constant confining pressure carbonate samples range from calcite limestones containing dolomite to pure calcite mudstones which allow us to study how mineral composition and microstructure affect the magnitude of the observed changes the elastic properties of both the saturated and dry rock i e rock frame show a gradual loss of sstrength upon injection as testified by the continuous decrease in the dry p and s wave velocity and by proxy the bulk and shear moduli the magnitude of the observed changes in the elastic properties varies among the samples with a maximum relative change of 24 7 871 m s and 21 2 443 m s in p and s wave velocities respectively the decrease is also accompanied by a relative increase in permeability up to 495 and porosity up to 19 the observed changes likely derive from a change in microstructure which is monitored over time via scanning electron microscopy the variation in porosity results from two competing and interdependent processes the chemical dissolution of calcite and the resulting mechanical compaction under pressure the results of this study show that upon injection of fluids that are in chemical disequilibrium with the hosting rock the variation of the elastic properties cannot be described by existing rock physics models

teleseismic p wave tomography beneath the pantanal parana and chaco parana basins se south america delimiting lithospheric blocks of the sw gondwana assemblage

micro continuum approaches for modeling pore scale geochemical processes the recent profusion of microscopic characterization methods applicable to earth science materials many of which are described in this volume anovitz and cole 2015 this volume noiriel 2015 this volume suggests that we now have an unprecedented new ability to consider geochemical processes at the pore scale these new capabilities offer the potential for a paradigm shift in the earth sciences that will allow us to understand and ultimately quantify such enigmas as the apparent discrepancy between laboratory and field rates white and brantley 2003 and the impact of geochemical reactions on the transport properties of subsurface materials steefel and lasaga 1990 1994 steefel and lichtner 1994 xie et al 2015 it has only gradually become apparent that many geochemical investigations of earth materials have suffered perhaps inadvertently from the assumption of bulk or continuum behavior leading to volume averaging of properties and processes that really need to be considered at the individual grain or pore scale for example a relationship between reaction induced porosity and permeability change can perhaps be developed based on bulk samples but ultimately a mechanistic understanding and robust predictive capability of the associated geochemical and physical processes will require a pore scale view the question still arises do we need pore scale characterization and models in geochemistry and mineralogy the laboratory field rate discrepancy or enigma is a good example of where a pore scale understanding may provide insights not easily achievable with bulk characterization and models if the reasons for this apparent discrepancy between laboratory and field rates cannot be explained then it appears unlikely that scientifically defensible and quantitative models for a number of important earth science applications ranging from chemical weathering and its effects on atmospheric co2 to subsurface carbon sequestration to nuclear waste storage to contaminant remediation and transport

reactive transport under stress permeability evolution in deformable porous media abstract we study reactive transport in a stressed porous media where dissolution of the solid matrix causes two simultaneous competing effects pore enlargement due to chemical deformation and pore compaction due to mechanical weakening we use a novel mechanistic pore scale model to simulate flooding of a sample under fixed confining stress our simulations show that increasing the stress inhibits the permeability enhancement increasing the injected volume required to reach a certain permeability in agreement with recent experiments we explain this behavior by stress concentration downstream in the less dissolved hence stiffer outlet region as this region is also less conductive even its small compaction has a strong bottleneck effect that curbs the permeability our results also elucidate that the impact of stress depends on the dissolution regime under wormholing conditions slow injection i e high damkohler number da the development of a sharp dissolution front and high porosity contrast accentuates the bottleneck effect this reduces transport heterogeneity promoting wormhole competition once the outlet starts eroding the extreme focusing of transport and hence dissolution characteristic of wormholing becomes dominant diminishing the bottleneck effect and hence the impact of stress at breakthrough in contrast at high flow rates low da incomplete reaction upstream allows some of the reactant to traverse the sample causing a more uniform dissolution the continuous dissolution and its partial counteraction by compaction at the outlet provides a steady gradual increase in the effect of stress consequently the impact of stress is more pronounced at high da during early stages low permeability and at low da close breakthrough our work promotes understanding of the interplay between dissolution and compaction and its effect on the hydromechanical property evolution with important implications for processes ranging from diagenesis and weathering of rocks to well stimulation and carbon sequestration

the cordillera blanca batholith a study of granite intrusion and the relation of crustal thickening to peraluminosity the cordillera blanca batholith peru is emplaced axially over the thickened crustal keel of the andes 60 km it is made up of tonalite and leucogranodiorite the latter having an s type character it is the terminal component in continental growth with ages as young as 3 ma however the crustal character of the leucogranodiorite relates to fluid interaction with the surrounding aluminous graphitic basinal rocks and not to any deep seated source material chemistry isotopes and geological evidence suggest the source is enriched mantle akin to that seen further south in chile and variations in composition e g increases in sr cen ybn and ta hf ratios of granitoids from the trench into the continent relate to changes in this source

adakite like signature of porphyry granitoid stocks in the meiduk and parkam porphyry copper deposits ne of shahr e babak kerman iran constrains on geochemistry abstract the middle miocene porphyry granitoid stocks of meiduk and parkam porphyry copper deposits are intruded in the north western part of the dehaj sarduiyeh volcano sedimentary belt in the south eastern extension of the urumieh dukhtar magmatic arc udma in iran the porphyritic to microgranular granitoids are mainly consist of quartz diorite granodiorite and diorite the whole rock geochemical analyses of these rocks reveals sub alkaline calc alkaline meta peraluminous and i type characteristics their geochemical characteristics such as al 2 o 3 content of 13 51 17 05 wt high sr concentration mostly 400 ppm low yb an average of 0 74 ppm and y an average of 9 02 ppm contents strongly differentiated ree patterns la yb 20 lack of eu anomaly eu eu 1 are indicative of adakitic signature their enrichment in low field strength elements lfse and conspicuous negative anomalies for nb ta and ti are typical of subduction related magmas detailed petrological studies and geochemical data indicated that meiduk and parkam porphyry granitoids were derived from amphibole fractionation of hydrous melts at a depth of 40 km in a post collisional tectonic setting

deposition of magnetic pyrrhotite during alteration of crude oil and reduction of sulfate abstract crude oil seepage is widespread in carbonate cap rock and in overlying sediments of the damon mound salt dome brazoria county texas scanning electron microscopy of calcite lined cavities provides insight to the latest stage of mineral deposition in this salt dome environment solid crude oil residues and microbes occur in association with surface minerals such as gypsum and barite deposition of pyrrhotite pyrite marcasite sphalerite and elemental sulfur is attributed to microbial sulfate reduction and deposition of aragonite and calcite is related to microbial oxidation of crude oil hydrocarbons the c1 c4 hydrocarbons in pyrrhotite bearing cap rock samples are a biodegraded residue derived from crude oil higher molecular weight crude oil components in cap rock are altered by water washing and to a lesser extent by biodegradation hydrocarbon compositions could be explained by limited aerobic biodegradation but a component of anaerobic biodegradation cannot be excluded some damon mound samples are highly magnetic because of abundant ferrimagnetic pyrrhotite this study provides new evidence of a link between crude oil migration microbial activity and diagenetic magnetic anomalies in rocks

detailed compositional analysis of gas seepage at the national carbon storage test site teapot dome wyoming usa abstract a baseline determination of co2 and ch4 fluxes and soil gas concentrations of co2 and ch4 was made over the teapot dome oil field in the naval petroleum reserve no 3 npr 3 in wyoming usa this was done in anticipation of experimentation with co2 sequestration in the pennsylvanian tensleep sandstone underlying the field at a depth of 1680 m the baseline data were collected during the winter 2004 in order to minimize near surface biological activity in the soil profile the baseline data were used to select anomalous locations that may be the result of seeping thermogenic gas along with background locations five 10 m holes were drilled 3 of which had anomalous gas microseepage and 2 were characterized as background these were equipped for nested gas sampling at depths of 10 5 3 2 and 1 m depths methane concentrations as high as 170 000 ppmv 17 were found along with high concentrations of c2h6 c3h8 n c4h10 and i c4h10 much smaller concentrations of c2h4 and c3h6 were observed indicating the beginning of hydrocarbon oxidation in the anomalous holes the anomalous 10 m holes also had high concentrations of isotopically enriched co2 indicating the oxidation of hydrocarbons concentrations of the gases decreased upward as expected indicating oxidation and transport into the atmosphere the ancient source of the gases was confirmed by 14c determinations on co2 with radiocarbon ages approaching 38 ka within 5 m of the surface modeling was used to analyze the distribution of hydrocarbons in the anomalous and background 10 m holes diffusion alone was not sufficient to account for the hydrocarbon concentration distributions however the data could be fit with the addition of a consumptive reaction first order rate constants for methanotrophic oxidation were obtained by inverse modeling high rates of oxidation were found particularly near the surface in the anomalous 10 m holes demonstrating the effectiveness of the process in the attenuation of ch4 microseepage the results also demonstrate the importance of ch4 measurements in the planning of a monitoring and verification program for geological co2 sequestration in sites with significant remaining hydrocarbons i e spent oil reservoirs

