precambrian accretionary tectonics in the bou azzer el graara region anti atlas morocco ophiolites of the anti atlas of morocco occur as highly dismembered slivers of mafic and ultramafic rocks sandwiched between a precambrian craton and a magmatic arc tectonic slices of accretionary melange ophiolites and forearc basins have been juxtaposed by oblique subduction and transpression in the forearc region of a late proterozoic subduction zone synkinematic to postkinematic calc alkalic magmatism in these terranes indicates migration of the magmatic front over the deformed forearc terranes

thick skinned deformation observed on deep seismic reflection profiles in western argentina one of the first deep seismic reflection profiles in south america has resulted from reprocessing nearly 90 km of 96 channel vibroseis data that were originally collected by ypf the argentine national oil company across the sierra de la huerta in the sierras pampeanas of western argentina reprocessed sections show the following features in the upper 20 km of crust 1 prominent layering within carboniferous to quaternary strata and 2 a complex suite of faint reflections which are interpreted to define both extensional and compressional faults the mesozoic strata are bounded by one or more west dipping normal faults one or two east dipping thrust faults shallow with depth a geometry similar to the rocky mountain foreland of the western u s a one thrust has a clear surface trace although lacking evidence of surface scarps or clear fault plane reflections panels of constant dip domain seen in the seismic data justify the application of fault bend folding techniques to suggest that a second deeper fault becomes nearly flat at 15 20 km cumulative cenozoic offset on the thrusts beneath the sierra de la huerta is about 13 km a well located earthquake hypocenter lies within the footwall block of a major thrust and is presumed to represent susidiary thrust motion along a secondary high angle fault gravity models suggest that the thrust ramps may nucleate at an anomalous dense mass in the middle or lower crust interpretation of lower crustal structure is hampered by the lack of clear reflections presumably due to small scale and low amplitude impedance contrast variations of lithologic units at those depths consideration of recorrelation limitations and signal penetration constraints provide likely rationale for the scarcity of clear reflections in the basement rocks of the region but low reflectance of mafic metamorphic rocks remains a possible explanation

wave spectrum retrieval from airborne sunglitter images abstract reconstruction and evolution of two dimensional spectra of surface waves in the gulf of mexico are derived from airborne sun glitter imagery as the proposed method is based on a linear transfer function deduced from the shape of the sunglitter brightness the absolute wavenumber elevation spectrum does not require any additional assumption or information about sky brightness wind or wave energy the detailed description of the airborne image processing method is given as demonstrated retrieved spectra agree well with nearby ndbc buoy data both for spectrum shape level and energy angular distribution the 180 degree wave direction ambiguity inherent to image derived spectra is eliminated by using cross correlation analysis between two consecutive images a case study corresponding to the spectral evolution with increasing distance from shore in slanting fetch conditions is then considered energy level and peak position transformation are consistent with established approximations and laws of wind sea development the technical requirements flight altitude image resolution view angles etc and applicability of the suggested methodology are also discussed these results demonstrate the potential efficiency of high resolution sea state monitoring from drones or light aircrafts using sunglitter imagery

non recognition of continental tholeiites using the ti y zr diagram continental tholeiites cannot be identified using the ti y zr diagram two hundred and thirteen analyses from 16 localities representing 7 continental tholeiitic provinces all plot outside the within plate basalt field of this diagram

evolution of intra oceanic arc trench systems abstract intra oceanic island arc trench systems represent a locus of crustal thickening through the addition of mantle derived magmas which form the volcanoes and subjacent stratiform plutons the crust of immature island arcs and adjacent back arc basins is oceanic in chemistry in that it is dominated by mafic e g tholeiitic magmas a magma sequence is recognized in many arc systems which progress from boninite to arc tholeiite to calc alkaline and in some arcs to shoshonites the adjacent back arc basins are formed of morb like tholeiite arc systems may be dominated by extension rather than compression if the subducted and overriding plates are decoupled extension of the arc system may be associated with seaward retreat or roll back of the trench axis alternate cycles of extension and compression of the arc system can lead to both tectonic erosion of the fore arc and accretion of fragments from the incoming sea floor the izu bonin mariana tonga yap and palau trench arc systems are examples of different stages in a dynamic process which starts when subduction begins along old fracture zone traces crustal extension causes both fore arc subsidence and opening of back arc basins island arc volcanic chains and back arc basins are new crust formed largely of fractional melts of the mantle the evolution of an arc system may stop when blocking of the subduction zone or changes in relative plate motion causes overthrusting of back arc lithosphere on to former volcanic arcs many ophiolites preserve remnants of arc system crust and upper mantle

