long lived localized magmatism in central eastern part of the pernambuco alagoas domain borborema province ne brazil implications for tectonic setting heat sources and lithospheric reworking magmatic activity circumscribed to a restricted area lasting for tenths of millions of years allows to link changes in magma chemistry to deformation and thus to determine the tectonic evolution of a region here we describe an area of about 180 40 km in eastern borborema province northeastern brazil where several pulses of magmatism occurred in the time interval 656 562 ma the oldest pulses are represented by orthogneisses bearing a flat lying foliation resulting from deformation and metamorphism of granitoids emplaced in three stages 656 645 ma 632 618 ma and 606 597 ma the oldest and most important unit 656 652 ma old has large length width ratio c 10 alkali calcic signature lacks pronounced negative ta anomalies whereas showing positive hf and zr anomalies in normalized trace element diagrams and yields crystallization age similar to the deposition age of surrounding dominantly clastic metasedimentary units these characteristics point to emplacement in an extensional setting and most probably fit in a rift sag basin model with thermal thinning of the lithosphere generating regional subsidence and magmatism rocks related to the 632 618 ma old stage are represented by orthogneisses with petrographic and geochemical characteristics akin to i type granites indicating a shift to an orogenic setting at around 640 630 ma increased radioactive heating during continued thickening promoted anatexis of metasedimentary rocks at mid crustal levels and segregation of small volumes of peraluminous melts at 606 597 ma the subsequent period of magmatic activity at 587 581 ma witnesses the transition to a transcurrent regime with intrusion of large granitic and syenitic batholiths with high k calc alkalic to shoshonitic affinity intrusion of these plutons added to heat resulting from strain heating to produce peraluminous granites at 573 566 ma through partial melting of metasedimentary sources the final magmatic episode at c 562 ma suggests involvement of the lithospheric mantle and crustal sources perhaps related to localized delamination generally younger orthogneisses and plutons contain zircons inherited from previous magmatic events pointing to important crustal reworking together with the geochemistry of the rocks these results imply only modest addition of juvenile material and thus no net crustal growth in eastern borborema province during the 660 560 ma time interval

diatreme breccias at the kelian gold mine kalimantan indonesia precursors to epithermal gold mineralization early miocene volcanism associated with a maar diatreme breccia complex preceded main stage epithermal gold mineralization at the kelian gold mine kalimantan indonesia prior to brecciation andesite intrusions 19 7 0 06 ma were emplaced into a package of felsic volcaniclastic rocks and overlying carbonaceous sand stones and mudstones and a weakly mineralized geothermal system was established intrusion of quartz phyric 19 8 0 1 ma and quartz feldspar phyric rhyolite 19 5 0 1 ma into the active geothermal system triggered widespread fragmentation and formation of the maar diatreme complex subsurface phreatomagmatic and phreatic explosions disrupted the preexisting hydrothermal system producing three composite diatreme breccia bodies the tepu runcing and burung breccias the diatremes consist of polymict breccias and sandstones that contain abundant carbonaceous matrix a distinctive facies association comprising coherent rhyohte jigsaw fit rhyolite breccia and matrix rich breccias that contain wispy to blocky juvenile rhyolite clasts define the root zones of the diatremes the surficial products of maar diatreme volcanic activity at kelian are preserved as large blocks of well stratified breccias they contain accretionary lapilli and were deposited by a combination of wet pyroclastic basesurge fallout and cosurge fallout processes evidence for syneruptive resedimentation of the pyroclastic deposits is preserved in poorly stratified breccia beds megablocks of phreatomagmatic base surge deposits were dropped down several hundred meters from the maar environment into the underlying diatremes volcanism in the kelian maar diatreme complex was dominated by a combination of phreatomagmatic and phreatic processes with subordinate hydraulic tectonic and dry magmatic fragmentation the carbonaceous matrix rich diatreme breccias acted as aquicludes during subsequent hydrothermal activity focusing fluid flow into the wall rocks adjacent to the diatremes where epithermal gold mineralization and hydrothermal brecciation occurred 2008 society of economic geologists inc

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

the oldest uhp eclogites of the world age of uhp metamorphism nature of protoliths and tectonic implications coesite bearing eclogitic rocks from the pan african belt in northern mali were dated by multichronological methods rb sr sm nd and ar ar rb sr and sm nd isotope analyses on whole rock and mineral separates of an omphacite kyanite micaschist and a mafic eclogite yielded concordant ages of about 620 ma this is interpreted as the time of the eclogitisation which represents the oldest ultrahigh pressure uhp metamorphic event so far recorded in the continental crust the nearly identical rb sr and sm nd ages suggest very rapid exhumation as observed in many phanerozoic uhp rocks 40ar 39ar dating of phengites from two uhp micaschists gave much older ages of 1045 and 760 ma phengite from a quartzite collected from the same site yielded a plateau date of 623 3 ma which is in agreement with the rb sr and sm nd dates the two 40ar 39ar ages older than 625 ma testify once more the recurrent problem of excess ar in ultrahigh pressure phengites the occurrence of the mali uhp eclogites indicates that by the late precambrian the earth has cooled enough to sustain the formation and preservation of deeply subducted uhp metamorphic rocks the new ages of ca 620 ma also constrain the amalgamation in nw gondwana to have occurred in the latest proterozoic but not in the middle cambrian this is supported by several recent chronological data for the southern segment of the major suture zone in the dahomeyide orogenic belt

p t evolution of eclogites from the agualada unit ordenes complex northwest iberian massif spain implications for crustal subduction abstract eclogite lenses in the agualada unit western ordenes complex spain contain the peak mineral assemblage garnet prograde rim alm 48 mol prp 30 mol omphacite jd max 36 mol quartz rutile and rare zoisite which equilibrated at t 700 c and p 12 14 kbar garnet shows discontinuous growth zoning with a pyrope poor intermediate zone alm 51 mol prp 10 mol between a core zone where pyrope is slightly higher alm 46 mol prp 16 mol and areas just inward from the rims where the maximum pyrope contents alm 48 mol prp 30 mol are recorded in atoll interiors garnet contains inclusions of a first generation of omphacite jd max 40 mol this omphacite is replaced in the matrix by a second generation jd max 36 mol with higher fe fe mg ratio the compositions of garnet and omphacite suggest a complex syneclogitic tectonothermal evolution for the agualada unit characterized by 1 eclogite facies metamorphism t 585 c p 12 13 kbar followed by 2 cooling during a slight decompression t 500 c p 11 12 kbar and 3 a final increase in p and t to form the garnet rim matrix omphacite mineral assemblage the agualada unit is part of a subduction complex which affected the gondwana margin at the beginning of the variscan cycle the p t evolution of the agualada eclogites is closely related to the structural evolution of the accretionary complex and the whole orogenic wedge the cooling event recorded by the agualada eclogites may have resulted from the accretion of a new colder crustal slice under the unit whereas the final progradation reflects the emplacement of the agualada unit directly under the mantle wedge this evolution fits well with previously presented the retical models both for the tectonothermal evolution of accretionary complexes and for the dynamic evolution of orogenic wedges p t paths such as the one for the agualada unit eclogites probably reflect a prolonged structural evolution although evidently rarely preserved such paths are probably the rule rather than the exception during plate convergence

fissure fills along faults variscan examples from gower south wales the extent to which persistent rather than transient fissures wide planar voids can exist along upper crustal faults is important in assessing fault permeability to mineral and hydrocarbon bearing fluids variscan late carboniferous faults cutting dinantian lower carboniferous limestones on the gower peninsula south wales host clear evidence for fissures up to several metres wide evidence includes dendritic hematite growth and elongate calcite growth into open voids spar ball and cockade breccia formation laminated sediment infill and void collapse breccias detailed mapping reveals cross cutting geometries and brecciation of earlier fissure fills showing that fissures were formed during rather than after active faulting fissures therefore probably formed by geometric mismatch between displaced fault walls rather than by solution widening along inactive faults

tightening up ne brazil and nw africa connections new u pb lu hf zircon data of a complete plate tectonic cycle in the dahomey belt of the west gondwana orogen in togo and benin the dahomey belt in togo and benin is an important segment of the larger west gondwana orogen here we review the geodynamic evolution of the dahomey belt and discuss new u pb and lu hf zircon data in light of similar data previously acquired on the geologically related northern borborema province in ne brazil eighteen samples from different tectonic settings and regions within the belt were collected for zircon isotopic investigation passive margin deposits of the atacora structural unit and lower units of the volta basin have detrital zircon signatures compatible with the flanking west africa craton the arc related magmatism resulted from the east dipping subduction of the goias pharusian oceanic lithosphere and is represented by a variety of granitoids emplaced in the benino nigerian shield between 670 and 610 ma these granitoids were mainly sourced from crustal reservoirs with subordinate juvenile input detrital zircon ages from syn orogenic deposits in benino nigerian shield suggest that arc development could have started as early as 780 ma the main period of melting in the internal part of the belt the benino nigerian shield is related to crustal thickening and occurs only ca 30 m y after initiation of the continental collision marked by the ca 610 ma ultra high pressure uhp metamorphism recorded at lato hills foreland development represented by the upper units of the volta basin developed soon after continental collision and persisted with the development of the west verging thrust front synchronously with the main period of crustal melting due to collision at ca 580 ma the subvertical transbrasiliano lineament in south america that corresponds to the kandi lineament in africa provides a present day fit between nw africa and ne brazil restoration of the movement of the transbrasiliano kandi lineament strike slip plate boundary places the dahomey belt and borborema province ne brazil along the same section of the west gondwana orogen this configuration would explain some of the misfits previously discussed in the literature and aligns the uhp eclogites in togo and ne brazil