magnetic forward models of cement oil field oklahoma based on rock magnetic geochemical and petrologic constraints magnetic forward models of the cement oil field oklahoma were generated to assess the possibility that ferrimagnetic pyrrhotite related to hydrocarbon seepage in the upper 1 km of permian strata contributes to aeromagnetic anomalies at cement six bodies having different magnetizations were constructed for the magnetic models based on geology and on petrologic and geochemical results supplemented by rock magnetic measurements of shallow core and outcrop samples the column of rock through which hydrocarbons have passed is divided into three sulfide zones on the basis of pyrrhotite content and the column is capped by a 30 m thick zone that contains ferric oxide minerals formed mainly from oxidized pyrite red beds unaffected by sulfidization as well as a zone of rock depleted in hematite but lacking sulfide surround sulfidic zones the synthetic magnetic profiles are controlled mainly by pyrrhotite bearing strata at depths of 200 500 m the magnetizations of these bodies are calculated from 1 pet

the proterozoic history of eastern bolivia abstract about 100 000 km2 of the previously unmapped bolivian sector of the central brazil shield has been studied by proyecto precambrico an anglo bolivian technical cooperation programme the lower proterozoic is represented by the lomas maneches granulite group and the bulk of the chiquitania paragneiss complex which were formed during the trans amazonic orogenic cycle 2000 ma the middle proterozoic spans the orogenic cycles of san ignacio 2000 1300 ma and sunsas unmetamorphosed upper proterozoic and possibly cambrian sediments on the southern and eastern flanks of the shield represent marine transgressions related to the intra continental braziliano orogenic cycle east trending dolerite dykes were probably intruded during this period within the shield

relationship between magnetic anomalies and hydrocarbon microseepage above the jingbian gas field ordos basin china in this study soil magnetic measurements susceptibility and hysteretic parameters and soil hydrocarbon analyses were conducted on samples from three profiles profiles i and ii run across and profile iii runs parallel to the trend of the jingbian gas field in the ordos basin central china to determine the relationship between the magnetic anomalies e g volume specific magnetic susceptibility k and the hydrocarbon seepage environments the results document a strong correlation between magnetic susceptibility and soil gas hydrocarbon concentration furthermore the spatial distribution of k and hydrocarbon anomalies correlate with those of the gas field in addition magnetic minerals in the soils with higher susceptibility are predominantly magnetite with little or no substitution of titanium compared to that of samples with lower susceptibility 7 105 si international unit of susceptibility these results provide strong evidences for the formation of highly magnetic minerals in close association with hydrocarbon seepage recognition of such seepage induced magnetic anomalies can be used to facilitate the exploration for oil and gas in china and elsewhere

proterozoic crustal evolution at the southern margin of the amazonian craton in the state of mato grosso brazil evidence from rbsr and k ar data abstract geologic mapping on a 1 100 000 scale in the sao jose dos quatro marcos sjqm area southwest of the state of mato grosso central western brazil showed a remarkable variety of precambrian lithologic units predominant are grey tonalitic gneisses with a rb sr isochron age of 1971 770 ma an initial 87 sr 86 sr ratio of 0 7071 and a k ar biotite cooling age of 1500 ma the gneisses gneisses show interlayered first generation amphibolite lenses and bands k ar amphibole age of 1500 ma and some rare relicts of grey gneissic granulites together with associated pink augen gneisses rare outcrops of deformed banded calc silicate rocks are recrystallized by contact metamorphism these units are intruded by a granitoid suite constituted by early usually somewhat deformed tonalites and granodiorites and late massive pink granodiorites and monzo and syegogranites rb sr isochron age 1472 19 ma i r 0 7037 they show enclaves of grey gneisses and second generation amphibolites a large amphibolite body to the west is also considered part of this second generation basic sequence scattered rhyolite outcrops to the east represent the southernmost remnants of the volcanic rio branco group the grey gneisses here formally identified as the sao jose dos quatro marcos gneiss and probably derived from lower proterozoic tonalites were complexly deformed and recrystallized at 2 0 ga into predominantly amphibolite facies rocks the first and second generation amphibolites document two distinct commingling episodes separated by at least 0 5 ga each directly associated with the corresponding host rock granitoid magmatism the porphyritic granites were converted into pink augen gneisses before 1500 ma and are thus not contemporaneous to the 1380 ma syn kinematic san ignacio granitoids from eastern bolivia calc silicate rocks are interpreted as remnants of supracrustal sequences perhaps related to the quatro meninas group the 1500 ma granitoids formally named the jaboti intrusive suite invaded under a syn to late kinematic regime determining a unique thermal magmatic event in this region

u pb age data from the sunsas region of eastern bolivia evidence for the allochthonous origin of the paragua block abstract ion microprobe u pb dating of detrital zircons from the chiquitania and san ignacio groups of eastern bolivia show that these rocks were derived mostly from a 1765 ma source and were deposited after 1690 ma the emplacement of the lomas maneches granitoid suit dated at 1690 1660 ma accompanied the deposition of these metasediments these rocks which collectively define the basement of eastern bolivia were subsequently deformed and metamorphosed during two discrete periods of orogenesis the oldest of these events the san ignacio orogeny deformed the terrane through three successive phases of deformation and was accompanied by voluminous syn tectonic granite intrusion and migmatisation of the 1690 ma sediments and intrusions u pb age data from igneous and metamorphic zircon constrain the san ignacio orogeny to between 1340 and 1320 ma while the orientation of structures suggests deformation was the result of w nw directed shortening a 300 million years hiatus during which the overlying sunsas group was deposited separated the san ignacio orogeny from the sunsas orogeny shortening during sunsas orogenesis resulted in an intense nw trending structural overprint in the southern and western extremes of the study area the intensity of deformation decreased toward the northeast such that in the san ignacio region sunsas tectonism resulted in distinct dome and basin fold interference patterns late syn to post tectonic granites were emplaced in the region of highest sunsas strain one of these intrusions the taperas granite yielded an age of ca 1075 ma when taken collectively these new data highlight a number of important points 1 the deposition of the chiquitania and san ignacio groups and the synchronous emplacement of the lomas maneches granitoid suite 1690 1660 ma have no temporal equivalents in the either the rondonia and mato grosso regions of brazil to the north or east or the proterozoic pre andean inliers to the west 2 the trend of the structures attributed to the san ignacio orogeny are oriented at a high angle to the nw trending pre sunsas margin of amazonia and 3 the sunsas orogeny represents the only geological event with a common age and direction of crustal shortening recorded in both the amazon craton and the rocks of eastern bolivia these data are interpreted to suggest that the rocks of eastern bolivia represent a geologically distinct microcontinent the paragua craton that was accreted to the southern margin of amazon craton during the sunsas orogeny

three tier transition of neoarchean ttg sanukitoid magmatism in the beit bridge complex southern africa abstract neoarchean ttg sanukitoid associations of contrasting scales occur within the beit bridge complex terrane of the limpopo complex in southern africa these include the smaller 2 65 2 63 ga avoca granitoid and the voluminous 2 73 2 64 ga alldays granitoid this study characterizes the wide compositional spectrum preserved in these two granitoids the elliptical avoca pluton consists of a biotite amphibole orthopyroxene clinopyroxene bearing core that is dominantly trondhjemite with less dominant tonalite and granodiorite variants and a thin amphibole biotite bearing granite rim with local occurrence of two pyroxene bearing metabasite boudins while both the core and rim rocks exhibit a linear fabric the granite in addition preserves a penetrative foliation field relations of granite enclaves in the core rocks together with available ages indicate that the core rocks intruded the granite the foliated biotite amphibole bearing alldays granitoid contains inclusions of older supracrustals and rocks of the messina layered intrusion and is widely distributed compositionally it include tonalites and granodiorites and to a lesser extent trondhjemites both the avoca core and rim rocks are characterized by difference in mineral chemistry with the mafic minerals mg rich in the ttg core while they are fe rich in the granite and metabasite in comparison biotite is mg rich and amphibole is fe rich in the alldays granitoid two groups of alldays ttg can be delineated in terms of whole rock geochemical characteristics and are comparable to the low to medium pressure ttg groups delineated by moyen 2011 while the avoca ttg is similar to the high pressure ttg group the lowest silica samples from each group of granitoid have geochemical characteristics comparable to archean sanukitoids with those from the avoca granitoid similar to low ti sanukitoids and those from the alldays granitoid similar to low ti and high ti sanukitoids separate petrogenetic models are suggested for different phases of the avoca core with the trondhjemite tonalites considered as high pressure melts of metabasalt while the granodiorite with lower sio2 content higher k2o and mgo contents and higher incompatible element contents than the trondhjemite tonalites is a product of hybridization of earlier ttg melts and peridodite granite from the avoca rim are low pressure melts of pre existing crustal lithologies the two groups of alldays ttg with lower sr y ratios than the avoca ttg are considered as low to medium pressure melts of metabasalt whose progressive interaction with peridotitic mantle at shallower angles account for the unique composition of alldays low ti and high ti sanukitoids taken together with their spatial and temporal transition from southeastern 2 73 2 72 ga low pressure ttg low ti sanukitoid to central 2 65 2 64 ga medium pressure ttg high ti sanukitoid to northwestern 2 63 ga high pressure ttg low ti sanukitoid parts of the beit bridge complex the three tier transition of ttg sanukitoid magmatism argues for the southern margin of the beit bridge complex to represent an active arc in the neoarchean