regional o sr and pb isotope relationships in late cenozoic calc alkaline lavas of the andean cordillera quaternary recent volcanism in the andes has occurred in three regions 45 33 s 28 16 s and 2 s 5 n each of which has a distinct plate tectonic setting and contains volcanic suites with different chemical and isotopic characteristics isotope ratios of o and sr are lowest and those of pb least variable in the southern volcanic zone svz where medium k lavas have isotopic characteristics equivalent to volcanics from intraoceanic arcs where continental crust is absent the svz lavas were probably derived from an asthenospheric mantle source above a shallow benioff zone parental basaltic magmas rose largely unmodified through thin continental crust where differentiation occurred by low pressure crystal fractionation without concurrent modification of isotopic composition the slight enrichment of 206 pb in the northern volcanic zone nvz where the crust is thin and the benioff zone deep suggests a greater subduction zone component for parental medium k magmas the slight 18 o enrichment suggests a small amount of lower crustal interaction isotope ratios of o and sr are highest and those of pb most variable in the central volcanic zone cvz parental magmas in the cvz were probably generated within the same mantle source region as those in the svz and nvz subsequently during transit through the exceptionally thick continental crust of the central andes high k magmas were produced by a combination of bulk contamination in the lower crust and later low pressure fractional crystallization assimilation processes in the upper crust which altered both chemical and isotopic compositions

proterozoic collisional basins in a pan african suture zone anti atlas mountains morocco abstract the anti atlas mountains constitute a late proterozoic suture zone produced by northward subduction of oceanic lithosphere culminating in the pan african orogeny southward migration of thrust slices associated with the destruction of the forearc terrane resulted in the uplift and erosion of previously deposited basin sediments these sediments were subsequently reincorporated into collisional basin deposits of the tiddiline formation the tiddiline formation consists of coarsening upwards sequences of maroon siltstones sandstones and intraformational conglomerates these rocks unconformably overlie metamorphosed volcaniclastic rocks of the relict forearc basin and accretionary terrane syn and post depostional deformation has resulted in folding about gently plunging fold axes folds were subsequently cut by strike slip faults that strike at a high angle to the basin axis deformation of the tiddiline formation is attributed to transpressional suturing of the relict forearc terrane to the west african craton to the south collisional basins of the anti atlas mountains serve as ancient analogs for the destruction of forearc basins in an obliqueconvergent margin setting such as those of the western pacific region

field and textural evidence of two migmatization events in the sierras de cordoba argentina abstract in the metamorphic basement of the central eastern sierra de comechingones sierras pampeanas of cordoba argentina two types of migmatites are found tala cruz metatexites and canada del sauce diatexites rafts of deformed metatexites in the diatexites are interpreted as dismembered and partially assimilated remnants of the tala cruz metatexites two migmatization events are recognized the m2 metamorphic peak produced high grade paragenesis and the first anatexis generating the metatexites 810 840 c and 8 5 9 kbar were calculated for m2 the m4 decompression event produced the diatexites by a second melting of the metatexites 725 780 c and 5 5 6 kbar were estimated for m4 a dynamic contractional m3 event in high ductility and high temperature conditions was recognized between the two migmatization events a correlation could be established between the second migmatization event m4 and the event dated as cambrian 520 530 ma in other areas in the sierras de cordoba in which case the first migmatization event m2 would have been older perhaps as old as neoproterozoic the tectonic evolution of the sierra de comechingones would imply the occurrence of extension shortening cycles interpreted as pull push cycles mirroring the subduction and collision of pampia terrane against the western gondwana margin these cycles would have controlled the evolution of the eastern sierras pampeanas during the neoproterozoic early palaeozoic times

basic volcanic rocks and tectonic setting a discussion of the zr ti y discrimination diagram and its suitability for classification purposes the zr ti y diagram cannot discriminate between normal and anomalous oceanic ridge basalts the latter tend to plot as within plate basalts this indicates that it is the mantle anomaly rather than the tectonic setting that characterizes the zr ti y interelement ratios in such rocks it is shown that rocks with sio2 56 of a tholeiitic series indicate a differentiation trend from ocean floor via within plate to calc alkaline basalts this feature shows that great care should be taken when data for altered or metamorphic rocks are selected for classification by this method

volcanic glass compositions of the troodos ophiolite cyprus fresh volcanic glass is preserved throughout the extrusive section of the troodos ophiolite which indicates that the lavas have not been pervasively metamorphosed glass compositions reveal the existence of two major magma suites apparently corresponding to distinct stratigraphic intervals the basal 400 500 m of the sequence consists of an andesite dacite rhyolite assemblage containing abundant hyaloclastites the remainder of the section comprises a basalt basaltic andesite assemblage with high mgo and low tio 2 and total iron the lower sequence is interpreted as an evolved arc tholeiite suite the upper has some similarities to boninitic lavas the close association in time and space of these two suites is similar to that observed in the mariana and bonin arcs and suggests that all of the troodos lavas were erupted in a subduction zone environment most probably in an incipient arc or fore arc