a complete wilson cycle recorded within the riacho do pontal orogen ne brazil implications for the neoproterozoic evolution of the borborema province at the heart of west gondwana abstract the riacho do pontal orogen rpo is part of the extensive brasiliano pan african orogenic system in northeastern south america borborema province this region is crucial for understanding the neoproterozoic evolution of west gondwana because it occupies a strategic position between three of its most important constituents namely the sao francisco congo amazon and west africa cratons based on lithostratigraphic lithochemical geochronological and isotopic data a five stage model for the geodynamic evolution of the rpo and adjacent areas in the borborema province is proposed 1 rift phase development of a triple junction rift system at ca 900 820 ma ago probably related to the emplacement of a mantle plume and leading to intense mafic ultramafic magmatism of the brejo seco layered intrusion ca 900 ma this was followed by the development of a continental rift system testified by the paulistana complex metavolcanosedimentary sequence ca 882 ma 2 drift phase with continued lithospheric thinning the rift system further evolved to a broad passive margin in the northern sao francisco craton edge represented by the barra bonita formation platformal sediments the drift phase culminated in the development of true oceanic crust represented by the monte orebe ophiolite around 820 630 ma ago 3 convergence phase at circa 630 620 ma onset of subduction in the borborema province caused the inversion of the basins obduction of oceanic crust slices and sedimentation of the mandacaru formation syn orogenic greywackes 4 collisional phase continental collision between the sao francisco craton lower plate and the pernambuco alagoas block upper plate occurred around 620 590 ma with the stacking of the casa nova nappes upon the lower plate crustal thickening deformation metamorphism melt generation and intrusion of the syn collisional rajada suite two mica granites 5 lateral escape phase the collisional stage was followed by a lateral escape tectonics stage around 590 530 ma that generated the western branch of the e w trending pernambuco shear zone which truncates the northern part of the orogen and displaces its counterparts in the transversal zone in a dextral sense shear this phase was accompanied by extensive alkaline magmatism in the form of the serra da aldeia caboclo suites thus the rpo represents a complete plate tectonic cycle that spans the whole neoproterozoic and preserve excellent time markers for both the break up and dispersion of the rodinia supercontinent fragments and for the re amalgamation of those fragments in west gondwana the interpretation of a complete wilson cycle in this area challenges current views of the borborema province as a coherent block since paleoproterozoic times part of the atlantica supercontinent suggesting instead a dynamic setting where multiple plates interacted to form the heart of west gondwana

geochronology of granitoid and metasedimentary rocks from togo and benin west africa comparisons with ne brazil abstract the westernmost part of the pan african c 600 ma dahomeyide orogen in togo and benin consists of a complex stack of thrust sheets mainly composed of palaeoproterozoic eburnean c 2000 ma granitoid rocks and younger volcanic and metasedimentary successions this thrust stack comprises the suture between the west african craton and the benino nigerian province to the east this eastern province consists largely of migmatites supracrustals and granitoid rocks rb sr and sm nd isotope data for four granites from the western domain confirm the palaeoproterozoic age of these rocks found by earlier studies one of these granites yielded a zircon u pb age of c 2060 ma twenty samples from the eastern region yield rb sr and sm nd data indicating that both migmatites and granitoid rocks are of neoproterozoic pan african age zircon u pb data on six of these samples yielded ages ranging from c 650 to c 550 ma suggestions that parts of the migmatites might be of eburnean age could not be confirmed the neoproterozoic granitoid rocks may represent a pan african arc overlying an eastward dipping subduction zone and sm nd data indicate that they contain significant proportions of older crustal material age probability diagrams for detrital zircons from three metasedimentary rocks one from nigeria and one s type quartz diorite from the eastern region show prominent peaks at c 1900 ma and 2400 2200 ma a provenance of the zircons from sources in the borborema province in ne brazil or lateral equivalents thereof is suggested a few neoproterozoic zircons are present in three of the samples on the basis of th u ratios a distinction can be made between neoproterozoic zircons of metamorphic and detrital origin the zircon age probability diagram of a quartzite from northern benin exhibits one peak at c 1900 ma different from the other metasedimentary samples this quartzite might be correlated with sedimentary strata of the volta basin that overlie the eburnean basement in ghana togo and burkina faso correlation of the rocks in the study area with those in the borborema province is discussed

coesite inclusions in garnet from eclogitic rocks in western tianshan northwest china convincing proof of uhp metamorphism coesite inclusions in garnet have been recognized in eclogitic rocks from western tianshan northwest china the coesite grains exhibit distinct radial cracks in host porphyroblastic garnet some coesite relics are well preserved whereas others are partially replaced by quartz coesite has been identified optically and then confirmed by in situ raman spectroscopy showing the characteristic band at 522 cm 1 and subsidiary bands at 428 326 271 178 151 and 121 cm 1 the eclogitic rocks contain garnet omphacite and na ca amphibole and they are rich in white mica 30 and graphite peak conditions of 570 630 c and 2 7 3 3 gpa are constrained by garnet clinopyroxene geothermometry and the occurrence of coesite the presence of coesite and widespread quartz inclusions in garnet with radial cracks indicative of former coesite in these unique graphitic rocks confirms the previous suggestion of the uhp terrane for the western tianshan china

u pb age of the coesite bearing eclogite from nw borborema province ne brazil implications for western gondwana assembly abstract the late neoproterozoic assembly of western gondwana played an important role in the subduction of oceanic and continental lithospheres such event was also a source of arc magmatism reworking of cratonic margins and development of ultra high pressure uhp suture zones in the borborema province ne brazil we have described for the first time uhp rocks enclosed within gneiss migmatite and calc silicate rocks they bear coesite included in atoll type garnet from metamafic rocks identified by petrographic study and raman microspectroscopy analysis u pb zircon dating of the leucosome of the migmatites and the calc silicate rock displays concordant ages of 639 10 ma and 649 7 5 ma respectively here interpreted as the minimum age of the eclogitization event in the region u pb zircon dating of the coesite bearing rock defined a concordia age of 614 9 7 9 ma that comprised the retrograde eclogitic conditions to amphibolite facies the uhp rocks mostly retrograded to garnet amphibolites occur enclosed in the paleoproterozoic continental block composed of calc silicate rocks migmatized sillimanite gneiss mylonitic augen gneiss and granitic and tonalitic gneiss along a narrow n s oriented belt between the santa quiteria magmatic arc and the transbrasiliano lineament this block was involved in the subduction to uhp eclogite depths and was retrogressed to amphibolite during its exhumation and thrusting our data indicate an important neoproterozoic transcontinental suture zone connecting the pharusian belt with borborema province and probably with the brasilia belt in central brazil

ultradeep metamorphic rocks the retrospective viewpoint ultradeep or ultra high pressure uhp metamorphic rocks formed from crustal protoliths within the stability field of coesite at pressures 2 5 3 0 gpa corresponding to depths 80 120 km occur locally though regionally distributed in at least five continental areas their recognition is solely based on characteristic minerals and mineral assemblages calibrated by experimental high pressure studies detailed petrographic and microprobe work especially on mineral inclusions in favorable cases allows the derivation of prograde pt paths during subduction and of retrograde ones during exhumation commonly the gneisses adjacent to the uhp rocks do not exhibit signs of ultradeep metamorphism apparently because the kinetics of their mineral reactions are sufficiently fast to allow complete reequilibration to shallower pt conditions during the retrograde path it is also possible however that uhp equilibria were not attained throughout the rock volumes subducted but only along zones of shearing and fluid introduction if it is true that not all uhp metamorphic rocks return to the crustal orogenic belts but some continue to be subducted to greater mantle depths the classical geochemical pattern of a one way mass transfer from mantle to crust throughout the earth s history is at stake the assumed gradual growth of continents may have had a counterpart of continent destruction during collision events most recent experimental studies at high pressures and relatively low temperatures show that at least three new hydrous mg a1 silicates exist that were not found in nature thus far but may be characteristic minerals in the cold portions of old subduction zones thus extending the water retentivity of subducting slabs to greater and hitherto unexpected depths