geological evolution of the basement rocks in the east central part of the rondonia tin province sw amazonian craton brazil u pb and sm nd isotopic constraints on the basis of geologic petrologic and u pb geochronologic data the basement rocks in the east central part of the rondonia tin province rtp southwestern amazonian craton are grouped into five lithologic associations 1 tonalitic gneiss 1 75 ga 2 enderbitic granulite 1 73 ga 3 paragneiss 4 granitic and charnockitic augen gneisses 1 57 1 53 ga and 5 fine grained granitic gneiss and charnockitic granulite 1 43 1 42 ga the first three are related to development of the paleoproterozoic rio negro juruena province and represent the oldest crust in the region the tonalitic gneisses and enderbitic granulites show calc alkaline affinities and nd isotopic compositions initial end 0 1 to 1 5 tdm of 2 2 2 1 ga that suggest a continental arc margin setting for the original magmas the paragneisses yield tdm values of 2 2 2 1 ga suggesting that source material was primarily derived from the ventuari tapajos and rio negro juruena crusts but detrital zircon ages and an intrusive granitoid bracket deposition between 1 67 and 1 57 ga the granitic and charnockitic augen gneisses show predominantly a type and within plate granite affinities but also some volcanic arc granite characteristics the initial end values 0 6 to 2 0 indicate mixing of magmas derived from depleted mantle and older crustal sources these rocks are correlated to the 1 60 1 53 ga serra da providencia intrusive suite that reflects inboard magmatism coeval with the cachoeirinha orogen located to the southeast the fine grained granitic gneiss and charnockitic granulites represent the first record of widespread magmatism at 1 43 1 42 ga in northern rondonia their geochemical signatures and the slightly positive initial end values 0 7 to 1 2 are very similar to those of the most evolved granites of the calc alkaline santa helena batholith farther southeast u pb monazite and sm nd whole rock garnet ages demonstrate that a high grade tectonometamorphic episode occurred in this region at 1 33 1 30 ga this episode attained upper amphibolite conditions and is interpreted as the peak of the rondonian san ignacio orogeny the u pb and sm nd data presented here and data published on rapakivi granites elsewhere indicate that the east central part of the rtp is a poly orogenic region characterized by successive episodes of magmatism metamorphism and deformation between 1 75 and 0 97 ga

reactive transport modeling an essential tool and a new research approach for the earth sciences abstract reactive transport modeling is an essential tool for the analysis of coupled physical chemical and biological processes in earth systems and has additional potential to better integrate the results from focused fundamental research on earth materials appropriately designed models can describe the interactions of competing processes at a range of spatial and time scales and hence are critical for connecting the advancing capabilities for materials characterization at the atomic scale with the macroscopic behavior of complex earth systems reactive transport modeling has had a significant impact on the treatment of contaminant retardation in the subsurface the description of elemental and nutrient fluxes between major earth reservoirs and in the treatment of deep earth processes such as metamorphism and magma transport active topics of research include the development of pore scale and hybrid or multiple continua models to capture the scale dependence of coupled reactive transport processes frontier research questions that are only now being addressed include the effects of chemical microenvironments coupled thermal mechanical chemical processes controls on mineral fluid reaction rates in natural media and scaling of reactive transport processes from the microscopic to pore to field scale

the serra da graciosa a type granites and syenites southern brazil part 2 petrographic and mineralogical evolution of the alkaline and aluminous associations abstract the serra da graciosa granites and syenites comprise five distinct plutons in the brasiliano pan african graciosa a type province southern brazil six petrographic series can be identified in these plutons 1 alkaline series 1 composed of amphibole bearing alkali feldspar syenites with varied mafic mineralogy and quartz contents from alkali feldspar syenites with calcic amphibole clinopyroxene olivine and allanite to alkali feldspar quartz syenites with sodic calcic amphibole and chevkinite perrierite and to alkali feldspar granites with sodic amphibole 2 alkaline series 2 characterized by amphibole bearing alkali feldspar granites with limited modal variations but amphibole compositions also varying from calcic to sodic 3 alkaline series 3 of limited occurrence which includes alkali feldspar syenites with olivine and clinopyroxene and no amphibole 4 aluminous series 1 of widespread occurrence with various petrographic facies of biotite granites with amphibole 5 aluminous series 2 characterized by alkali feldspar granites with biotite and only minor amphibole 6 monzodiorites typically with biotite calcic amphibole and augitic clinopyroxene partially mingled with granitic magmas the mafic minerals present are in general fe rich with correspondingly low mg and al contents in alkaline series 1 amphiboles crystallized in progressively more oxidizing and alkaline conditions while in alkaline series 2 the initial conditions were somewhat more oxidizing and shifted to reducing in the final stages in aluminous series 1 and aluminous series 2 amphiboles are calcic and comparatively homogeneous the amphiboles in the monzodioritic rocks while also homogeneous are more mg rich and show compositions quite distinct from the calcic varieties in the other associations and this is also the case for clinopyroxene mg in biotite decreases from the monzodioritic rocks to aluminous series 1 and further to aluminous series 2 contrasting evolution of the various associations suggests that several coeval magmatic series are present in the serra da graciosa granites

lattice boltzmann based approaches for pore scale reactive transport important geoscience and environmental applications such as geologic carbon storage environmental remediation and unconventional oil and gas recovery are best understood in the context of reactive flow and multicomponent transport in the subsurface environment the coupling of chemical and microbiological reactions with hydrological and mechanical processes can lead to complex behaviors across an enormous range of spatial and temporal scales these coupled responses are also strongly influenced by the heterogeneity and anisotropy of the geologic formations reactive transport processes can change the pore morphology at the pore scale thereby leading to nonlinear interactions with advective and diffusive transport which can strongly influence larger scale properties such as permeability and dispersion therefore one of the greatest research challenges is to improve our ability to predict these processes across scales doe 2007 the development of pore scale experimental and modeling methods to study reactive processes involving mineral precipitation and dissolution and biofilm dynamics allows more fundamental investigation of physical behavior so that more accurate and robust upscaled constitutive models can be developed for the continuum scale a pore scale model provides fundamental mechanistic explanations of how biogeochemical processes and pore scale interfacial reactions alter flow paths by pore plugging and dissolving under different geochemical compositions and pore configurations for example dissolved co2 during geological co2 storage may react with minerals in fractured rocks confined aquifers or faults resulting in cementation and or dissolution and altering hydrodynamics of reactive flow this can be observed in a natural analogue where primary porosity in sandstone is cemented by carbonate precipitates affecting dissolved co2 flow paths at the little garde wash fault utah e g fig 1a b several other examples demonstrating macroscopic characteristics of calcium carbonate caco3 precipitation in figure 1 include an elongated concretion along the groundwater flow direction caco3 precipitation along the vertical pathway sealed

geology of the campanian m1 sandstone oil reservoir of eastern ecuador a delta system sourced from the amazon craton abstract the campanian m1 sandstone member of the napo formation is one of the main reservoirs of the capiron tiputini oil play system of the eastern part of the oriente basin of ecuador this study presents a geological model for this reservoir at the apaika and nenke oil fields included within the block 31 and the eden yuturi oil field operated by petroamazonas ep the geological characterization of the m1 sandstone is based on an integrated study which includes core description electric log analyses and seismic data interpretation the sedimentological descriptions from available cored sections show that m1 sandstone records a variety of fluvial to shallow marine processes including mass flow and delta front turbidity currents not previously documented the stratal stacking pattern interpreted from electric logs indicates that the succession reflects deltaic progradation rather than the backstepping pattern typically displayed by the underlying albian to santonian sediments of the napo formation seismic rms amplitude defines clinoform geometries with sigmoidal sand body architecture prograding from east to west correlation with neighboring regions in colombia suggest that the m1 sandstone was probably part of a large river dominated delta system sourced from the east during major campanian progradation the location of the source area of sediment was determined using u pb detrital zircon dating provenance ages are uniform within the m1 sandstone cluster in a population that ranges from 1 4 to 1 6 billion years these ages indicate that sands were derived mostly from erosion of the lithotectonic rio negro jurena province located to the northeast of the studied area detrital ages from the overlying maastrichtian tena formation include ages between 80 and 200 ma pointing to an important change in the source areas located in the eastern cordillera of ecuador the change in sediment supply from east to west recorded in the tena formation coincide with the initial episodes of the andean orogeny and may be driven by the collision of fragments of the caribbean large igneous province clip against the continental margin of northwestern south america

migmatization and secondary granitic magmas effects of emplacement and crystallization of primary granitoids in southern bohemia austria the sauwald and muhl zones of the prebatholithic moldanubian middle crust in northern austria contain metapelites and metaluminous to weakly peraluminous metagreywackes respectively both zones were affected by low pressure high temperature metamorphism and anatexis the metapelites of the sauwald zone became in situ diatexites probably by fluid absent reactions involving the breakdown of muscovite and the partial breakdown of biotite the biotite plagioclase quartz gneisses of the muhl zone experienced only slight melting following this event and while the mid crust was still hot additional heat was locally advected into the muhl zone by the intrusion of the weinsberg granite this brought about fluid present partial melting of the biotite plagioclasequartz gneisses producing relatively large volumes of metaluminous to weakly peraluminous i type schlieren granite this cool wet restite rich magma remained close to its site of generation thus infracrustal i type granitoids may be formed anywhere in the crust and not always at high t under special circumstances the heat and fluids from granitic magmas can spawn secondary granites also relatively low initial 87sr 86sr values of around 0 707 in i type rocks do not necessarily indicate either lower crustal magma sources or mixing with mantle derived magma the weinsberg granite magma came from the lower crust p probably 700 mpa where widespread fluid absent breakdown of biotite plagioclase quartz assemblages occurred the necessary high heat flow was probably provided by newly underplated mafic magmas however these seem not to have mixed or mingled with the crustally derived weinsberg magmas deep equivalents of the muhl zone metagreywackes may have formed the weinsberg protolith fluid absent experiments show that the melting temperature probably exceeded 850 c and that a garnet bearing orthopyroxene rich residue should be present in the lower crust fluid present experiments demonstrate that the availability of free h2o can radically alter the characteristics of the partial melts from apparent s type mineralogy with fluid absent melting to i type mineralogy with wet melting