structural and geochronological constraints on the evolution of the bou azzer neoproterozoic ophiolite anti atlas morocco abstract the bou azzer ophiolite in the moroccan anti atlas represents the suture of the neoproterozoic pan african orogen the bou azzer inlier includes from ne to sw three main geological units a volcanic arc an ophiolite and a continental platform several subduction related plutons intrude the ophiolite and the volcanic arc and syn to late orogenic deposits unconformably cover the three terranes the eburnean 2 ga continental basement observed in other inliers of the anti atlas appears not to crop out at bou azzer where rocks traditionally attributed to that old basement are in fact neoproterozoic rocks either metagranitoids of the continental margin or ophiolitic metagabbros the age of a metagabbro of the ophiolite is reported here for the first time as 697 8 ma shrimp u pb on zircons u pb geochronological data are also presented for two syn kinematic subduction related plutons which together with previous data establish a time span of ca 655 635 ma for these arc type rocks moreover the oumlill granite previously attributed to the eburnean basement has been dated as 741 9 ma the earliest tectonometamorphic event affecting the ophiolite is recorded as a relic foliation and a garnet rutile metamorphic assemblage which might attest to an early peak pressure exceeding the 5 6 kbar medium pressure reported so far thus we suggest a first stage of ophiolite underthrusting below a volcanic arc followed by ophiolite exhumation and the concomitant development of the main low grade planar linear tectonic fabric the ages of the calc alkaline syn kinematic subduction related plutons 655 635 ma might be taken as the end of subduction and the beginning of the ophiolite exhumation obduction the author s work in the bou azzer region supports the ophiolite continental platform interpretation of leblanc 1975 and leblanc and moussine pouchkine 1994 rather than the tectonic melange interpretation proposed by saquaque et al 1989b and others a recumbent anticlinal and the basal thrust of the ophiolitic terrane are the main macrostructures formed during the obduction stage characterized by top to the w sw kinematics this deformational event is also recorded in the rocks of the underlying continental platform a syn orogenic unit namely the tiddiline formation unconformably overlies the arc volcanics the ophiolite and the continental platform further collisional deformation affected the tiddiline sediments giving way to ese wsw striking upright folds and oblique reverse faults the latest structures in this neoproterozoic suture were left lateral strike slip faults pointing to an increase in the left lateral component of the continental collision

petrology chemistry and isotopic composition of submarine volcanoes in the southern mariana arc four submarine volcanoes in the southern and central mariana arc were dredge sampled the recovered lavas were analyzed for major and trace element abundances including rare earth elements ree mineral compositions and sr and nd isotopic ratios the samples exhibit a great range of chemical and mineral compositions relative to subaerial exposures in the central arc zealandia bank produces high magnesium basalts mgo 8 9 wt enriched in ni and with forsteritic olivine as much as fo 88 this lava composition is probably parental to more fractionated basalts and basaltic andesites exposed on the adjacent islands of guguan and sarigan the southernmost seamounts in the mariana arc south of lat 15 n produce dacitic pumice a composition that is not common in the active arc ree data preclude their derivation from a basaltic parent by fractional crystallization they may be derived by direct partial mantle melting under water saturated conditions at low pressure nd and sr isotopic data 143 nd 144 nd 0 5130 0 5131 87 sr 86 sr 0 7032 0 7035 indicate a mantle source for the mariana arc lavas similar in isotopic characteristics to sources for ocean islands significant contributions to these arc magmas from subducted oceanic crust or sediment are not evident a spatial distribution in magmatic characteristics coincides with a structural break in the arc near 16 n south of this submarine seamounts produce incompatible element enriched lavas with relatively high 87 sr 86 sr 0 7034 0 7035 these lavas were produced by a relatively low degree of partial melting 5 15 of a shallow spinel bearing mantle to the north zealandia bank and the central arc islands are characterized by lower incompatible element abundances and lower 87 sr 86 sr 0 7032 0 7033 these lavas were produced by higher degrees of partial melting 15 25 of a deeper garnet bearing mantle

distribution and origin of igneous rocks from the landward slopes of the mariana trench implications for its structure and evolution the landward slope of the mariana trench is composed largely of igneous rocks serpentinites and serpentinized ultramafic rocks occur at nearly all structural levels on the slope from depths of 8000 to 1200 m seamountlike features on the trench slope break are the surface expression of serpentinite diapirs cumulate and massive gabbros are found several varieties of volcanic rocks are common including boninites altered and metamorphosed basalts andesites and dacites the chemical characteristics of the volcanic rocks indicate that nearly all are products of island arc volcanism together with the gabbros these volcanic rocks represent what is probably a late eocene arc complex these rocks were probably the first volcanic products to result from the subduction of the pacific plate beneath the phillippine sea plate their exposure on the trench slope today implies a significant amount of tectonic erosion of the landward slope since eocene time most of this removal of material appears to have occurred during the early stages of subduction there are isolated occurrences on the landward slope of rock assemblages including alkalic basalts chert hyaloclastites upper cretaceous siliceous sediments and shallow water limestones these assemblages are very similar to rocks dredged from seamounts on the offshore flankmore of the trench and their presence on the landward slope suggests that since the cessation of vigorous tectonic erosion there has been episodic accretion of seamount fragments to the landward slope less