geology and geochemistry of gunung subang gold deposit tanggeung cianjur west java indonesia abstract the gunung subang gold deposit is located in the sunda arc volcanic belt in west java indonesia the ore bodies are associated with volcanic rocks from the miocene koleberes and bentang formation to constrain the significance of the ore genesis and mineralization this study was carried out with the aim of examining the deposit using methods such as detailed field observation petrography petrochemistry zircon u pb dating hf isotope fluid inclusion analysis and sulfur and in situ pb isotope analyses the results suggest the following 1 the volcanic rocks in gunung subang belong to the basalt andesite group with 48 66 79 17 average 61 04 sio2 the trace element patterns of the volcanic rocks exhibit relatively low hree content flat hree patterns and weak negative or positive eu anomalies which are similar to those of typical arc magmatic rocks 2 most zircons selected from the volcanic rocks are magmatic and only a few are inherited the magmatic zircons have a weighted mean u pb age of 17 0 0 4 ma and the inherited zircons are from the precambrian dated at 887 2379 ma the magmatic zircons have ehf t values ranging from 1 24 to 13 59 indicating that the magmatism may be contaminated predominantly with juvenile crust but also with a small percentage of ancient crust 3 the sulfur isotopic compositions are fairly uniform and the 34s values of pyrite galena and sphalerite range from 0 53 to 2 97 suggesting a magmatic origin 4 the pb isotopic compositions of pyrite and galena are clustered and some include relatively high amounts of radiogenic lead the mean ratios of 206pb 204pb 207pb 204pb and 208pb 204pb are 18 59 0 007 15 65 0 007 and 38 89 0 017 respectively indicating that the ore forming material is highly related to miocene to pliocene volcanic rocks 5 fluid inclusions in the quartz from the ores are liquid and vapor liquid phases with components of the nacl h2o system with homogenization temperatures of 240 320 c and salinities of 0 2 6 2 wt nacl the coexistence of vapor and liquid rich fluid inclusions suggests that boiling occurred throughout the deposition of the ore 6 the results of electron probe microanalysis epma indicate that pyrite is the major au bearing mineral combined with the observation of scanning electron microscope sem gold is primarily present as au ag telluride minerals and there may also be native gold au0 finally we consider the gunung subang is an epithermal gold deposit related to the mid miocene magmatism

genesis of cockade breccias in the tectonic evolution of the cirotan epithermal gold system west java the pliocene cirotan gold deposit provides an exceptional example of mineralized breccias and in particular cockade breccias analysis of these breccias approached through both field observation and image analysis on photographs made it possible to determine their method of graded formation the mineralized structure as much a 25 m thick in places is a right lateral strike slip fault that evolved to a normal fault at the end of the system s development it consists of juxtaposed breccia bodies separated by major slip or septa which formed at a major inflection of the host fault the breccia bodies all show a similar internal organization from footwall to hanging wall one passes from a siliceous breccia with angular clasts to a thick infill of cockade breccia showing a reverse grading with small centimetre size cockades aspect ratios of 1 10 passing to larger up to 1 m more equant cockades aspect ratios 1 3 in contact with the hanging wall the different stages of cockade formation demonst

palaeogeographical reconstructions of the pan african brasiliano orogen closure of an oceanic domain or intracontinental convergence between major blocks abstract a reconstruction of the pan african brasiliano orogen has become possible thanks to new data from the southern pan african belt and by using published data geological mapping which included structural thermobarometric and geochemical studies in togo and neighbouring countries have enabled to define the existence of a preorogenic continental rift whose closure followed soon after its opening continental crust convergence induced an oblique collision that caused nappe stacking this involved external nappes thrust over the west african craton intermediate nappes including metabasic rocks with eclogitic assemblages and internal nappes composed of highgrade anatectic gneiss and intrusive granite and charnockite comparison with data from other regions hoggar adrar des iforas northeastern brazil nigeria central africa has led to the recognition of a pre orogenic continental rift that bordered the west africa sao francisco congo cratons in the southern part of the chain farther north in the hoggar and adrar des iforas this continental rift opened on a wider oceanic domain which involved a passive margin along the west african craton and an active margin to the east after these rifting and subduction stages continental collision followed that was dominated by strike slip movement oblique to the plate boundaries the general organization of the pan african brasiliano belt can be considered as the result of a confrontation between three major continental domains these were the west african and sao francisco congo cratons and a reworked shield composed in its inner part of polycyclic basement rocks and bounded by a fringe of monocyclic metasedimentary thrust belts that faced the cratons

timing of the ht lp transpression in the neoproterozoic serido belt borborema province brazil constraints from upb shrimp geochronology and implications for the connections between ne brazil and west africa abstract the serido schist belt is a classical domain of the borborema province ne brazil recording crustal melting at ht lp conditions in a tectonic setting dominated by dextral transpressive deformation a comparable tectonometamorphic setting has been described in the pan african igarra and ilesha schist belts of south nigeria africa the ht transpression of the serido has previously been imprecisely dated by using a range of isotopic methods on a suite of syntectonic granites we provide new u pb zircon ages of the acari granite and santa luzia migmatite that constrain the ht lp metamorphism of the serido at c 575 ma anisotropy of magnetic susceptibility of the santa luzia migmatite shows that the magnetic fabric records the ne trending dextral kinematics similar to those previously described in the acari pluton on the other hand the u pb ages of the totoro pluton provide evidence for an older mostly mafic to intermediate magmatism intruding the pelitic schist that precedes the ht lp metamorphism by approximately 15 m y in contrast with the acari pluton and santa luzia migmatite the magnetic fabrics of totoro show a concentric steep dipping structure apparently related to magma ascent thermobarometric studies in the serido igarra and ilesha schist belts indicate that a large domain in the northern borborema and south of the nigeria shields was deformed at a high temperature and at relatively shallow depths although the tectonic setting for such a post collision ht event remains an open issue it allows us nevertheless to refine the across atlantic geological correlation between the continents

origin of atoll garnets in eclogites and implications for the redistribution of trace elements during slab exhumation in a continental subduction zone detailed electron and ion microprobe analyses were carried out on atoll shaped and normal garnets in ultrahigh pressure uhp metamorphic eclogite from dabie east central china compositional profiles of both normal garnets and rings of atoll garnets show well preserved growth zoning with a decrease in mn ca and heavy rare earth elements hrees and an increase in mg toward rims manganese and middle rare earth element mree enrichments are observed near garnet rims island and peninsula shaped garnet inside atolls are homogeneous in major elements and show the same composition as garnet rims whereas the hree concentrations are similar to those of the normal garnet cores electron back scatter diffraction ebsd analyses show that the island and peninsula shaped garnet fractions inside atolls have crystallographic orientations identical to that of the atoll rings these observations suggest that the atoll garnets were formed by the consumption of earlier formed cores by fluid released from both hydroxyl exsolution from the nominally anhydrous minerals nams and lawsonite decomposition at the onset of exhumation i e garnet breakdown was from the inside and re growth from outside to inside though somewhat restricted this study reveals that because garnet and zircon act as sinks for hrees and zr respectively the majority of released hrees and zr were likely re incorporated into newly grown garnet and zircon during the dabie uhp metamorphic slab exhumation

reply to comment by cinthia labails and walter r roest on breakup of pangaea and plate kinematics of the central atlantic and atlas regions summary in their comment labails and roest have raised several points against the kinematic model of schettino and turco for the central atlantic region which requires the existence of an independent moroccan plate during the oligocene and the early miocene we thank these authors for giving us the opportunity to clarify some controversial points but disagree with most of their objections and show that they are based either on a different interpretation of existing data or a distortion of the facts presented by schettino and turco

chert in the palaeoproterozoic bartle member killara formation yerrida basin western australia a rift related playa lake and thermal spring environment abstract palaeoproterozoic chert in the bartle member of the killara formation yerrida group yerrida basin of western australia contains an anomalous association of crystal structures and rock fabrics that are difficult to interpret the association appears to indicate an evaporitic pyroclastic thermal spring environment associated with rifting at about 2 2 ga the chert member contains silica pseudomorphs after evaporite minerals that in places enclose relict isolated crystals and aggregates of crystals of gypsum and anhydrite the evaporite minerals are associated with minerals such as barite and analcime the association of these minerals together with palaeoenvironmental evidence invites comparison with rocks of the afar region and lake magadi in the east african rift system the bartle member locally contains anomalous gold up to twelve times the average crustal values of 4 ppb and barium it also contains finely disseminated kerogen together with numerous spheroids classified here as microdubiofossils and some curious structures showing organisation into complex petal like structures classified here as putative microfossils the bartle member chert has many characteristics of playa lake and thermal spring deposits and may host epithermal precious metal mineralisation