early cretaceous na rich granitoids and their enclaves in the tengchong block sw china magmatism in relation to subduction of the bangong nujiang tethys ocean abstract the na rich intermediate to felsic granitic rocks provide insights into the generation of magmas in subduction zones this paper presents zircon la icp ms u pb ages as well as whole rock geochemical mineral chemical and in situ zircon hf isotopic data on na rich granitic rocks from the tengchong block sw china the granodiorites and associated mafic magmatic enclaves mmes from the menglian batholith yield zircon u pb ages of 116 1 0 8 to 117 8 0 6 ma and 117 7 0 7 ma respectively both host granodiorites and enclaves show calc alkaline and sodium rich nature enrichment in large ion lithophile elements liles and variable depletion in zircon hf isotopic compositions euhedral amphiboles in both granodiorites and associated enclaves are magnesian hornblende with high mg and ca and contain euhedral plagioclase inclusions of labradorite to andesine an36 57 composition the granodiorite was most likely derived through the mixing of partial melts derived from juvenile basaltic lower crust and a minor evolved component of ancient crustal sources the quartz monzodiorite granodiorites and associated mmes from the xiaotang mangdong batholith yield zircon u pb ages of 120 3 1 3 to 122 6 0 8 ma and 120 7 1 5 ma these rocks are also sodium rich and show calc alkaline trend with negative zircon hf isotopic compositions 5 55 to 0 58 the mmes in the host intrusions are monzogabbro with variable and depleted zircon hf isotopic compositions the amphiboles in the both host intrusions and the enclaves show al rich ferro tschermakite composition we infer that the quartz monzodiorite granodiorites were derived from magmas generated by the melting of ancient basaltic rocks in the lower arc crust induced by the underplating of mantle derived mafic magmas the formation of the different types of na rich granitic rocks is correlated to the subduction of bangong nujiang tethyan ocean a comparison with magmatism in the northern magmatic belt suggests that mantle derived magmas played an important role in the genesis of early cretaceous intrusions from tengchong to lhasa blocks although crustal melting is the dominant contributor

origin of rectangular and aligned lakes in the beni basin of bolivia abstract the origin of aligned rectangular shaped lakes in the beni basin of bolivia has been investigated utilizing landsat imagery shuttle imaging radar and the available scientific literature the lakes tend to line up in two preferred orientations northwest and northeast they occur in cenozoic sediments of the beni basin situated between the brazilian shield and the andean foredeep available evidence suggests that the shapes and orientations of the lakes are primarily controlled by a nearly orthogonal fracture pattern propagated upward from the underlying granitic brazilian shield basement these orientations and shapes may reflect a reactivated ancient global fracture pattern or may be the result of an e w greatest principal compressive stress 1 and subsequent downwarping and uplifting of the beni basin during the cenozoic andean orogeny the lakes themselves may occupy down dropped basement blocks induced in the gently upwarped basement while unusual these lakes are not unique this suggests that transmission of basement structure through overlying sediments may occur in other regions but that special conditions such as little or no lateral compression of the sediments the absence of high trees and vegetation stability of surface material from drifting and a high water table are necessary to preserve and enhance such surface features

orogenic development of the argentinian chilean andes during the palaeozoic the geological development of the basement of the argentinian chilean andes during the palaeozoic is characterized by an almost continuous structural metamorphic and plutonic mobility ultrabasic magmatism occurred only in a few regions terrigenic psammopelitic sequences are widespread at all times metamorphism is mostly of low to intermediate pressure but of variable temperature late palaeozoic high pressure metamorphism only took place near the present pacific coast there are about four culminations of orogenic activity varying in space and time from the mid cambrian to the mid ordovician from the silurian to the mid devonian during the early carboniferous and at the permian triassic boundary fold structures strike in variable directions at different times on the whole the pacific margin of gondwana consolidated continuously from the shield region in the e towards the pacific during the palaeozoic

the rock physics and geochemistry of carbonates exposed to reactive brines when carbonate rich rocks are brought into contact with an acidic brine their mechanical and acoustic responses depend on many factors including ph porosity permeability effective stress and time these complexities hinder the understanding of processes such as hydrothermal fluid circulation seismicity and deep burial diagenesis the present study addresses how different lithofacies exposed to the same reactive brine undergo varying degrees of transformation and whether it is possible to remotely detect these phenomena in the earth results are presented from fluid injections carried out on a large and varied set of calcareous rocks under hydrostatic stress the output brine was analyzed for dissolved mineral concentrations and the rock porosity permeability axial strain ultrasonic velocity and images from electron microscopy were contrasted before and after injection microbialites were found to be the lithofacies most vulnerable to changes in their transport properties however all samples irreversibly compacted with the greatest strain in the most porous and permeable cores the most extreme structural changes discovered through imaging were the welding of microporous zones grain sliding and the fracturing of various phases observations are consistent with a chemically enhanced weakening of the rock frame that generated compliant pores the associated decrease in velocity of the dry rock can be approximated with linear relations that depend on both porosity and effective stress

plutonic rock types series discrimination of various granitoid series and related rocks abstract the streckeisen qap diagram discriminates four characteristic type series among the large variety of granitoids associated in intrusions three of them integrate varied compositions from basic to acid terms 1 the calcalkaline series with its low k or trondhiemite medium k or granodioritic high k or monzonitic variants 2 the alkaline series with its characteristic syenitic members 3 the tholeiitic series deprived of k feldspars the fourth 4 corresponds to mobilizates associated with migmatites of varied composition but always quartz rich the generalization in the qapf system allowing the plotting of undersaturated alkaline series provides an excellent visual display of the major associations and trends for the plutonic rocks

mode and timing of terrane accretion in the forearc of the andes in ecuador the volcanic basement of the ecuadorian western cordillera pallatanga formation and san juan unit is made up of mafi c and ultramafi c rocks that once formed an oceanic plateau radiometric ages from these rocks overlap with a hornblende 40 ar 39 ar plateau age of 88 1 6 ma age obtained for oceanic plateau basement rocks of the pinon formation in coastal ecuador and with ca 92 88 ma ages reported for oceanic plateau sequences in the caribbean and western colombia these results suggest that the oceanic plateau rocks of the western cordillera and fl at forearc in ecuador are derived from the late cretaceous caribbean colombia oceanic plateau intraoceanic island arc sequences rio cala group overlie the plateau in the western cordillera and yield crystallization ages that range between ca 85 and 72 ma the geochemistry and radiometric ages of island arc lavas from the rio cala group combined with the age range and geochemistry of their turbiditic volcaniclastic products indicate that the arc was initiated by westward subduction beneath the caribbean plateau they are coeval with island arc rocks of coastal ecuador las orquideas san lorenzo and cayo formations and colombia ricaurte arc these island arc

deep seismic profile of the amazonian craton northern brazil deep seismic profiling of the amazonian craton northern brazil reveals previously unknown deep structural features buried beneath a thin relatively flat phanerozoic sedimentary cover amazon basin the seismic signature of the precambrian crust is characterized by a transparent shallow basement down to 8 9 km followed by a highly reflective middle and lower crust middle and lower crustal events are dominated by abundant dipping asymmetric arcuate reflections and diffractions the shallowest of which occur beneath a structural arch jutai arch although wide angle refraction data are unavailable a decrease in the number of coherent reflections at about 15 s two way travel time 45 km and small drops in amplitude and increases in average frequency are interpreted to mark the moho the deep amazonian seismic line is located near the westernmost boundary of the amazonian craton imaging the rondonian province one of the youngest geochronological provinces of the craton middle proterozoic to the south the rondonian basement crops out revealing schists gneisses granulites and important volumes of anorogenic granites the prominent laterally extensive arcuate reflections observed within the middle and lower crust may be related to this anorogenic magmatic activity perhaps corresponding to layered intrusive complexes on the other hand intrusion of mantle derived material during an early paleozoic rifting as well as the mesozoic episodes of mafic intrusion may also have contributed to this seismic signature similar seismic signatures found in eastern australia and on cocorp profiles of the central u s craton suggest that such cratonic terranes may have undergone similar tectonic evolution in the upper crust probable fault plane reflections associated with ene wsw reverse faults of jurassic early cretaceous age may sole into the top of the reflective zone suggesting that the lower limit of the transparent upper crust may represent a rheological boundary

recognising biodegradation in gas oil accumulations through the δ13c compositions of gas components a large suite of natural gases 93 from the north west shelf and gippsland and otway basins in australia have been characterised chemically and isotopically resulting in the elucidation of two types of gases about 26 of these gases have anomalous stable carbon isotope compositions in the c1 c4 hydrocarbons and co2 components and are interpreted to have a secondary biogenic history the characteristics include unusually large isotopic separations between successive n alkane homologues up to 29 pdb and isotopically heavy co2 up to 19 5 pdb irrespective of geographic location these anomalous gases are from the shallower accumulations 600 1700 m where temperatures are lower than 75 c the secondary biogenic gases are readily distinguishable from thermogenic gases 74 of this sample suite which should assist in the appraisal of hydrocarbons during exploration where hydrocarbon accumulations are under 2000 m while dissolution effects may have contributed to the high 13c enrichment of the co2 component in the secondary biogenic gases the primary signature of this co2 is attributed to biochemical fractionation associated with anaerobic degradation and methanogenesis correlation between biodegraded oils and biodegraded dry gas supports the concept that gas is formed from the bacterial destruction of oil resulting in secondary biogenic gas furthermore the prominence of methanogenic co2 in these types of accumulations along with some isotopically depleted methane provides evidence that the processes of methanogenesis and oil biodegradation are linked it is further proposed that biodegradation of oil proceeds via a complex anaerobic coupling that is integral to and supports methanogenesis