geochemistry of precambrian ophiolites from bou azzer morocco the upper proterozoic ophiolite complex of bou azzer morocco includes ultramafic rocks cumulate gabbros sheeted dykes pillow lavas and diorite quartz diorite intrusions and an overlying volcano sedimentary sequence the gabbroic cumulates basaltic flows and dykes have compositions similar to recent ocean floor rocks n and or t type among other features they have comparable light ree depleted patterns and relations of ti zr and la nb although fractional crystallization played an important role in the evolution of these rocks the large variations in their chemical compositions require generation from a heterogeneous upper mantle source and or by a dynamic partial melting process diorites quartz diorites and the volcanic rocks of the overlying sequence are calc alkaline genetically unrelated to the tholeiitic suite and indicative of an island arc setting a possible tectonic model for the ophiolite complex is a marginal basin just behind a still active island arc

petrological and geochemical characterization of proterozoic ophiolitic melange nellore khammam schist belt se india abstract the ophiolitic melange occurs close to a major fault within the metavolcanics of kanigiri area in the western margin of the khammam nellore schist belt andhra pradesh india the unique feature of kanigiri melange is the occurrence of ophiolitic bodies of various sizes most of which display petrological and geochemical characteristics typical of supra subduction zone oceanic crust the ophiolitic melange is composed mainly of blocks of variably deformed and metamorphosed pillow basalts serpentinized ultramafic rocks meta gabbros cherts and pelagic sediments in a pervasive scaly argillite matrix the melange has a tectonic contact with a granite intrusion and numerous thrust faults cut across the melange exposures primary magmatic features such as drainage cavities in pillow basalts magmatic layering in gabbros and clinopyroxene cumulates in ultramafic rocks are well preserved in low strain domains mafic components enclosed in the melange are characterized by low pressure secondary mineral assemblages retrograded from amphibolite down to greenschist facies as commonly observed in subducted and exhumed orogens all amphiboles in the studied fragments are calcic and cluster in the range from winchite actinolite to hornblende amphibole compositions are characterized by al 2 o 3 2 4 13 8 feo 5 13 k 2 o 0 04 0 7 and x mg 0 67 0 77 the metabasalt and wherlite samples exhibit light rare earth element lree enriched patterns and negative high field strength element hfse zr and ti consistent with a suprasubduction zone geodynamic setting a notable feature of both metabasalt and wherlite samples is their pronounced negative ce anomaly and a positive eu anomaly as observed in a number of modern island arc suites as the petrological features and the geochemical characteristics of the remnant fragments we analyzed in this study are reminiscent of their phanerozoic ophiolite counterparts we suggest that these rocks were derived from a proterozoic oceanic plate belonging to a suprasubduction ophiolite assemblage subduction zone geochemical signatures and ophiolite like crustal lithologies in the study region are consistent with the accretion of juvenile oceanic fragments to the pre existing continental blocks through a forearc continent collision

geology geochemistry and tectonic significance of mafic ultramafic rocks of mesoproterozoic phulad ophiolite suite of south delhi fold belt nw indian shield abstract a thick sequence of mafic ultramafic rocks occurs along a major shear zone phulad lineament running across the length of aravalli mountain range for about 300 kms it has been suggested that this sequence may represent a fragment of ophiolite or a rift related metavolcanic suite made up of basalts and fractionated ultramafics the geological and tectonic significance of the complex is assessed using field relationships petrography and geochemistry structurally the lowest part of the complex comprises a discontinuous band of plastically deformed harzburgite mantle component followed by layered cumulus gabbroic rocks crustal component a complex of non cumulus rocks comprising hornblende schists gabbros sheeted dykes and pillowed basalts structurally overlies layered gabbros huge bodies of diorite intrude volcanics geochemical classification suggests that all non cumulus mafic rocks are sub alkaline basalts except one variety of dykes which shows mildly alkaline character the sub alkaline rocks are tholeiite to calc alkaline with boninite affinity tectono magmatic variation diagrams and morb normalised patterns suggest a fore arc tectonic regime for the eruption of these rocks the mafic rocks of phulad ophiolite suite are zoned across the strike in terms of their distribution from west to east the hornblende schists and basalts are exposed at the westernmost margin followed by gabbros and dykes the alkaline dyke occurs at the easternmost part the rocks of phulad suite are juxtaposed with shallow water sediments in the east followed by platformal sediments and then continental slope sediments in the further east indicating gradual thickening of the crust from west to east and an eastward subduction the geochemical interpretation presented in this study together with discussion of lithological association is used to decipher the tectonic evolution of the mesoproterozoics of nw indian shield

proterozoic blueschist bearing melange in the anti atlas mountains morocco abstract blueschists from the bou azzer inlier provide compelling evidence for late proterozoic subduction in the anti atlas mountains of morocco high pressure low temperature metabasites containing blue amphibole minerals crossite and magnesioriebeckite record pressures in excess of 5 kbar together with regional relationships the geologic setting of the blueschists constrains the polarity of pan african subduction in this region which occurred from 750 to 600 ma blueschist facies rocks crop out in a heterogeneous assemblage of variably deformed and metamorphosed tectonic slices of ophiolitic fragments enclosed in a schistose serpentinite matrix the melange belt containing the blueschist facies rocks is intruded by a number of diorite plutons one of which has yielded a u pb radiometric age of 650 ma together with transaharan belt to the southeast the anti atlas suture zone exposed within the bou azzer inlier contains among the oldest known blueschist bearing ophiolitic melanges in the world