the neoproterozoic brasiliano orogeny in northeast brazil 40ar39ar and petrostructural data from ceara abstract the borborema province in ne brazil comprises paleoproterozoic basement and its proterozoic cover that experienced a complex tectonometamorphic history during the brasiliano pan african orogeny in ceara four major domains separated by continental scale steeply dipping dextral shear zones have been identified 40 ar 39 ar step heating of mineral separates and laser probe fusion of single grains have been used in conjunction with petro structural observations to delineate the cooling tectonic history of these domains to the north of the senador pompeu shear zone spsz amphibole and biotite pairs from low pressure granulite belts exposed along the granja shear zone and spsz record plateau ages in the range 575 560 ma corresponding to cooling rates close to 20 c m y 40 ar 39 ar laser probe muscovite dates from the chaval augen gneiss point to a similar age for shearing responsible for the exhumation and fast cooling of the granulite belts a muscovite biotite pair from the central ceara high pressure nappes lying between the shear zones documents a lower cooling rate in the interval 534 525 ma related to the decompression and uplift of previously deelly buried rocks to the south of the spsz only moderate crustal thickening occurred within the various schist belts a distinct cooling pattern is indicated for these belts and the western patos shear zone attesting to a different crustal development 40 ar 39 ar dates of amphibole phlogopite muscovite and biotite range from 550 to 522 ma recording a postmetamorphic cooling rate of 6 9 c m y slower cooling rates of 3 4 c m y between amphibole and biotite closure ages in the range 540 500 ma are reported for the eastern part of the palos shear zone system the diachronous cooling recorded along this shear zone is interpreted to reflect the variations in thickness of the high temperature mylonitic belts earlier more rapid cooling occurred where the belts are narrow in the west late slower cooling occurred in wider zones to the east these thermochronological data reveal an age and cooling rate distribution at the scale of the borborema province that provides new constraints on the development of the brasiliano orogeny between 580 and 500 ma

pan african subduction collision event evidenced by high p coronas in metanorites from the agou massif southern togo the ultrabasic basic complexes from se ghana and togo to n b enin are generally considered as an unique lithotectonic unit which may decorate the suture zone of the pan african collision belt in the southern region of togo the agou massif is one of these massifs mainly consisting of granulite facies rocks metagabbronorites and metagabbros outcrop in the innermost part of the agou igneous complex aic whereas in the outermost part relict boudins of metagabbronorites are surrounded by related retrogressed rocks such as layered grt bearing amphibolites and amphibolites the latter are intruded by leucocratic to melanocratic veins including dykes of microgabbros post solidus cooling of the metagabbronorites leads to the production of ht mp granulites m1 granulite stage opx1 cpx1 pl1 ilm sil t 850 c p 8kbar following this an evolution toward high pcoronitic granulites is indicated by early cpx and grt coronite developed around opx1 and pl1 granulite 2 stage cpx2 grt2 pl2 qtz sil rt 770 c 10kbar three successive stages of retrogression are then observed from i hornblende granulite facies 700 750 c 9kbar ii to amphibolite facies 700 c 6 7kbar and iii finally greenschist facies conditions 550 500 c 3 4kbar the inferredp tpath defines an unusual anti clockwise loop characterized by a slight pressure increase according to their calc alkaline signature and their metamorphic evolution the granulites of agou are derived from deep seated basic intrusion equilibrated in the lower continental crust under mp ht conditions and never experienced an early eclogite stage this contrasts with granulites derived from eclogites that are also described in sw togo in order to explain the existence of two types of metabasites in togo we propose that the agou massif corresponds to basic magmas emplaced and cooled at the base of an active continental margin western part of the benino nigerian shield whereas the former eclogites reflect subduction of the passive margin of the west african carton during subduction process or at the transition from subduction to collision at c 610ma burial of the edge of the active margin in the subduction zone led to the formation of coronitic granulites during the pan african orogeny at c 580ma the coronitic granulites together with the eclogites were exhumed under amphibolite facies conditions 2004 elsevier b v all rights reserved

proterozoic links between northeastern brazil and west africa a plate tectonic model based on gravity data abstract using the bullard predrift reconstruction for the southern atlantic analysis of the major gravimetric trends of the west african craton 2000 ma and the surrounding mobile belts allows us to delineate the upper proterozoic shield boundary and to put forward an evolutionary model for late precambrian time over the cratonic area gravimetric anomalies are related to a basement framework or to sedimentary basins whose geometry is linked to reactivation of ancient faults gravity anomalies and landsat imagery show a ne sw lineament in west africa which can be correlated with a variety of geologic features the largest being the late precambrian gourma aulacogen at the margins of the craton the deformation patterns and gravity signatures associated with the 600 ma orogeny were largely controlled by the geometry of the stable craton itself on the eastern edge pan african belt and brazilian belt east of the sao luis and amazonian cratons the distribution of the anomalies and deduced structures can be explained by a continental collision process on the western and southern edge brazilian belt between the sao luis and amazonian cratons rokelide belt mauritanide belt the gravity signature is different suggesting a large amount of intensely tectonized basic rocks a limited spreading process followed by an intense compressional episode is suggested

cirotan west java indonesia a 1 7 ma hybrid epithermal au ag sn w deposit the cirotan gold deposit dated at 1 7 or 0 1 ma is a mineralized fault with ore shoots contained within a system of fractures cutting a miocene volcano sedimentary series 9 5 ma intruded by pliocene microdiorite 4 5 ma the mineralization hosted by a right lateral strike slip fault that evolved into a normal fault has been partly eroded the deposit is of the adularia sericite type with strong vertical ore zoning ag au 64 at the top and only 7 in depth and resulted from a single mineralizing event with five stages of deposition silicification stage a siliceous breccias stage b cockade breccias with polymetallic sulfide ores and local rhodochrosite rhodonite stage c precious brecciated ore including bonanzas stage d and late vuggy quartz stage e this evolution was accompanied by an increase in w sn au and bi reflected in the late appearance of abundant ferberite scheelite gold cassiterite and rare minerals such as te bearing canfieldite and lillianite fluid inclusion studies indicate a slight increase in temperature 235 degrees 255 degrees c and salinity 2 89 7 15 wt nacl equiv from the initial silicification to the bonanza ore and an absence of volatiles other than h 2 o that this could reflect an increased participation of magmatic fluids is supported by the sulfur isotope compositions of pyrite and galena avg delta 34 s 2 2 and 1 0ppm respectively close to the 0 per mil isotopic value of magmatic sulfur lead isotope results establish a common origin for both the cirotan gold deposit 206 pb 204 pb 18 77 and 207 pb 204 pb 15 67 and the pliocene andesitic dacitic magmas to which the gold is related and indicate a remobilization of precambrian continental basement although cirotan shows strong affinities with classic epithermal gold deposits the late geochemical and mineralogical evolution with w sn and bi is extremely unusual if not unique in the context of the malaysia sumatra tin belt this is considered to reflect a genetic link with porphyry tin deposition and is explained by a progressive upward migration of pliocene magma favored by strike slip faulting the occurrence of the two types of mineralization within a single structure thus makes cirotan a hybrid deposit

geology of the northern borborema province ne brazil and its correlation with nigeria nw africa the borborema and benin nigeria provinces of ne brazil and nw africa respectively are key areas in the amalgamation of west gondwana by continental collision during the brasiliano pan african orogenies both are underlain by complex basement nigeria has c 3 05 ga archaean crust but no known palaeoproterozoic rocks 2 0 ga in ne brazil 2 6 3 5 ga archaean rocks form small cores within palaeoproterozoic gneiss terrains affected by plutonism at c 2 17 ga both regions exhibit late palaeoproterozoic c 1 8 ga rift related magmatism and metasedimentary sequences overlying the basement the serido group of ne brazil 0 65 ga is similar to the igarra sequence in sw nigeria the ceara group which may date back to c 0 85 ga is a passive margin deposit on crust thinned during initiation of an oceanic domain in both provinces basement and sedimentary cover were involved in tangential tectonics that resulted in crust thickening by nappe stacking associated with closure of this ocean frontal collision between c 0 66 and 0 60 ga later evolved to an oblique collision generating north south continental strike slip shear zones at c 0 59 ga in ne brazil the main pan african suture is probably buried beneath the parnaiba basin the transbrasiliano lineament interpreted as the prolongation of the kandi 4 50 lineament in hoggar may represent a cryptic suture