analysis of light hydrocarbons in soil gases lost river region west virginia relation to stratigraphy and geological structures analyses of 471 near surface soil gas samples for light hydrocarbons c1 c4 c2l c3l and h2 from the lost river gas field in hardy county west virginia reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases which occur directly over the faulted eastern limb of the whip cove anticline compositional changes in the soil gases data clearly define major changes in the maturity and locations of potential source beds grids placed on botanically defined anomalies confirm a possible correlation between these two independent indicators statistical analysis shows that samples from 45 sites contain anomalously large concentrations of light hydrocarbons in the soil gas constituents large concentrations coupled with high saturate to olefin ratios further confirms that this active seepage is near macroseep levels variations in soil gas compositional trends separate the soil gas data into two domains with oilier compositions to the west and gassier compositions to the east although the composition of the shallow soil gases above the lost river gas field are oilier than the reservoir gases they occur directly over the eastern faulted limb of the producing anticlinal structure suggesting that the dry gases from the oriskany reservoir are probably mixed with oilier gases from organic rich strata among devonian shales the eastern anomalies are much gassier and are very similar to the oriskany gases produced by the lost river gas field the eastern anomalies directly overlie near vertical beds of devonian and older age formations that are likely conduits for deeper mature thermal gases

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

evidence for a relationship between hydrocarbons and authigenic magnetite establishing a relationship between hydrocarbon migration and the precipitation of authigenic magnetite in sedimentary rocks is of significant interest with respect to 1 elucidating mechanisms for remagnetization and establishing the origin of secondary magnetizations residing in magnetite 2 developing a method to date hydrocarbon migration events by determining the time of remanence acquisition by palaeomagnetic methods and 3 evaluating whether searching for anomalous concentrations of diagenetic magnetic minerals and or aeromagnetic anomalies is justified as a relatively inexpensive exploration tool the direct association of hydrocarbon migration and the precipitation of authigenic magnetite has however not been established in this paper we report palaeomagnetic rock magnetic petrographic and geochemical results of a study of samples from permian spelebthems and gilsonite found in the ordovician arbuckle group in southern oklahoma the results indicate that there is a genetic relationship between hydrocarbon migration and the precipitation of authigenic magnetite

the neogene evolution of the outer carpathian flysch units poland ukraine and romania kinematics of a foreland fold and thrust belt system abstract various techniques of geometric and kinematic analyses including cross section balancing and forward modeling have been applied successfully to the outer carpathian flysch units along regional geologic profiles crossing the belt from its undeformed foreland to the pieniny klippen belt western carpathians poland or to the central east carpathian allochthon eastern carpathians ukraine and romania the amounts of neogene shortening as well as the mode and the location of basement consumption are discussed on the basis of balanced cross sections compiled along the three most representative regional traverses in addition the complete kinematic evolution of complex carpathian structures such as the fore dukla triangle zone southeastern poland the borislav pokut duplexes ukraine and the marginal folds romania are described in more detail by using a forward modeling approach major results concern the tectonic heritage of laramian inverted structures mesozoic normal faults reactivated as reverse faults during the late cretaceous paleocene compression or transpression in the development of the neogene outer carpathian edifice pre eocene deformation and erosion effectively induce discontinuities in the mid cretaceous blackshale detachment horizon and help to localize the neogene deformations in complex triangle zones the thrust sequence itself evolves alternatively in a piggyback or in an out of sequence mode clearly the critical taper of the outer carpathians is preserved by a constant balance between erosion and thrust reactivations in the inner part of the wedge and sedimentation and tectonic accretion at its front

petrogenetic appraisal of early palaeozoic granitoids of kinnaur district higher himachal himalaya india abstract the early palaeozoic felsic magmatism of kinnaur district is represented by more or less equivalent akpa granitoids 477 29 ma and rakcham granitoids 453 9 ma which form an integral part of the central crystalline zone in the southern portion of the tethys himalayan tectogen both akpa granitoids ag and rakcham granitoids rg intrude the rocks of the vaikrita group substantially low magnetic susceptibility values c 0 016 0 187 10 3 si of these early palaeozoic granitoids suggest their nature similar to as ilmenite series reduced type granitoids emplaced in a syn collisional tectonic setting the modal compositions of ag mainly correspond to two mica muscovite biotite monzogranite belonging to the granitoids generated by crustal fusion based on mineral assemblage ms bt tur ap associated skarn type tungsten mineralization occurrence of metasedimentary enclaves and whole rock geochemistry sio2 70 75 72 95 wt tio2 0 02 0 17 wt k2o na2o 1 05 2 32 molar a cnk 0 93 to 1 33 cipw corundum 0 17 to 3 82 wt av sr 304ppm initial 87sr 86sr 0 7206 0 00235 the ag can be typically characterized as peraluminous s type granite as similarly noted for rg higher rb sr ratio and initial 87sr 86sr 0 737 0 002 of rg compared to that of ag however indicate involvement of relatively more crustal component in generation of rg the ag is enriched in ba and light rare earth elements lree the chondrite normalized ree patterns of ag appear weakly fractionated lan lun 1 06 6 43 and show pronounced negative eu anomaly eun eu 0 03 the ag and rg represent typical s type granite suites that largely evolved due to differential degree of anatexis partial fusion of sedimentary protoliths prior to caledonian but after the pan indian thermal orogenic event

miogeoclines and suspect terranes of the caledonian appalachian orogen tectonic patterns in the north atlantic region the caledonian appalachian orogen was formed by the closing of a paleozoic iapetus ocean the continental margins of iapetus are identified in the deformed early paleozoic miogeoclines of the caledonian appalachian orogen ophiolitic vestiges of iapetus its oceanic plateaus microcontinents and volcanic arcs are caledonian appalachian suspect terranes these were assembled in interior parts of the orogen and locally they were emplaced structurally upon the adjacent miogeoclines the modern north atlantic ocean opened along an axis that traversed the paleozoic orogen longitudinally its opening dispersed the elements of the paleozoic orogen and led to the present arrangement of disjunct paleozoic miogeoclines and suspect terranes throughout the north atlantic borderlands the western or north american margin of iapetus is represented by the miogeoclines along the west flank of the north american appalachians and caledonides of east greenland a small north american miogeoclinal segment occurs in the britis

evolution in composition of granitic rocks controlled by time dependent changes in petrogenetic processes examples from the archaean of eastern finland abstract the grey gneisses of eastern finland form the basement on which the archaean greenstone belts were developed they are composed of tonalites trondhjemites and granodiorites ttg emplaced during two distinct magmatic episodes at 2 86 ga kivijarvi gneisses and 2 65 ga naavala petrogenetic models based on trace element behaviour indicate that the ttg are ultimately derived from the mantle by a mechanism involving two stages 1 1 melting of the mantle to form a tholeiitic crust and 2 2 re melting of these tholeiites transformed into garnet amphibolites to yield ttg rocks the low yb contents of archaean ttg are explained by the influence of residual garnet and hornblende in the source rocks involved in magma genesis after 2 5 ga the role of these minerals becomes progressively less important and this is interpreted in terms of a cooling earth model at present the subducted oceanic crust is relatively cold and the geothermal gradient at its interface with the overlying mantle wedge is low in addition the descending lithospheric slab undergoes dehydration before partial melting is able to occur and the liberated fluids pass upward into the mantle wedge or base of the continental crust where partial melting may begin these magmas have calc alkaline affinities and have equilibrated with residual olivine and pyroxene in the source consequently the liquids derived from partial melting have high yb contents during the archaean subducted oceanic crust was relatively hot and the geothermal gradients in the interface zone with the mantle were high c 20 25 c km 1 thus partial melting of the subducted oceanic material was possible at temperatures between 650 and 700 c before dehydration would take place residual phases in the source would include garnet and hornblende thus explaining the low yb contents of the ttg association it is concluded that there has been a change in the site of cale alkaline magma genesis since archaean times