chromite pods in a lherzolite massif collo algeria evidence of oceanic type mantle rocks along the west mediterranean alpine belt abstract the high temperature lherzolite massifs of ronda southern spain and beni bousera northern morocco are ascribed by various authors to sub continental mantle diapirs emplaced within a granulite metamorphic aureole in continental rifts along transcurrent lithospheric faults the collo plagioclase lherzolite massif with associated kinzigite and related anatectic rocks belongs to the same type of geodynamic context about 17 chromitite bodies are found in the collo lherzolite massif their structural textural and geochemical characteristics are those of chromitite pods that are typical of ophiolite complexes i e of an oceanic lithosphere the collo massif is an atypical high temperature lherzolite massif displaying both continental and oceanic characteristics it could correspond to a mantle intrusion in a transition zone from continental to oceanic rift type

hydrothermal alteration of a 1 km section through the upper oceanic crust deep sea drilling project hole 504b mineralogy chemistry and evolution of seawater basalt interactions deep sea drilling project hole 504b penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes alteration of the crust at site 504 occurred in four stages related to the movement of the crust away from the spreading axis 1 dikes reacted with seawater 200 300 c in the upwelling zone of an axial convection cell at the spreading axis resulting in the formation of greenschist facies parageneses in veins and host rocks mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologic transition zone causing a steep temperature gradient at the base of the pillow section secondary minerals formed in the lower pillow section from the resultant reducing mixed fluids at temperatures of around 100 c at the same time the initial effects of seafloor weathering began in the upper 320 m of the pillow section at low temperatures 50 c and under conditions of open circulation of oxidizing seawater 2 following refracturing of the dikes a second stage of axial upwelling occurred hydrothermal fluids 200 380 c were probably similar to those presently sampled from spreading ridges on the seafloor mixing of these fluids with seawater in the lithologic transition zone caused deposition of quartz epidote and sulfides in veins and formation of a sulfide rich stockworklike zone within this transition zone alteration of the pillow section proceeded under the prior conditions with the effects of seafloor weathering extending progressively downward into the crust 3 seawater recharge penetrated to depths of at least 1075 5 m subbasement and deposited anhydrite locally in veins 4 off axis alteration of the dikes was characterized by formation of zeolites calcite and prehnite in veins and host rocks from more highly evolved and lower temperature 100 250 c fluids alteration in the upper pillow section evolved from seafloor weathering conditions to more reducing and rock dominated as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment zeolites and calcite were the last phases to form throughout the pillow section sealing of the crust to convective cooling also allowed conductive reheating of the crust from below calculations of seawater crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt seawater interactions are a sink for seawater mg and k and a source for si and ca

structural interplay between plutons during the construction of a batholith cerro aspero batholith sierras de cordoba argentina magnetic fabric patterns and microstructures of granitic rocks provide evidence of structural modifications as a result of the coalescence of two plutons alpa corral and talita during the construction of a large granitic batholith cerro aspero in the sierras pampeanas of cordoba argentina the alpa corral granite is a small and nearly circular pluton while the larger talita pluton displays a marked n s elongation the nw quadrangle of the alpa corral pluton underwent a deformation resulting in foliation trajectories that outline a crescent moon pattern whose inner arc is concordant with the alpa corral talita contact the highest values for the magnetic susceptibility and its anisotropy are concentrated in the coalescence zone between both plutons the magmatic and sub magmatic nature of the microstructures demonstrates that such a deformation was caused by the southward overriding of the largest talita pluton while the alpa corral pluton was not yet fully crystallized rb sr geochronology indicates that this process took place 369g9 ma ago long after the famatinian ordovician to early devonian deformation of the country rocks which confirms a synmagmatic rather than a regional deformational event q 2006 elsevier ltd all rights reserved

buoy measurements of wind wave relations during hurricane matthew in 2016 abstractstudies suggested that neutral stability wind speed at 10 m u10 9 m s 1 and wave steepness hs lp 0 020 can be taken as criteria for aerodynamically rough ocean surface and the onset of