an alternative early opening scenario for the central atlantic ocean abstract the opening of the central atlantic ocean basin that separated north america from northwest africa is well documented and assumed to have started during the late jurassic however the early evolution and the initial breakup history of pangaea are still debated most of the existing models are based on one or multiple ridge jumps at the middle jurassic leaving the oldest crust on the american side between the east coast magnetic anomaly ecma and the blake spur magnetic anomaly bsma according to these hypotheses the bsma represents the limit of the initial basin and the footprint subsequent to the ridge jump consequently the evolution of the northwest african margin is widely different from the northeast american margin however this setting is in contradiction with the existing observations in this paper we propose an alternative scenario for the continental breakup and the mesozoic spreading history of the central atlantic ocean the new model is based on an analysis of geophysical data including new seismic lines an interpretation of the newly compiled magnetic data and satellite derived gravimetry and recently published results which demonstrate that the opening of the central atlantic ocean started already during the late sinemurian 190 ma based on a new identification of the african conjugate to the ecma and on the extent of salt provinces off morocco and nova scotia the identification of an african conjugate magnetic anomaly to bsma the african blake spur magnetic anomaly absma together with the significant change in basement topography are in good agreement with that initial reconstruction the early opening history for the central atlantic ocean is described in four distinct phases during the first 20 myr after the initial breakup 190 170 ma from late sinemurian to early bajocian oceanic accretion was extremely slow 0 8 cm y at the time of blake spur 170 ma early bajocian a drastic change occurred both in the relative plate motion direction from nnw sse to nw se and in the spreading rate an increase to 1 7 cm y after a small increase between chron m25 154 ma kimmeridgian and chron m22 150 ma tithonian the spreading rate slowed down to about 1 3 cm y and remained fairly constant until chron m0 125 ma barremian aptian boundary in addition kinematic reconstructions illustrate a significant spreading asymmetry during the early history of the central atlantic ocean the accretion rates were higher on the american side and led to the formation of more oceanic crust on this plate we infer that this asymmetry could be related to the fact that the thermal anomaly responsible for the significant magmatism of the central atlantic magmatic province camp was preferentially located below the african plate

transition from subduction to collision recorded in the pan african arc complexes mali to ghana abstract the 1000 km long suture zone of the dahomeyide belt exposed from southeast ghana to south mali corresponds to a narrow and lithologically diverse area with symptomatic coronitic hp granulitic massifs based on a review of published petrological geochemical and geochronological data along the dahomeyide belt we propose a global scenario for the closure of the pharusian ocean between the west african craton wac and the benino nigerian shield during the end of the neoproterozoic the onset of a long lived oceanic subduction by 800 780 ma is recorded by early magmatism in the amalaoulaou intra oceanic arc in mali contemporary to the gourma and adrar des iforas to the north and in brazil to the south the first occurrence of tonalitic plutons dated at 720 ma and the development of a forearc system around 650 ma mark the onset of active margin subduction beneath the benino nigerain shield and its northward prolongation in mali oceanic subduction beneath the active margin ended between 640 and 630 ma with the onset of subduction of the wac continental margin while subduction related magmatism continues till ca 600 ma on the upper plate during a short period between 620 and 610 ma the forearc system and the tip of the active continental margin were buried synchronously the positive bouguer anomaly observed to the east of the suture zone in benin and not beneath the suture zone itself supports the occurrence of a massive mafic body at the base of the crust that could correspond to the underplated buried forearc ultimately the exhumation and partial amphibolitization of the suture zone and a shift from a calc alkaline magmatism to anatectic magmatism along with the onset of strike slip faulting in the upper plate marks the transition from continental subduction to continental collision between 610 and 580 ma this work highlights the importance of the pharusian suture zone s l to our knowledge it represents a unique example in the world in where the forearc system is buried and partly exhumed at the transition from subduction to collision

seismic cycle recorded in cockade bearing faults col de teghime alpine corsica abstract few fault rocks are known to be associated undoubtedly with seismic faulting here we investigated the formation mechanism of cockade breccias found in transtensional faults cutting marbles and quartzites from the col de teghime area alpine corsica france field surveys coupled with detailed microanalytical investigations indicated that i the core clasts of the cockades are composed of host rock fragments 310 m in size that are suspended in the slipping zones and arranged in inverse grading ii the concentric rims of the cockades show a cyclic zoning made of saddle dolomite mg calcite goethite anatase iii the cockade bearing veins are associated with minor fault veins filled with fine fragments we propose that the cockade bearing faults formed at shallow crustal depths

chaotic breccia along the dent fault nw england implosion or collapse of a fault void a body of chaotic breccia along the reverse oblique dent fault zone is ascribed to hanging wall collapse into persistent voids created by geometric mismatch of fault walls although some implosion into transient voids is a possibility the breccia comprises a 20 m wide body of hanging wall lithologies with a chaotic clast supported fabric that contrasts with the fitted fabric breccias typical of the dent fault damage zone the breccia body has crude bedding defined by clast shape and size contrasts the void fill is cut by variscan fault strands which together with its ferroan calcite and barite cement prove its late carboniferous rather than recent age it is shown that any fault void transient or persistent had a smaller aperture than the final width of the breccia body and no more than 5 m a span that can be supported to depths of 2 or 3 km however cement zonation in the breccia fill suggests that the void opened in multiple increments each of an aperture compatible with the maximum displacement in any one event along the dent fault the dent fault example highlights the possible general importance of fault void collapse but also the problems in distinguishing it from implosion processes

cockade breccia product of mineralisation along dilational faults cockade breccias are fault fills in which individual clasts are completely surrounded by concentric layers of cement they occur particularly in low temperature near surface hydrothermal veins at least six mechanisms have been proposed for the formation of cockade breccia like textures but only two repeated rotation accretion and partial metasomatic replacement of clast minerals have been supported by detailed evidence a typical example of cockade breccia from the gower peninsula south wales shows clear evidence for the rotation accretion mechanism in particular overgrown breakage points in cement layers where cockades were previously touching each other and rotated geopetal infills of haematitic sediment based on the available evidence it is proposed that cockade textures result from low rates of cement growth compared to high rates of dilational fault slip seven criteria are given for the correct identification of cockade breccias

deformation and structural evolution of mantle peridotites during exhumation on transform faults a forced transition from ductile to brittle regime abstract the exhumation of peridotite rocks in oceanic transform zones passes by the rheological transition between the ductile and brittle deformation until the complete emplacement in the oceanic lithosphere the sao pedro and sao paulo archipelago spspa in the equatorial atlantic records the deformational products of ductile brittle and the rocks fluid interaction generating specific structures in each domain the deformational stages are related to the transpressional and transtensional geodynamics of sao paulo transform fault firstly during transpression exhumation occurs associated with the ductile domain causing intense mylonitization in temperatures between 700 and 800 c defined by olivine and orthopyroxene recrystallization the interaction with fluids initially originated from the mantle generates amphibole and oxide rich layers marking the passage to a semi brittle deformation the continuation of peridotite exhumation associated with an nw se shortening and transpressional led to a higher availability of hydrothermal fluids as a consequence four serpentinization episodes are recorded which are associated with semi brittle to brittle transition under temperatures between 300 and 400 c finally the complete exhumation and establishment of brittle mechanisms led to carbonatation phase near the surface with temperatures ranging from 300 to 150 c the active nw se tectonic stress generated e w strike slip faults that were filled by carbonates recording the final exhumation stage

chaotic breccia zones on the pembroke peninsula south wales evidence for collapse into voids along dilational faults chaotic breccias and megabreccias locally called gash breccias hosted within the pembroke limestone group visean mississippian lower carboniferous of southwest wales are re mapped along with spatially related crackle and mosaic breccias of thirteen studied megabreccia bodies seven lie along steep nnw or nne striking strike slip faults originating during north south variscan late carboniferous shortening though reactivated during later extension four bodies are conformable with e w striking steeply dipping bedding and two have irregular or indeterminate margins the bedding parallel zones are interpreted as the dilational tips of listric normal faults and the cross strike faults as transtensional transfer zones sub horizontal clast fabrics suggest brecciation by gravitational collapse into opening fissures rather than by cataclasis along the faults most fissures have geometrically matched margins produced by this dilational faulting and only locally have the indented margins indicating solutional processes the most likely age for the main fissure extension and fill is late triassic based on analogous dated fills at the eastern end of the bristol channel basin the pembroke megabreccias blur the distinction between fault rocks formed by deformation and those formed by redeposition along fault zones

high pressure granulites from carire borborema province ne brazil tectonic setting metamorphic conditions and u pb lu hf and sm nd geochronology abstract in the northwest region of the ceara central domain borborema province an elongate area of mylonitic diorites and granodiorites is structurally controlled by ne sw trending strike slip faults granitic material and layers and lenses of felsic and mafic granulites occur along the main shear zones we present and discuss petrographic geothermobarometric and geochronological la mc icpms u pb and lu hf in zircon and sm nd whole rock data for these rocks felsic granulites are composed of orthopyroxene clinopyroxene plagioclase quartz hornblende garnet biotite feldspar while the mafic granulites comprise clinopyroxene plagioclase garnet hornblende rutile quartz the p t conditions recorded range from 9 4 to 8 3 kbars and from 804 to 870 c for the felsic granulites and from 13 6 to 10 2 kbars and 910 to 750 c for the mafic granulites u pb zircon ages of granodiorite host rocks are 2157 and 2044 ma the felsic granulites yield a u pb age of 2110 ma while the mafic granulites yield u pb zircon ages of 613 and 589 ma for the metamorphic recrystallization sm nd and lu hf t dm ages for the mafic granulites are similar 1 6 1 3 ga and share positive values of e nd and e hf which suggests that these mafic rocks were derived from a juvenile mesoproterozoic crust in addition the felsic granulites and the granodiorites have similar t dm nd and u pb ages with positive values of e nd indicating a paleoproterozoic mantle source for these rocks