detailed compositional analysis of hydrocarbons in soil gases above multi horizon petroleum deposits a case study from western poland abstract a surface geochemical survey was carried on in an area with a cluster of petroleum deposits in western poland the largest of these accumulations is the paproc cicha gora dry gas deposit reservoired in the rotliegend sediments other similar gas deposits are the paproc gas field discovered in the top part of the carboniferous formation and the paproc w paproc n and nowy tomy l gas fields localized in the zechstein limestone reef structures moreover the paproc gas condensate and the jastrzebsko oil deposits were found at shallower depths in the main dolomite formation this paper aims to analyze the variability of composition and distribution of near surface gaseous hydrocarbons in relation to hydrocarbon accumulations reservoired in various stratigraphic units at depths the occurrence of ch 4 its heavier homologues and unsaturated hydrocarbons was measured in 267 soil gas samples the sampling sites were located along six survey lines a a b b c c d d e e and f f at a 200 m spacing concentrations of ch 4 total alkanes c 2 c 4 and total alkenes c 2 c 3 reached maxima of 0 26 vol 5 32 ppm and 5 88 ppm respectively soil gas alkanes heavier than ch 4 were interpreted as being derived from subsurface hydrocarbon accumulations these compounds penetrated to the near surface zone by means of active and passive microseepage source strength of alkanes microseepage from subsurface accumulations to the land surface determined by the c 2 h 6 c 2 h 4 ratio is not a factor that substantially changes the patterns of surface anomalies analysis of the values of ch 4 c 2 h 6 and c 2 h 6 c 3 h 8 coefficients confirmed the presence of compositionally diversified deep hydrocarbon accumulations the study revealed that even small accumulations of gas condensate produce very strong surface geochemical signatures contrasting with dry gas deposits the reason may be a specific effect of hydrocarbon migration within the existing petroleum system the main dolomite which is both the source and the reservoir rock in the fore sudetic monocline plays the role of a filter which modifies the flux of gases ascending from deep seated reservoirs as a result of such filtering the geochemical signature of hydrocarbons recorded at the surface indicates the dominance of gas condensate and oil gas accumulations at depths giving rise to obviously incorrect interpretations these interpretations were evidenced by compositional analysis of soil gases which did not explicitly confirm the existence of a dry gas deposit in the rotliegend reservoir paproc cicha gora gas field on the other hand the identified anomalies generally reflect the outline of this accumulation it supports the applicability of surface geochemical surveys to hydrocarbon exploration in the fore sudetic monocline the presence of the main dolomite at shallower depth must be taken into account in the interpretation of data used for positioning of seismically identified rotliegend structures the results of this project indicate that the main dolomite may obscure the geochemical signature of near surface gases derived from dry gas accumulations in both the deeper rotliegend and carboniferous reservoirs

origin of the bolivian andean orocline a geologic study utilizing landsat and shuttle imaging radar abstract the origin of the major andean orocline in bolivia where the north trending southern andes abruptly change direction by 45 and continue northwestward into peru has been investigated by photogeologic techniques utilizing landsat imagery and shuttle imaging radar sir a in conjunction with published literature and maps the evidence suggests that this orocline might have been formed beginning in the miocene by the pacific coastal region being bent around the resistant brazilian shield as a result of the relative motions between the eastward moving coastal region and the westward moving south american continent the eastward motion of the coast was further increased about 10 m y b p when the opening of the east pacific rise increased the rate of eastward motion of the subducting nazca plate the brazilian shield with its deep roots provided greater resistance to this motion resulting in the decoupling of the andes along a wide left lateral strike slip fault zone extending east west from the altiplano through santa cruz and into the interior indicated left lateral motion across this fault decreases from 180 km on the east side of the andes to 50 or 60 km in the altiplano the difference being taken up in compressing the eastern cordillera and sub andean foredeep from an average width of 350 km in southern bolivia and argentina to an average width of 220 km in northern bolivia and peru the resistance of the shield may also have resulted in counter clockwise rotation of and lateral angular shear in the andes north of the break zone

the geology and mesozoic collisional history of the cordillera real ecuador abstract the geology of the metamorphic rocks of the cordillera real of ecuador is described in terms of five informal lithotectonic divisions we deduce that during the mesozoic repeated accretionary events occurred and that dextral transpression has been of fundamental importance in determining the tectonic evolution of this part of the northern andes the oldest event recognised of probable late triassic age may be related to the break up of western gondwana and generated a regional belt of s type plutons during the jurassic major calc alkaline batholiths were intruded following this in latest jurassic to early cretaceous time a volcano sedimentary terrane of possible oceanic or marginal basin origin the alao division and the most westerly gneissic chaucha arenillas terrane were accreted to continental south america the accretion of the oceanic western cordillera took place in latest cretaceous to earliest tertiary time this latter event coincided with widespread thermal disturbance as evidenced by the large number of young k ar mineral ages recorded from the cordillera real

causes and spatial distribution of anomalous magnetization in hydrocarbon seepage environments in diagenetic environments containing hydrocarbons the most important magnetic minerals formed appear to be magnetite and pyrrhotite whereas the most important magnetic mineral dissolved or replaced is hematite hence hydrocarbon seepage and migration may result in positive absent or negative anomalous magnetization magnetic contrasts relative to the total magnetization prior to hydrocarbon invasion depending on the amounts of magnetite and pyrrhotite formed relative to the amounts of hematite destroyed where anomalous magnetization is formed by hydrocarbon seepage the magnetic contrasts are located in relatively reducing geochemical plumes such plumes are located above leaking subsurface hydrocarbon accumulations if buoyancy is the predominant driving force for hydrocarbons commonly however migration of the escaping hydrocarbons is dominated by advection hydrocarbon seepage therefore may be vertical or may have any direction other than vertical depending on the position of the leaking hydrocarbon accumulation in the prevailing flow system and on the distribution of high permeability pathways such as carrier beds and faults in these cases magnetic contrasts are deflected or displaced and they are not likely to be vertically above the hydrocarbon source anomalous magnetization magnetic contrasts has been documented from some hydrocarbon seepage environments and can be used for hydrocarbon exploration in association with other surface exploration methods similar magnetic contrasts should also be detectable in core and could be used for evaluation of subsurface proximity to hydrocarbon traps the magnitude and spatial distribution of such magnetic contrasts whether at the surface or in the subsurface depend on geochemical microbial geophysical sedimentologic and hydrogeologic processes these factors must be taken into consideration in aeromagnetic and ground based prospecting for hydrocarbons in particular the use of anomalous magnetization as an exploration tool should always be accompanied by a hydrogeologic study the results of this study suggest that magnetic contrasts could also be used in environmental studies for example to detect hydrocarbons leaking from storage tanks or waste disposal sites on the other hand magnetic contrasts may be generated also by natural and anthropogenic processes that have no genetic relationships to hydrocarbons consequently anomalous magnetization may or may not be genetically related to natural or anthropogenic hydrocarbon accumulations

permeability impairment of a limestone reservoir triggered by heterogeneous dissolution and particles migration during co2 rich injection 1 a co2 rich brine core flood experiment in calcite limestone for conditions representative of underground storage p 12 mpa and t 100 c was performed in order to explore the dissolution mechanisms arising at moderate co2 partial pressure 0 3 mpa an increase of the total porosity t accompanied by a persistent permeability k decrease was measured the mechanisms controlling this atypical anticorrelated k t relationship were investigated from the analysis of high resolution x ray microtomography images of the sample acquired before and after the experiment all the evidences converge to the conclusion that the ubiquitous decrease of permeability measured during the 44 h of dissolution is due to the clogging of a fraction of the macroporosity by microporous material triggered by the rearrangement of the detached undissolved particles this mechanism results in the development of low permeability zones bridging the macroporosity

surface geochemical exploration for hydrocarbons in the area of prospective structures of the lublin trough eastern poland abstract the search for crude oil and natural gas deposits is dominated by expensive seismic methods however less expensive and faster surface methods are available which may improve the effectiveness of petroleum exploration one of these methods is the surface geochemical survey the objective of this method is to detect record and analyse the trace amounts of hydrocarbons in soil gas samples our paper presents the results of such research carried out over the prospective structures of the lublin trough in the eastern poland three hundred fifty five soil gas samples were collected from 1 2 m depth below the surface which were analysed by gas chromatographic analysis for their molecular composition sampling sites were arranged along 5 sampling lines with 200 m spacing maximum concentrations of ch4 total alkanes c2 c4 total alkenes c2 c4 h2 co2 and o2 detected in our samples were 0 16 vol 0 95 ppm 6 79 ppm 0 37 vol 5 01 vol and 21 6 vol respectively the results demonstrate that alkanes recorded in the soil gas which are heavier than methane originated from the deeper hydrocarbon accumulations these components migrated to the near surface zone due to their active microseepage along the tectonic discontinuities and cracks in the overburden weak correlation of h2 and co2 with alkanes were found which precludes the application of both components as additional prospective indicators in the study area diverse activity of partial oxidation of alkanes to alkenes perhaps due to different rate of microseepage of hydrocarbons has been determined using the c2h6 c2h4 c2 c2 ratio the presence of numerous tectonic dislocations intensifies the microseepage and increases the amplitudes of geochemical anomalies recorded at the surface normalization of concentrations of methane total alkanes c2 c4 and total alkenes c2 c4 with their of background values which were calculated for separate subsets of c2 c2 ratio values significantly reducing the ranges and the amplitudes of hydrocarbons anomalies it allowed for modification of the anomalous pattern due to diverse activity of partial oxidation of alkanes to alkenes statistical analysis of c2h6 c3h8 c2 c3 ratio values showed that in the study area accumulations of gas condensate and dry gas dominate with the strongest surface geochemical signals the surface geochemical results were integrated with the structural map of the top of devonian formations as well as we when displayed as trend integrated with geological cross sections the surface alkane anomalies generally occur along the major nw se trending chain of structural uplifts located between abramow glinnik and nasutow this zone is most prospective and indicates the highest petroleum potential changes in normalized concentrations of total alkanes c2 c4 allowed the identification of 9 anomalous zones indicating the presence of deep hydrocarbon sources the no 2 anomaly which was localized at the intersection of the b b and the e e sampling lines seems to be the most prospective the accumulations of dry gas or gas condensate likely occur in the anticline formed by devonian and or carboniferous strata this zone should be opened for confirmation by drilling the geochemical lead should now be integrated with all available geological seismic data for prospect development our study demonstrates that the results of surface geochemical surveys combined with the results of geological studies and geophysical surveys provide a valuable source of comprehensive information about the explored area