unusual massive magnetite veins and highly altered cr spinels as relics of a cl rich acidic hydrothermal event in neoproterozoic serpentinites bou azzer ophiolite anti atlas morocco abstract if magnetite is a common serpentinization product centimetric massive and pure magnetite veins are rarely observed in serpentinites unique example in the ait ahmane ultramafic unit bou azzer neoproterozoic ophiolite anti atlas morocco allows to assess the hydrothermal processes that prevailed at the ending precambrian in this study rock magnetism petrography mineral and bulk chemistry are combined to assess iron behavior in these meta ultramafics in order to constrain the serpentinites alteration and magnetite veins formation processes very high cr low mg high fe 3 and low ti content of cr spinels cores reflect a supra subduction zone origin for the ait ahmane serpentinites precursor typical lizardite chrysotile pseudomorphic texture in fresh serpentinites reveals an initial oceanic like serpentinization involving temperature 200 c magnetic measurements reveal a lower magnetite content in hydrothermalized serpentinites hosting the magnetite veins with lowest values down to 0 58 wt for bleached serpentinites constituting the wall rock of the veins these magnetic data are consistent with bulk rock chemistry showing a lower total iron content in hydrothermalized serpentinites hysteresis parameters and thermomagnetic measurements denote a magnetic grains size that increases with the alteration associated with the emergence of a new magnetic phase cr magnetite produced by cr spinels alteration a new proxy based on thermomagnetic measurements the crm m ratio provides a quantification of its contribution to the magnetic susceptibility mineral chemistry allowed to identify the cr spinels alteration sequence and reveals an important chlorine enrichment in serpentine phases from hydrothermalized serpentinites these results suggest that a cl rich acidic hydrothermal event involving temperatures below 350 c produced an intense magnetite leaching in the host serpentinite and an advanced cr spinels alteration to ferritchromite and cr magnetite iron provided by this leaching have led to the formation of unique magnetite veins in the ait ahmane ultramafic unit two different settings are proposed for the hydrothermal event 1 a continental hydrothermal system as advanced for the co ni as ores in the bou azzer inlier or 2 an oceanic black smoker type hydrothermal vent field on the neoproterozoic seafloor

evolution of the sierras de cordoba argentina abstract located in the southeastern part of the sierras pampeanas argentina the pampean ranges of cordoba show the evolution at the western margin of the rio de la plata craton from extensional stages during neoproterozoic times to compressive stages during the pampean orogeny the cumbre de gaspar group represents distal deep sea clastic sedimentation deposited in a passive margin environment very possibly during a time of incipient oceanization the santa sabina group representing shelf sedimentation rests conformably on the cumbre de gaspar group stocks of granites and granodiorites of calc alkaline affinity intruded the cumbre de gaspar group during the early pampean or earlier the polyphase pampean orogeny involves several distinct metamorphic and orogenic events metamorphism is discussed using the chemical composition of minerals the major effect of the pampean orogeny was the formation of a pile of westward thrust blocks the rio de la plata craton thrust above its marginal basin forms the uppermost unit the intrusion of the el pilon granite dated at 520 5 ma post dates these tectonic and metamorphic events after this main orogenic event the domain remained permanently located on the cratonic side of younger famatinian and andean orogenic events the mermela group probably corresponds to the molasse of the pampean orogeny and to the base of a new sedimentary cycle it is difficult to distinguish famatinian tectonics in the sierras de cordoba because the main orogeny occurred further west however this orogenic cycle is marked by the intrusion of the large achala batholith during the late palaeozoic and mesozoic the sierras de cordoba remained a stable platform the whole region was rejuvenated by the andean orogeny which induced tilted blocks in its foreland and the pocho volcanics of shoshonitic affinities

geochemistry and significance of metavolcanic rocks from the bou azzer el graara ophiolite morocco abstract the bou azzer el graara ophiolite has been interpreted as oceanic or backarc crust overlain by a calc alkalic arc sequence or as a composite forearc and accretionary melange seventy nine new major and trace element whole rock analyses of the metavolcanic rocks from throughout the complex support the latter interpretation the results of this study indicate that the bou azzer metavolcanics comprise three petrographically and geochemically distinct groups 1 a a siliceous metavolcanic series ranging from basaltic andesites to rhyolites with calc alkalic characteristics is the principal component of the northern section of the complex it includes distinct quartz bearing rhyolite flows isolated massive pyroclastic layers are typically associated with this series these rocks are interpreted as forearc basin volcanics and pyroclastics produced during late proterozoic north dipping subduction 2 b low ti metabasalts which display island arc tholeiitic affinities occur as isolated flows or blocks in most of the ophiolite these rocks may represent fragments of the forearc basement below the siliceous series 3 c high ti metabasalts which range from enriched tholeiitic zr y 4 compositions are the principal volcanic block in the southern melange like terrane of the complex these have high ba and sr contents 100 200 ppm average ba 260 ppm average sr they are identical to basaltic compositions from tachdamt and bleida on the southern margins of the ophiolite which have previously been interpreted as a rifted continental margin sequence these rocks could represent seamount volcanic products which have been thrust into an accretionary prism along the north dipping subduction zone or rift margin basalts which have been assembled in a tectonic melange as a consequence of a forearc continent collision both the low ti and high ti metabasalt groups include samples previously identified as ocean ridge or back arc volcanics however there is no compelling evidence for the occurrence of these lava compositions in this region the distribution of rock types within the ophiolite is most consistent with a previously proposed model which views the complex as three allochthonous terranes which were juxtaposed during collision to form the bou azzer el graara complex