hydrothermal breccias in vein type ore deposits a review of mechanisms morphology and size distribution abstract breccias are among the most widely distributed rock textures found in hydrothermal vein type deposits previous studies have mainly been interested in developing qualitative descriptive approaches leading to a confusing profusion of terms brecciation originates in numerous ways resulting in highly complex classification systems and frequent misinterpretations of facies field observations are difficult to reconcile with physical theories of fragmentation partly due to the fact that few satisfactory quantitative tools have been developed a review of the main brecciation processes occurring in hydrothermal vein type deposits allows for the discrimination between chemical and physical mechanisms including tectonic comminution wear abrasion two types of fluid assisted brecciation hydraulic and critical volume expansion or reduction impact and collapse each of these mechanisms can be distinguished using nonscalar parameters that describe breccia geometry including fragment morphology size distribution of the fragments fabric and dilation ratio the first two parameters are especially important because 1 the morphology of the fragments allows chemical and physical mechanical breccias to be distinguished and 2 the particle size distribution psd is a function of the energy input during breccia formation the slope of the cumulative psd fractal dimension ranges from high values for high energy brecciation processes to low values for low energy processes indicated by an isometric distribution the evolution of a vein system can be divided into three stages propagation wear and dilation these stages are separated by one threshold of mechanical discontinuity and one of hydraulic continuity these two thresholds also mark the transition between different types of brecciation mineralization occurs during all three stages and may display different textures due to pressure variations the use of quantitative parameters in fault related hydrothermal breccias allows a better understanding of the physical parameters related to a vein environment including structural setting and crustal level as well as fluid rock interactions recognition of the different breccia types could also be important during the early stages of mineral exploration

comment on breakup of pangaea and plate kinematics of the central atlantic and atlas regions by antonio schettino and eugenio turco summary schettino and turco recently proposed a revised model for the continental closure of pangaea and the early opening history of the central atlantic ocean based on a quantitative method in combination with a range of geological and geophysical data their discussion and final results do not properly consider field data and moreover their analyses violate certain plate kinematic principles the comments we offer concern in particular the estimated independent motion of the moroccan block

relict coesite exsolution in omphacite from western tianshan eclogites china exsolution rods of relict coesite together with quartz were identified in omphacite in eclogites from western tianshan china they are oriented along the c axis of the host clinopyroxenes and have grain size up to 30 m long and 2 3 m wide raman spectra of exsolved lamellae yield consistent but weak bands at 521 270 181 151 and 118 cm 1 typical for coesite in addition to those of quartz and the host omphacite such occurrences together with textured observations suggest a two stage evolution of sio 2 exsolution rods in omphacite lamellae of coesite were apparently exsolved from supersilicic omphacite at p 5 0 gpa and the transformation from coesite to quartz occurred during retrograde metamorphism

two stage terrane assembly in western gondwana insights from structural geology and geophysical data of central borborema province ne brazil abstract combined geophysical and structural data from the transversal subprovince of the borborema province ne brazil highlight the internal structure of and interrelationships between the constituent terranes radiometric and magnetic maps show distinctive signatures for the archean paleoproterozoic alto moxoto and meso to neoproterozoic alto pajeu and pernambuco alagoas terranes mapped radiometric and first and second order magnetic lineaments associated with euler deconvolution enable correlation between geophysical data and major structures in addition to early related deformation of the alto moxoto terrane combined analysis of late transposition foliations lineations and kinematic criteria reveal a complex structural evolution marked by two distinct assembly stages the first phase is characterized by thrust tectonics with top to the south vergence resulting in the juxtaposition of the allochtonous alto pajeu terrane with the structurally underlying alto moxoto terrane the terrane boundary is delineated by the serra de jabitaca shear zone which is associated with low dipping collisional granitic sheets and ca 1 0 ga mafic ultramafic rocks interpreted as obtudcted ophiolite remnants later strike slip movements strongly folded and obliterated thrust related markers and the continental scale e w pernambuco lineament is interpreted as the result of lateral assembly between the composite alto pajeu alto moxoto terranes and the pernambuco alagoas terrane during the metamorphic peak of the brasiliano orogeny evolution of the borborema province reflects accretionary processes during the assembly of western gondwana

modelling the evolution of columnar joints abstract cooling cracks developing from the margins of solidifying lava flows or intrusive dykes or sills often propagate inward and divide the rock into prismatic columns it has been shown that 1 the initial crack pattern that forms at the surface of the body is not as regular as the pseudo hexagonal pattern which evolves later 2 columnar structures develop by repeated step wise crack advances forming cm to dm transverse bands on the column faces 3 even within well developed colonnades the polygonal outlines continue to shift slightly from one crack advance to the next we propose that each new crack should propagate parallel to the highest thermal gradient ahead of the current crack tip when adjacent columns are of unequal size the local asymmetry of the isotherms drives the new crack toward the biggest and hottest column we present a geometrical model and algorithm that follows this rule and mimics the successive crack advances as the cooling front migrates into the lava sheet or dyke the model polygonal pattern obtained consists of convex irregular polygons with a variable number of sides but in which pentagons and hexagons predominate the algorithm successfully reproduces the rapid evolution from an initial immature crack pattern to a pseudo hexagonal mature one in which the average number of sides approaches six it predicts also the predominance of y type crack junctions the rapid evolution toward columns of approximately equal sizes and the persistent changes in length and diverging orientation of growth steps observed along the sides of columns the model polygonal patterns exhibit geometrical properties which are very similar to those observed in well developed columnar jointed flows such as the giant s causeway

neoproterozoic crustal accretion in central brazil recent geochronological studies have demonstrated the existence of several neoproterozoic orthogneiss and metavolcanic are suites ca 900 to 600 ma in the southern part of the tocantins province a major neoproterozoic orogenic area in central brazil exposed between the amazon and sao francisco cratons nd and sr isotopic characteristics of these metamorphic suites are primitive initial 87 sr 87 sr ratios vary from 0 7024 to 0 7042 and initial nd values are positive varying between 0 2 and 6 9 the most primitive initial sr and nd isotopic compositions are very similar to model depleted mantle compositions at the time of formation of the original magmas the mantle like characteristics of these are suites in western goias contradict previous ensialic evolution models for the tocantins province and reveal that crustal accretion processes were important in large areas of central brazil during neoproterozoic time

the lu hf isotope geochemistry of chondrites and the evolution of the mantle crust system earth planet sci lett 148 1997 243 258 1

the mafic layered complex of the kabye massif north togo and north benin evidence of a pan african granulitic continental arc root abstract between the predominantly neoarchean paleoproterozoic west african craton wac and the saharian metacraton smc the dahomeyides suture zone represents a valuable witness of continental amalgamation during pan african times in the kabye massif northern togo and northern benin mainly granulitic metagabbros associated with al rich kyanite and garnet bearing felsic dykes are exposed as tectonic lenses within the pre pan african gneisses of the smc new geochemical data suggest that the high grade rocks granulites in the kabye massif originated in a mature continental arc setting afc calculations constrain the amount of contamination of mantle wedge derived magmas by crustal metasediments to about 10 reconstruction of initial magmatic stratigraphy has been carried out using cipw norms for cumulitic sequences within the massif indicating a normal igneous polarity from west to east published geochemical signatures along the pan african suture zone of the dahomeyides in ghana togo and benin constrain the origin of mafic granulitic and eclogitic complexes a distinction is made between bodies with mainly morb signatures originated from the subducted wac plate passive margin and those with magmatic arc signatures originated from the over riding plate mantle wedge this suggests that the closure of the oceanic domain between the wac and the smc from 640 to 610 ma was mainly accommodated by oceanic subduction beneath the active continental margin before pan african collision

size depth and related structures of intrusions under stratovolcanoes and associated geothermal systems abstract the subvolcanic structural configuration of faults stress field permeability controls and intrusion s shape size and depth are primary factors controlling the location and quality of a geothermal system subvolcanic stocks under stratovolcanoes are typically 1 to 3 km in diameter and emplaced at depths of 1 to 4 km exposed stocks in the western cascades of oregon are of similar size and are emplaced at depths of 1 to 2 km shallow magma chambers in general are typically 4 to 9 km deep and several times larger than the subvolcanic stocks likely subvolcanic structures are radial compression fractures around the magma chamber and above the chamber extension faulting stockwork fractures and breccia pipes pre existing or regional structures and stress field will influence the location and shape of magma bodies and associated thermal systems this study combines data from areas of active stratovolcanoes and older eroded volcanic areas to formulate a detailed structural model of the plumbing system under a stratovolcano the model is correlated with factors controlling associated geothermal systems a geothermal system may occur at depths of 1 2 km under a stratocone s base deeper systems associated with shallow magma chambers are likely to be present