predictions of oil or gas potential by near surface geochemistry recent advances in surface geochemical prospecting have enabled age old seep detection technology to be used to determine the gas versus oil character of a potential fairway extensive field work has demonstrated that the chemical compositions of near surface hydrocarbon soil gases measured by flame ionization gas chromatography are largely determined by the hydrocarbons in nearby underlying reservoirs by using compositions and ratios of the light hydrocarbons methane ethane propane and butane one may predict whether oil or gas is more likely to be discovered in the prospect area near surface hydrocarbons are best represented by normalized histograms of composition data these histograms are strongly correlative with those of reservoir gas and with compositions o gas from shows recorded in downhole mud logging this correspondence with the actual formation gases suggests that the upward migration of reservoired light hydrocarbons into near surface soils represents a viable mechanism allowing surface geochemical exploration to be utilized for regional hydrocarbon evaluations geochemical profiles over known production areas are shown for the sacramento and san joaquin basins in california and for the utah wyoming overthrust belt geochemical predictions were documented by subsequent drilling near the pineview field in utah the data imply that the pineview field should extend westward into an area containing a dry hole in addition a new gas discovery on the intersection of a landsat lineament and a large methane anomaly was made 6 5 km 4 mi northeast of the pineview field by amoco in 1981 most of the geochemical examples reported show direct anomalies over known fields however seeps can be laterally displaced in certain geologic settings in addition geochemical investigations indicate that seep magnitudes depend on tectonic activity to aid gas migration along faults and fractures which appear to provide the major migration pathways this fault association suggests that diffusion is of secondary importance geochemical prospecting must be used with caution and only in conjunction with geologic and geophysical tools because the location and shape of many geochemical anomalies are governed more by the local tectonic structure of the region than by the position and shape of the actual deposit regional groundwater flow is less significant thus geochemical prospecting when used alone cannot predict whether a particular soil gas anomaly is associated with a commercial deposit it can only be used to verify the presence of petroleum hydrocarbons and to predict whether gas or oil is likely to occur in a potential structure geochemical prospecting yields excellent regional evaluations of hydrocarbon potential

implications of pore microgeometry heterogeneity for the movement and chemical reactivity of co2 in carbonates abstractwe studied the heterogeneity of natural rocks with respect to their pore size distribution obtained from mercury intrusion capillary pressure micp tests at a scale about one fifth of the standard plug size 2 5 cm we investigated two fontainebleau sandstone and two limestone samples we found that at the scale of the micp tests heterogeneities are practically nonexistent still there are large differences in the capillary curves from one rock type to another also carbonate rocks unlike fontainebleau sandstone show heterogeneities at a scale smaller than the scale used in micp tests as seen by the complexity in the mercury saturation versus pressure curves we used this diversity between the capillary curves and this complexity within a single capillary curve to obtain information about the movement and chemical reactivity of co2 in carbonates the method consists of three steps first subdividing the carbonate pore system into microstructural facies each of them having a specific ra

mechano chemical interactions in sedimentary rocks in the context of co2 storage weak acid weak effects abstract due to the corrosive nature of dissolved co2 the potential short or long term alteration of rock properties represents a major issue in several sites where natural co2 circulation is observed as well as in reservoirs targeted for storage of anthropogenic co2 to date this has been primarily studied from a transport chemical perspective with laboratory evidence of microstructural modifications together with the consequences for flow properties compared to the transport chemical aspects the mechanical chemical aspects have been less investigated though it is to be expected that mechanical properties e g elastic properties failure parameters and time dependent mechanical behaviour could potentially be affected in a similar manner to hydraulic parameters yet since co2 is a weak acid the ph drop is expected to be moderate with a likely lower limit close to 4 0 the buffering of ph by calcite minerals present in most reservoirs targeted for storage may further limit the ph drop as well as confining it to a localized rock volume around the injection well this leads to the question of the magnitude and time spatial scales of chemically mediated mechanical processes during co2 sequestration the authors propose to address this issue by reviewing recent laboratory based studies restricted to sedimentary rocks namely reservoir rocks carbonate or sandstone intact or fractured caprocks and fault rocks key findings include the following 1 the short term impact on the elastic and inelastic behaviour of intact caprocks remains limited 2 shear strength weakening is likely to be respectively low and low to moderate for shale clay rich and anhydrite rich faults but without modifying slip stability in either case 3 the largest impact is located within carbonate reservoirs but with a broad range of reported responses depending on hydrodynamic conditions closed or open and on dissolution regime uniform or channelling and 4 creep experiments confirm that co2 induced dissolution may enhance long term compaction of carbonate reservoirs but the magnitude of acceleration varying from non significant to 50 times depends to a large extent on site specific conditions grain size ph temperature effective stress state etc which renders any direct extrapolation from laboratory to reservoir scale difficult finally some directions for future research studies are discussed

time lapse acoustic transport and nmr measurements to characterize microstructural changes of carbonate rocks during injection of co2 rich water abstractwe investigated how changes induced in the microstructure of carbonate rocks by the injection of co2 rich water affect pore network properties in particular we investigated from multiple perspectives the microstructural changes and types of porosity that alter the observable geophysical properties we thereby refined our understanding of induced modification of the pore network our experimental protocol included a suite of time lapse acoustic transport and nuclear magnetic resonance nmr measurements along with scanning electron microscopy sem and ct scan images these gave us complementary sensitivity to changes in different properties of the pore space induced porosity variations were smaller than in previous reported results because of chemomechanical compaction resulting from dissolution under pressure no porosity enhancements larger than 0 8 pu were observed results indicated that dissolution occured primarily in the grain coating cement and the microporosity of the micritic phase

structural and gold mineralizing evolution of the world class orogenic mana district burkina faso multiple mineralizing events over 150 million years abstract the mana district located in the northern part of the birimian hounde greenstone belt in western burkina faso is a world class paleoproterozoic orogenic gold district 8 moz including five gold deposits fofina nyafe siou wona kona and yaho these deposits are located in specific lithostratigraphic domains and gold is controlled by various structural features deposit and regional scale mapping intrusion age and geochemistry as well as airborne aeromagnetic and electrical resistivity geophysical data were used to decipher the tectonic evolution of each gold deposit and the district five deformational and four gold mineralizing events were recognized the first deformation event d1md e w oriented shortening affected the metamorphosed volcanic and sedimentary rocks of the lower birimian group this early deformation episode was correlated with the formation of gently n plunging folds f1md and n s striking thrusts faults coeval with emplacement of the pre to synkinematic wona kona and siou plutons dated at 2172 ma under greenschist facies metamorphism the quartz carbonate veins v1md at fofina and siou formed during d1md at eoeburnean time manifesting the first gold event at approximately 2172 ma the following deformation event d2md e w oriented extension is associated with the deposition of the upper birimian group mana basin overlying the lower birimian group the geometry of the mana basin is controlled by the mana and maoula shear zones the tarkwaian type rock formation overlying the upper birimian group controlled by the wona kona and siou shear zones is constrained at the end of d2md or at the beginning of the d3md event with a maximum deposition age at 2113 ma the third deformation event d3md e w to wnw ese transpression affected the entire supracrustal rock such event is correlated with the formation of map scale f3md folds and dextral shear zones during the eburnean orogeny 2113 2090 ma a second gold mineralizing event occurred during d3md and is manifested by quartz carbonate veins v3md and disseminated sulfides at the yaho fofina and nyafe and possibly wona kona deposits the fourth deformation event d4md nnw sse transpression is correlated with sinistral shearing along the major transcurrent faults and the development of asymmetric nne striking folds f4md associated with vertical fold axes syn d4md mineralization is characterized mainly by a strong silicification si4md with disseminated pyrite and arsenopyrite along the wona kona shear zone and by tiny quartz carbonate veinlets v4md this event is considered the main gold bearing event in the western margin of the mana district the fifth and last deformation event d5md is brittle in character and was responsible for the formation of e w subvertical crenulation cleavages and reverse faults under overall n s shortening this late deformation event is tentatively associated with a last gold event recorded as free gold associated with muscovite in brittle fractures developed in competent orebodies at the wona kona and siou deposits this event could be as young as 2022 ma the age obtained from ar ar datation of muscovite schists at the wona kona deposit our main contribution is that we decipher multiple gold mineralizing events at the district scale based on deposit and regional scale mapping it is interpreted that gold was introduced as early as 2172 ma and possibly as late as 2022 ma during at least 3 or even 4 shortening tectonic events in a timeframe not yet recognized at the district scale for all the birimian belts

palaeoproterozoic dome forming structures related to granulite facies metamorphism jequie block bahia brazil petrogenetic approaches abstract dome structures in the sao francisco craton northeast brazil have been identified in the regions of brejoes and santa ines within the granulitic jequie block domes are mainly composed of ch6 charnockites which intrude various granulites of the basement the brejoes dome is surrounded by a rim of supracrustal rocks the enderbite charnockite suites ch1 and ch2 which are partially included in the basement and were previously dated ca 2 8 2 7 ga are similar to modern calc alkaline suites and could be derived by partial melting of an enriched tholeiite degree of fractionation ca 24 23 an older archaean component is suggested by negative nd values at 2 8 2 7 ga which range between 1 and 4 and nd model ages with an average ca 3 1 ga these magmatic suites are ascribed to the amphibolite facies based on preserved prograde mineral assemblages and results of geochemical modelling the ch6 rocks formed in domes are of granitic composition they mainly derive from partial melting of the 2 7 ga ch2 basement suite the hercynite quartz assemblage in supracrustal rocks surrounding the brejoes dome suggests very high temperature 1000 c for the ch6 magmas which would have induced partial melting in adjacent supracrustal rocks forming garnet cordierite bearing granitic melts emplacement of domes is constrained by the age of the granulite facies metamorphism in the jequie block ca 2 1 ga and the average cooling age of monazite which occurred in the syn dome charnockite and the surrounding heterogeneous granulites of the basement at 2045 2 5 ma a southern dome exhibits a younger monazite age of 2026 1 ma which may suggest a broader age span for the doming event