an examination of fetch limited wave growth off the west coast of new zealand by a comparison with the jonswap results abstract a long term wave recording program based on the west coast of new zealand has allowed a number of fundamental properties of the ocean wave field to be studied this paper deals with wave generation processes and in particular compares the spectral properties of fetch limited seas with those found in the jonswap and other northern hemisphere studies

the structure of the precambrian and lower paleozoic basement of the central andes between 22 and 32 s lat three major diastrophic cycles defined by their structural style and their spatial distribution are recognized in the andean basement of this region the oldest structures are related to the panamerican orogeny 500 to 700 m a which produced in the central craton multiply deformed complexes of schists gneisses and granites that are covered discordantly by unmetamorphosed cambrian and ordovician beds west of the central craton ordovician sedimentary beds are folded with a simple structural style and intruded by granites both the sedimentry beds and the granites are covered discordantly by undeformed devonian sequences the folding of the ordovician is attributed to the ocloyic phase of the caledonian movements west of the ocloyic belt is another foldbelt consisting of strongly folded devonian beds attributed to the chanic phase hercynian the chanic belt is intruded by carboniferous and permian granites and covered discordantly by carboniferous and permian sequences

high frequency tail characteristics in the coastal waters off gopalpur northwest bay of bengal a nearshore modelling study over the years continued uncertainty amid 4 and 5 frequency exponent representation observed in the slope of the high frequency tail of a wind wave frequency spectrum is a major concern to comprehend the nature of the high frequency tail an effort has been made to assess the slope of the high frequency tail with measured data recorded for 3 years off gopalpur the study demonstrates that the high frequency slope of the spectra varied seasonally in the range of n 2 13 to 3 48 the swell and wind sea parameters calculated by separation frequency method shows that 64 6 of waves were dominant by swell and the rest 34 9 by sea annually single double and multi peaked spectra occur 12 23 71 80 and 15 37 annually to simulate wave spectra the nested wam swan model is forced with era interim winds and 1d wave spectra comparisons when performed proved to be encouraging from the comparisons of measured and theoretical spectra it is concluded that jonswap model could not describe the high frequency tail of measured spectrum as indicated by the very high scatter index ranging from 0 24 to 1 44 whether there exists a correct slope for the high frequency tail is still a question moreover the philosophy of a unique slope at any coastal location remains uncertain for the wave modelling community

rb sr geochronology of granitoid rocks from the pampean ranges argentina a reconnaissance rb sr geochronological investigation of granitoid rocks from the pampean range provides a framework whereby four culminations of igneous activity are delineated granitoids emplaced prior to 500 ma pre and synkinematic bodies ca 475 ma late kinematic bodies and ca 440 and 340 ma post kinematic bodies a number of the post kinematic granitoids have disturbed sr isotope systematics which is related to the formation of microcline megacrysts the carboniferous granitoids are distinguished by their high initial sr ratios i e 0 710 a feature compatible with an origin involving reworking of upper crust or crustal contamination there appears to be no systematic e w migration of granitoid emplacement during the ca 300 ma evolution of the basement rocks in the pampean ranges

structural evolution of the sierra de san luis eastern sierras pampeanas argentina implications for the proto andean margin of gondwana

the las chacras potrerillos batholith pampean ranges argentina structural evidences emplacement and timing of the intrusion within the southern part of the sierra pampeanas the sierra de san luis argentina a series of extensive intrusive bodies are regarded to post date the famatinian cycle but were emplaced during the achalian a period of heterogeneous deformation along crustal scale fault zones the largest of those is the las chacras potrerillos batholith that is situated at the northern end of the transpressive sinistral guzman shear zone this composite pluton exhibits three sub domains that comprise two granitoid sub units each the southern potrerillos stock muscovite bearing red granite and biotite bearing red granite and the central biotite porphyritic granite and giant porphyritic granite and northern domain equigranular granite and porphyritic granite of the las chacras stock the crystallisation ages of the biotite porphyritic granite is around 381 ma u pb on zircons and pb pb on sphene while the host rock was already cooled below 350 c at 420 ma thermal modelling approaches favour a pulsed intrusion with a duration of 1 5 ma the emplacement was followed by rapid cooling below the muscovite cooling temperature biotite cooling ages in different sub units reflect either a long lasting cooling history of approximately 30 ma which is supported by the modelling or a reheating effect at around 350 ma devonian age determinations on the fault rocks and granitoids point to a syn tectonic emplacement of the batholith the pluton is interpreted to be positioned at the crossover of sinistral shear zones the origin of this nne directed extensional setting in a transpressive regime seems to be related to the transfer of displacement along a secondary set of nnw trending sinistral faults the final emplacement is due to a subsequent ballooning of the batholith following the direction of space creation this model is based on the relative timing of the emplacement sequence and macroscopically visible planar fabrics in the field as well as magnetic fabric data our results indicate that the emplacement is syn kinematic with respect to the achalian deformation event

sedimentary and volcanic evolution of a neoproterozoic continental margin bleida anti atlas morocco abstract a tentative geodynamic evolution of the neoproterozoic continental margin of the northern border of the west african craton is reconstructed using both sedimentological observations and geochemical arguments from volcanic rocks the rifting stage was marked first by intermittent subsidence of a very shallow shelf environment leading to deeper water sedimentation conditions with local surges of deltaic channels this rifting stage was also characterized by the eruption of a 500 m thick pile of submarine basic lava flows with geochemical characteristics similar to those generally observed in basaltic lavas erupted during continental rifting this first rifting stage 790 ma clauer 1976 was succeeded by a drastic change both in sedimentation and volcanism the younger sediments are deep water distal turbidites with associated pyritic black shales they include a few highly differentiated tholeiitic flows and sedimentary exhalative copper deposits linked to felsic volcanism these changes are attributed to the development of deep water oceanic basins along the northern border of the west african craton the closure of this pan african oceanic domain occurred by transpression tectonics along this major lithospheric transform zone which is roughly e w trending

observation based source terms in the third generation wave model wavewatch iii updates and verification abstractthe observation based source terms available in the third generation wave model wavewatch iii i e the st6 package for parameterizations of wind input wave breaking and swell dissipatio

basement tectonics of the southern pampean ranges argentina upper precambrian to lower paleozoic crystalline basement of the southern pampean ranges is composed of 1 a narrow western zone having low to medium grade metamorphic rocks and relatively few granites and migmatites and 2 a wide central eastern hercynotype low pressure zone with abundant granitoids the internal structures fall into three distinct domains characterized by the following orientations dl nearly e w d2 nnw sse and d3 nne ssw the tectonic evolution is interpreted as having begun in an upper precambrian orogenic cycle commencing with lithospheric rifting followed by compression forming d1 structures next was a mobile belt cycle composed of two main stages the first one which generated the main tectonic features d2 of the igneous metamorphic complex coincided with a syntectonic to late tectonic granitic intrusive event with the principal metamorphic episode and with paleozoic andean tectonic trends related to the activity of the gondwanaland pacific margin the last one was a fracturing and an open weak folding d3 event with local low grade metamorphism it occurred during the silurian

mat related sedimentary structures in neoproterozoic peritidal passive margin deposits of the west african craton anti atlas morocco abstract proterozoic inliers in the central anti atlas mountains expose predominantly siliciclastic sedimentary successions deposited in peritidal zones along the neoproterozoic continental margin of the west african craton wac the low grade metamorphic and modestly deformed sediments contain a wealth of sedimentary structures related to the former presence and activities of microbial mats and respective physicobiological processes the well preserved structures include wrinkle structures erosion marks microbial sand chips spindle shaped and subcircular microbial shrinkage cracks and possibly gas domes and cabbage head structures thin sections exhibit mat fragments and dispersed grains of hematite limonite after pyrite in fine grained quartzitic storm deposits post storm layers frequently consist of matrix supported sand sized to silt sized grains and are overlain by argillaceous veneers including isolated silt sized grains and black carbonaceous laminae the muddy veneers are considered to represent compacted stacks of microbial mats biolaminites which colonized and biostabilized storm and post storm layers and thus prevented them from eroding in the absence of grazing and burrowing organisms and at suitable depositional and hydrodynamic conditions it may be expected that proterozoic microbial mats extensively grew from the supratidal to the intertidal zones and occasionally e g behind protective barriers in the subtidal zone and beyond mat related structures however need specific conditions for their formation and preservation wrinkle structures erosion marks and microbial sand chips require tractional currents and soon deposition and burial respectively such conditions are preferably met in intertidal and supratidal zones spindle shaped and subcircular cracks require mat shrinkage due to either desiccation or syneresis crack propagation implies progressive shrinkage while superposition of crack generations indicates repeated alternation between mat exposure and flooding respective conditions prevail in the upper intertidal zone gas domes and cabbage head structures are related to the production of gas from decaying organic material beneath a sealing cohesive mat they may be some of the first features formed during genesis of petee structures in the intertidal to lower supratidal zones mat related structures may serve as sensitive facies indicators once their modes of formation are revealed

petrology and geochemistry of late upper proterozoic volcanic rocks from the pan african belt in north western africa geodynamic implications and crustal evolution

isotopic and trace element study of the loon lake pluton grenville province ontario the loon lake pluton is a small reversely zoned pluton with subordinate isolated bodies of diorite and granodiorite gneiss this pluton is located within the hastings basin of the grenville provin

geochemistry of early mesozoic tholeiites from morocco abstract mesozoic dolerites from two areas of morocco the high atlas fold belt between marrakech and demnat and the anti atlas belt in the area of foum zquid are most high ti quartz normative tholeiites whichi in many respects resemble mesozoic dolerite dikes from eastern north america the dolerites display a wide range of major and trace element compositions some of which are due to fractional crystallization the doleritic sequences from high atlas also show vertical stratigraphic zonation which is characterized by a progressive depletion of lithophile elements toward the top this trend together with regularities of trace element ratio variations are indicative of a dynamic melting of an initially homogeneous source it is suggested that the continental upper mantle source for dolerites of morocco was enriched in several incompatible elements in comparison with the upper mantle source for ocean floor tholeiites