sedimentary fluids fault interaction during syn rift burial of the lodeve permian basin herault france an example of seismic valve mechanism in active extensional faults abstract during basin burial interstitial fluids initially trapped within the sedimentary pile easily move under thermal and pressure gradients as the main mechanism is linked to fluid overpressure such fluids play a significant role on frictional mechanics for fault reactivation and sediment deformation the lodeve permian basin herault france is an exhumed half graben with exceptional outcrop conditions providing access to barite sulfide mineralized systems and hydrocarbon trapped into syn rift roll over faults architectural studies show a cyclic infilling of fault zone and associated bedding parallel veins according to three main fluid events during dextral normal faulting contrasting fluid entrapment conditions are deduced from textural analysis fluid inclusion microthermometry and sulfur isotope geothermometer we conclude that a polyphase history of trapping occurred during permian syn rift formation of the basin the first stage is characterized by an implosion breccia cemented by silicifications and barite during an abrupt pressure drop within fault zone this mechanism is linked to the dextral strike slip motion on faults and leads to a first sealing of the fault zone by basinal fluid mineralization the second stage consists of a succession of barite ribbons precipitated under overpressure fluctuations derived from fault valve action this corresponds to periodic reactivations of fault planes and bedding controlled opening localized at sulphide rich micro shearing structures showing a normal movement this process formed the main mineralized ore bodies by the single action of fluid overpressure fluctuations undergoing changes in local stress distribution the last stage is associated with the formation of dextral strike slip pull apart infilled by large barite and contemporaneous hydrocarbons under suprahydrostatic pressure values this final tectonic activation of fault is linked to late basinal fluids and hydrocarbon migration during which shear stress restoration on the fault plane is faster than fluid pressure build up this integrated study shows the interplay action between tectonic stress and fluid overpressure in fault reactivation during basin burial that clearly impact potential economic reservoirs

a coupled micro and macrostructural approach to the analysis of fluid induced brecciation curnamona province south australia conditions during the formation of breccias in the curnamona province eastern south australia have been investigated through a detailed fractal microstructural structural and fluid inclusion study of the breccias syn tectonic quartz veins and surrounding rocks fractal analysis of clast shapes and clast size distributions were able to distinguish two styles of breccia and indicate that fluid pressure fluctuations played a significant role though to differing degrees in the initiation of the brecciation process approximate palaeostress orientations prevalent during brecciation and their relationship to the deformational history of the terrane have been reconstructed by combining the orientations of microfractures sealed microfractures and fluid inclusion planes with structural field data fluid pressure fluctuations of 80 mpa during the brecciation process have been quantified by structurally constrained microthermometric analysis of syn tectonic fluid inclusions coupled with an analysis of the metamorphic history of the area results from this study allow brecciation within the curnamona province to be placed within a structural framework and relate the process of brecciation to the interaction between regional hydrothermal alteration fluid flow systems shear zone formation and lithological contrasts within the upper crust during the palaeoproterozoic olarian orogeny these methods and findings provide a template for the study of brecciation processes in other terranes where fluid induced brittle deformation is a controlling factor in the localisation of major ore deposits

the algodoes amphibolite tonalite gneiss sequence borborema province ne brazil geochemical and geochronological evidence for palaeoproterozoic accretion of oceanic plateau back arc basalts and adakitic plutons abstract field relationships geochemistry and u pb zircon and sm nd whole rock geochronology are used to constrain the genesis of palaeoproterozoic massive amphibolites and orthogneisses of the algodoes sequence central ceara domain northeastern brazil the 2236 55 ma old algodoes amphibolites show trace element geochemical signature and positive e nd t values similar to phanaerozoic oceanic plateau basalts and less often to back arc basalts the amphibolites are intruded by tonalitic to quartz dioritic gneisses of the cipo unit which were dated at 2160 2170 ma and 2130 ma neodimium isotope data and whole rock geochemistry for these gneisses show they are akin to juvenile arc plutonics of the adakitic suite the likely occurrence of palaeoproterozoic plateau basalts and arc plutonics in this part of northeastearn brazil render comparisons with similar terranes elsewhere in south america and africa which in turn indicate a significant contribution of accretionary orogens in the early assembly of the columbia supercontinent

breakup of pangaea and plate kinematics of the central atlantic and atlas regions summary a new central pangaea fit type a is proposed for the late ladinian 230 ma together with a plate motions model for the subsequent phases of rifting continental breakup and initial spreading in the central atlantic this model is based on 1 a reinterpretation of the process of formation of the east coast magnetic anomaly along the eastern margin of north america and the corresponding magnetic anomalies at the conjugate margins of northwest africa and the moroccan meseta 2 an analysis of major rifting events in the central atlantic atlas and central mediterranean and 3 a crustal balancing of the stretched margins of north america moroccan meseta and northwest africa the process of fragmentation of central pangaea can be described by three major phases spanning the time interval from the late ladinian 230 ma to the tithonian 147 7 ma during the first phase from the late ladinian 230 ma to the latest rhaetian 200 ma rifting proceeded along the eastern margin of north america the northwest african margin and the high saharan and tunisian atlas determining the formation of a separate moroccan microplate at the interface between gondwana and laurasia during the second phase from the latest rhaetian 200 ma to the late pliensbachian 185 ma oceanic crust started forming between the east coast and blake spur magnetic anomalies whereas the morrocan meseta simply continued to rift away from north america during this time interval the atlas rift reached its maximum extent finally the third phase encompassing the time interval from the late pliensbachian 185 ma to chron m21 147 7 ma was triggered by the northward jump of the main plate boundary connecting the central atlantic with the tethys area therefore as soon as rifting in the atlas zone ceased plate motion started along complex fault systems between morocco and iberia whereas a rift drift transition occurred in the northern segment of the central atlantic between morocco and the conjugate margin of nova scotia the inversion of the atlas rift and the subsequent formation of the atlas mountain belt occurred during the oligocene early miocene time interval in the central atlantic this event was associated with higher spreading rates of the ridge segments north of the atlantis fz an estimate of 170 km of dextral offset of morocco relative to northwest africa in the central atlantic is required by an analysis of marine magnetic anomalies five plate tectonic reconstructions and a computer animation are proposed to illustrate the late triassic and jurassic process of breakup of pangaea in the central atlantic and atlas regions

the position of the amazonian craton in supercontinents abstract this paper examines the extensive regions of proterozoic accretionary belts that either formed most of the amazonian craton or are marginal to its southeastern border their overall geodynamic significance is considered taking into account the paleogeographic reconstruction of columbia rodinia and gondwana amazonia would be part of columbia together with laurentia north china and baltica forming a continuous continental landmass linked by the paleo to mesoproterozoic mobile belts that constitute large portions of it the rodinia supercontinent was formed in the mesoproterozoic by the agglutination of the existing cratonic fragments such as laurentia and amazonia during contemporary continental collisions worldwide the available paleomagnetic data suggest that laurentia and amazonia remained attached until at least 600 ma since all other cratonic units surrounding laurentia have already rifted away by that time the separation between amazonia and laurentia marks the final break up of rodinia with the opening of the lapetus ocean

internal vein texture and vein evolution of the epithermal shila paula district southern peru the epithermal shila paula au ag district is characterized by numerous veins hosted in tertiary volcanic rocks of the western cordillera southern peru field studies of the ore bodies reveal a systematic association of a main e w vein with secondary n55 60 w veins two directions that are also reflected by the orientation of fluid inclusion planes in quartz crystals of the host rock in areas where this pattern is not recognized such as the apacheta sector vein emplacement seems to have been guided by regional n40 e and n40 w fractures two main vein filling stages are identified stage 1 is a quartz adularia pyrite galena sphalerite chalcopyrite electrum mn silicate carbonate assemblage that fills the main e w veins stage 2 which contains most of the precious metal mineralization is divided into pre bonanza and bonanza substages the pre bonanza substage consists of a quartz adularia carbonate assemblage that is observed within the secondary n45 60 w veins in veinlets that cut the stage 1 assemblage and in final open space fillings the two latter structures are finally filled by the bonanza substage characterized by a fe poor sphalerite chalcopyrite pyrite galena tennantite tetrahedrite polybasite pearceite electrum assemblage the ore in the main veins is systematically brecciated whereas the ore in the secondary veins and geodes is characteristic of open space crystallization microthermometric measurements on sphalerite from both stages and on quartz and calcite from stage 2 indicate a salinity range of 0 to 15 5 wt nacl equivalent and homogenization temperatures bracketed between 200 and 330 c secondary co2 n2 and h2s bearing fluid inclusions are also identified the age of vein emplacement based on 40ar 39ar ages obtained on adularia of different veins is estimated at around 11 ma with some overlap between adularia of stage 1 11 4 0 4 ma and of stage 2 10 8 0 3 ma a three phase tectonic model has been constructed to explain the vein formation phase 1 corresponds to the assumed development of e w sinistral shear zones and associated n60 w cleavages under the effects of a ne sw shortening direction that is recognized at andean scale these structures contain the stage 1 ore assemblage that was brecciated during ongoing deformation phase 2 is a reactivation of earlier structures under a nw se shortening direction that allowed the reopening of the preexisting schistosity and the formation of scarce n50 e striking s2 cleavage planes filled by the stage 2 pre bonanza minerals phase 3 coincides with the bonanza ore emplacement in the secondary n45 60 w veins and also in open space in the core of the main e w veins our combined tectonic textural mineralogical fluid inclusion and geochronological study presents a complete model of vein formation in which the reactivation of previously formed tectonic structures plays a significant role in ore formation

nature and origin of collapse breccias in the zechstein of ne england local observations with cross border petroleum exploration and production significance across the north sea

ore related breccias in volcanoplutonic arcs an overview of breccias related to a variety of base metal precious metal and lithophile element deposits in volcanoplutonic arcs permits definition of six possible mechanisms for subsurface brecciation 1 release of magmatic hydrothermal fluids from high level hydrous magma chambers during second boiling and subsequent decompression generates a spectrum of breccia types in which fragments may suffer collapse and or ascent single or multiple intrusion related breccia pipes and pre and intermineral breccias in porphyry copper deposits provide widespread examples 2 magmatic heating and expansion of meteoric pore fluids may lead to brecciation commonly of late or postmineral age and including pebble dikes in porphyry type and related deposits magmatic heating of rocks saturated with seawater may cause submarine hydrothermal eruptions late in the emplacement histories of kuroko type massive sulfide deposits many of the resultant breccias underwent limited sedimentary transport overpressuring of heated fluids beneath semipermeable partly self sealed cap rocks may lead to brecciation and subaerial hydrothermal eruptions in shallow epithermal precious metal settings magmatic heating or tectonic disturbance may have triggered brecciation 3 interaction of cool ground waters with subsurface magma can generate phreatomagmatic explosions postmineral phreatomagmatic diatremes associated with porphyry systems and premineral diatremes with epithermal precious or base metal deposits were generated in this manner these attained the palcosurface to produce pyroclastic base surge and fall deposits that accumulated as tuff rings around maar craters 4 magmatic hydrothermal brecciation may lead to disruption of rocks through to the paleosurface decompression and fragmentation and eruption of the top part of an underlying magma chamber pre and postmineral magmatic diatremes of this sort are inferred to accompany a few porphyry type and other base and precious metal systems they were manifested at the palcosurface by accumulations of pyroclastic fall and flow deposits 5 breccias may result from mechanical disruption of wall rocks during subsurface movement of magma any intrusion related deposit may include such intrusion breccias 6 tectonic breccias resulting from fault displacement may accompany any type of ore deposit a continuum exists between many of these breccia types and it is difficult to identify unique criteria for their unambiguous distinction

structural aspects of fluid rock interactions in detachment zones metamorphic core complexes and associated detachment faults of the north american cordillera represent gently dipping normal displacement shear zones detachment zones along which hot deeper levels of the crust were transported upward and outward from underneath a brittlely distended upper plate structures that formed during the ductile to brittle evolution of detachment zones can be used to reconstruct the relative magnitudes of fluid pressure and deviatoric stress at different levels within the shear zones mylonitization which occurred along deeper segments of detachment zones below the brittle ductile transition was locally accompanied by tensile failure indicative of low deviatoric stress and high fluid pressure this condition persisted into the earliest phases of brittle deformation after which shear fractures formed due to both a reduction in fluid pressure and an increase in deviatoric stress brittle deformation of upper plate rocks occurred largely under conditions of low fluid pressure structural and geochemical data suggest that normal displacement on detachment zones results in establishment of two fluid systems 1 an upper plate system driven by convection and dominated by meteoric and connate fluids at near hydrostatic pressures and 2 a system within deeper levels of the shear zone where fluids are largely derived from igneous sources and fluid migration is aided by dilatancy pumping the late phases of normal displacement on detachment zones structurally juxtaposed rocks affected by the two fluid systems and locally caused the shear zone rocks to be overprinted by mineralization related to the upper plate fluid system

breccia formation by particle fluidization in fault zones implications for transitory rupture controlled fluid flow regimes in hydrothermal systems breccias in the rusey fault cornwall uk and the roamane fault porgera gold deposit papua new guinea provide insights about the dynamics of fluid flow and flow velocities when fault ruptures breach overpressured reservoirs of hydrothermal fluid these faults contain cockade like breccias in which rock fragments are mantled by spheroidal overgrowths of hydrothermal minerals at rusey the overgrowths are quartz dominant whereas in the roamane fault overgrowths are composed of calcite quartz or gold rich pyrite although none of the rock fragment cores of accretionary spheroids are in contact with their neighbours the spheroidal overgrowths do contact each other and are at least partially cemented together the hydrothermal overgrowths mostly comprise either outwards coarsening crystals that radiate from the surface of the core rock fragment or finer grained inequigranular to mesh like intergrowths concentric textural banding and oscillatory growth zones are present in some hydrothermal mantles the breccias occur as fault parallel layers and lenses each up to several tens of centimeters thick adjacent layers are characterized by texturally distinct ranges of clast sizes and different proportions of clasts to hydrothermal overgrowths in the rusey fault many texturally distinct breccia layers are present within a 3m wide fault core some layers truncate others and many breccia layers exhibit grainsize grading or banding clasts in the breccias include fragments of wall rock veins and various fault damage products including fragments of earlier generations of cemented breccia brecciation was episodic and separated by periods of cementation the distinctive textures of the breccias are interpreted to have formed by fluidization of fault damage products in a high fluid flux regime hydrothermal coatings developed while clasts were in a suspended state during fluid ascent through dilatant fault segments layered breccias record multiple episodes of particle fluidization and indicate that the faults provided conduits for repeated transitory fluid upflow particle size distributions indicate that fluid velocities during fluidization were in the range 0 1 ms 1 to 1 ms 1 the maximum flow rates correspond to fluid fluxes of 10 to 100 l s 1 per meter strike length of fault through dilatant fault apertures up to several tens of centimeters wide such high flow rates characteristically induce intense swarm seismicity rather than mainshock aftershock seismicity and have implications for the dynamics and rates of formation of fault related hydrothermal ore deposits

neoproterozoic garnet glaucophanites and eclogites new insights for subduction metamorphism of the gourma fold and thrust belt eastern mali

subduction ultrahigh pressure metamorphism and regurgitation of buoyant crustal slices implications for arcs and continental growth abstract intracontinental collisional belts that retain mineralogic relics of phanerozoic ultrahigh pressure uhp metamorphism are increasingly being recognized adjacent regions generally lack evidence of coeval calcalkaline volcanism plutonism following consumption of intervening oceanic lithosphere each orogen marks the site of a deeply subducted microcontinental promontory fragments of which later returned to midcrustal levels mafic and ultramafic rocks are volumetrically minor pressures in uhp complexes approached or exceeded 2 8 gpa at 600 900 c subduction zones involve low t prograde gradients and constitute the only plate tectonic realm where such conditions exist although internal portions of descending plates may have even lower geothermal gradients crustal upper margins are typified by p t trajectories of 5 10 c km attending underflow clinoamphibole rich metabasaltic rocks dehydrate at pressures 2 2 2 5 gpa this accounts for the kinetically favorable solid state production of mafic eclogites and partial fusion of morbs to form hydrous andesitic melts devolatilization of partly serpentinized mantle lithosphere also introduces h 2 o to the magmagenic zone as large masses of continental crust enter deeper portions of subduction zones mineralogic transformations slow or cease because micas the major oh bearing phases in such rocks retain volatile contents to pressures 4 gpa persistence of hydroxyl bearing layer silicates may explain the metastable preservation of low density assemblages in continental crust subjected to uhp conditions and the dearth of calcalkaline magmatism in most collision belts exhumed uhp terranes are intensely deformed and disaggregated all consist dominantly of quartzofeldspathic compositions mafic eclogites garnet peridotites constitute only a few volume percent eventual decoupling of the relatively low density subducted crust promotes buoyancy propelled transport to shallow depths along the subduction channel with the hanging wall lithosphere acting as a stress guide extensional collapse and erosional unroofing of rising masses result in the continued ascent of deeply subducted but buoyant crustal material although equidimensional massifs must return surfaceward large volume to surface areas render the contained uhp phase assemblages virtually unquenchable surviving uhp terranes thus consist of relatively thin slabs of continental crust allochthons that retain uhp relics apparently lost heat by conduction across both upper normal fault and lower reverse fault contacts based on present exposures estimated thicknesses of uhp sheets are qinling dabie sulu belt 5 km kokchetav complex 1 3 km dora maira massif 1 2 km western gneiss region 1 km slices evidently rose to midcrustal levels at exhumation rates approaching 10 mm per year back reaction attending decompression in all cases was nearly complete where uhp mineral inclusions persisted retrogression apparently was limited by the relative impermeability of the rocks to catalytic aqueous fluids and by declining temperatures the coarse grain size of host phases also disfavored complete retrogression