proterozoic early paleozoic evolution in western south america a discussion abstract the hypothesis of exotic terranes in peru bolivia argentina and chile generated discussions on the mode of transfer and extent of accretional events that may have occurred in the southern andes during the late proterozoic early paleozoic initially a tectogenesis based on autochthonous mobile fold belts was discussed following ideas emphasised the fragmentation of the supercontinent rodinia laurentia moving along the west gondwana border and colliding with the gondwana western margin the most important effect of this laurentia gondwana relationship was attributed to the argentine precordillera or cuyania terrane splitting off from laurentia and docking to gondwana in the early paleozoic in this study the most cited arguments for this laurentia precordillera relationship are discussed emphasising paleontological considerations it is shown that these arguments do not exclude a close original vicinity of the precordillera terrane to gondwana the precordillera terrane is suggested to be part of a hypothetical platform which developed between south america africa and antarctica safran platform and which was displaced to its actual position by transcurrent faults the collisional events in the sierras pampeanas ensued from strike slip movements and were responsible for the s and i type transpressional magmatism along the pampean and famatinian terranes the final result of this continent parallel movement of terrane slices is similar to that of a terrane split off from laurentia but the first named way of formation easier explains the general continuity of plate convergence at the western border of gondwana than the laurentia precordillera connection does

petrological constraints on crystallization conditions of mesoarchean sanukitoid rocks southeastern amazonian craton brazil we report petrological and geochemical data for the 2 87 ga rio maria sanukitoid granodiorite and associated rocks from mesoarchean granite greenstone terranes of the eastern amazonian craton brazil the dominant rocks have granodiorite to subordinate monzogranitic compositions with minor proportions of intermediate quartz diorites or quartz monzodiorites in addition to mafic endmembers occurring as layered rocks or as enclaves the mineral assemblage is dominated by amphibole plagioclase biotite and epidote minerals all of inferred magmatic origin pyroxenes being notably absent textural and compositional criteria indicate that amphibole is a principal mineral on the liquidus of all the rio maria rocks crystallization conditions have been derived from a comparison between natural phase assemblages proportions and compositions and experimental studies carried out on similar magma compositions the comparison shows that the parental magmas were water rich with more than 7 wt dissolved h2o with crystallization temperatures in the range 950 680 c the mg mg fe ratios of both amphibole and biotite indicate fo2 conditions in the range nno 0 5 to nno 2 5 where nno is nickel nickel oxide buffer therefore pointing to both water rich and oxidizing conditions for sanukitoid magmas amphibole compositions indicate emplacement at around 200 mpa and record a high pressure stage of magma crystallization around 600 900 mpa sanukitoid magmas share two of the principal characteristics of modern arc magmas elevated redox state and volatile contents which suggest that they may have formed in a geodynamic environment broadly similar to present day subduction zones

40ar 39ar constraints on the age and thermal history of the urucum neoproterozoic banded iron formation brazil urucum is one of the youngest banded iron formations bifs yet its exact age remains uncertain 40 ar 39 ar geochronology on late diagenetic to early metamorphic cryptomelane from the urucum sequence reveals a minimum depositional age of 587 7 ma metamorphic braunite age spectra yield flat segments defining apparent ages of 547 3 ma to 513 4 ma interpreted as recrystallization or cooling ages metamorphic muscovite grains from a meta arkose interbedded with the bif yield reproducible plateau ages of 513 3 ma cryptomelane ages are interpreted to record the minimum timing of the extensional tectonic events that generated the graben systems at the southeastern border of the amazon craton including the chiquitos tucavaca aulacogen and the jacadigo basin which hosts the urucum banded iron formation structurally controlled hydrothermal alteration resulted in crystallization of braunite and muscovite during the interval between 547 and 513 ma coeval with post collisional decompression that resulted in granite emplacement e g sao vicente granite in the paraguay belt subsequent uplift and erosion of the precambrian sequence possibly succeeded metamorphism a chemically distinct cryptomelane generation identified in surface samples yield ca 60 ma results these ages provide evidence for supergene recrystallization after exhumation in the mesozoic cenozoic 40 ar 39 ar systematics implies that the oldest cryptomelane samples experienced ar loss probably via thermally induced diffusion this suggests that the urucum bif was buried and heated to significant temperatures in the late proterozoic consistent with present mineralogy and isotopic composition that do not reflect the original characteristics of the depositional environment

the nevados de payachata volcanic region 18 s 69 w n chile ii evidence for widespread crustal involvement in andean magmatism volcanism extending over 11 ma is represented in the rocks of the nevados de payachata region culminating in the formation of two large composite stratocones within the last 500 000 years chemically distinct mafic magmas are erupted at a number of parasitic centers these cannot be related to each other by crystal fractionation and do not appear to be direct parents for the differentiated suites of the composite cones two distinct trends are defined by the intermediate and evolved rocks a high lile large ion lithophile element tio2 and ce yb lineage among the youngest rocks including the two major stratocones and a more typical calc alkaline trend among the older 1 ma rock types within individual volcanic centers differentiation involves fractionation of plagioclase pyroxene and hornblende with biotite and k feldspar in the more evolved rock types isotopic compositions sr pb nd o vary little with differentiation from basaltic andesite to rhyolite or with age contamination during differentiation from basalt to rhyolite may occur but the most mafic rocks erupted in the region are already enriched in incompatible trace elements and therefore may be insensitive to the effects of interaction with the crust the majority of data are similar to baseline compositions cenozoic parental magmas from other parts of the central andes and may reflect a relatively homogeneous magma source or source mixture throughout this central volcanic zone cvz which is distinct from the southern and northern andes and from island arc volcanic rocks

chemical and sr nd isotopic compositions and zircon u pb ages of the birimian granitoids from ne burkina faso west african craton implications on the geodynamic setting and crustal evolution abstract the west african craton was the site of massive juvenile crustal addition in the early proterozoic during the eburnean orogeny 2 1 ga in this study we determined chemical and isotopic compositions as well as zircon u pb ages of granitoid samples from ne burkina faso these data were then used to constrain the chemical characteristics emplacement age and tectonic environment of these rocks in the context of the west african craton evolution birimian paleoproterozoic granitoids of ne burkina faso are associated with volcanic and volcano sedimentary sequences of the greenstone belts analyses indicate that the na rich birimian granitoids of tonalite trondhjemite granodiorite ttg composition were generated by partial melting of a garnet bearing amphibolite crust granite intrusions derived from partial re melting of the deep basement of felsic calc alkaline nature with contribution of metasediments zircon u pb isotopic analyses yielded ages between 2122 15 ma and 2181 7 ma for the birimian na rich granitoids and 2151 10 ma for a biotite granite ij10 sr nd isotopic data show very low initial sr composition i sr t 0 7007 positive epsilon neodymium values nd t 0 7 to 2 5 and sm nd model ages t dm of 2363 2558 ma these data are similar to those of mafic rocks of the pissila greenstone belt i sr t 0 7003 0 7015 nd t 3 2 to 4 1 t dm 2336 2552 ma and further suggest that the na rich granitoids are of juvenile nature the present study together with published data on the birimian volcanics and meta sedimentary rocks of the west african craton preclude contamination of the juvenile birimian rocks of ne burkina faso by reworked archean materials the birimian granitoids of ne burkina faso were mostly emplaced during the early stage 2 19 2 15 ga of a large scale crustal growth event 2 2 2 0 ga where large amount of juvenile materials were added to the crust from the depleted mantle in a tectonic environment comparable to modern volcanic arcs

early paleozoic tectonic evolution of the chinese south tianshan orogen constraints from shrimp zircon u pb geochronology and geochemistry of basaltic and dioritic rocks from xiate nw china traditionally the chinese south tianshan has been regarded as a late paleozoic orogenic belt however little is known about the early paleozoic tectonic architecture of the region this paper presents the first evidence of cambrian ordovician morb type basalts and adakitic diorites on the southern margin of the yili plate in china basalts from xiate in southwestern tianshan show a typical transitional t morb and ferrobasalt composition which indicate a formation at a propagating spreading ridge the basalts give a weighted mean 206pb 238u crystallization age of 516 3 7 4 ma by shrimp u pb zircon dating and have experienced contact metamorphism due to the intrusion of a dioritic pluton the dioritic pluton has a weighted mean 206pb 238u crystallization age of 470 12 ma and geochemical characteristics resembling that of adakitic rocks the pluton is considered to have been formed by partial melting of garnet amphibolites from thickened lower crust in arc or continental collision settings the basalts and diorites are considered to outline the eastern extension of the early paleozoic suture zone the nikolaev line which stretches east west for hundreds of kilometers between the northern tianshan and central tianshan terranes of kyrgyzstan our findings substantiate that the yili and central tianshan plates were separated by the early paleozoic terskey ocean the terskey ocean probably closed during the early stage of the late ordovician lomize et al in geotectonics 31 6 463 482 1997 resulting in the final amalgamation of the yili and central tianshan plates consequently an early paleozoic suture zone is documented in the chinese tianshan region which is most likely represented by the north nalati fault

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry