structural tests of diapir hypotheses in archean crust of ontario detailed structural maps of two granitoid complexes in the wabigoon subprovince are used to test three diapir hypotheses advanced in earlier papers the gneiss masses of and individual domes within the complexes fail the test for solid state diapirism the gneiss domes also fail the test for tensile bending caused by hypothetical magmatic diapirs in the subsurface an oval pluton located near the best exposed gneiss dome proves to be a synformal sheet rather than a funnel shaped magmatic diapir this pluton could be a syenite diorite phacolith emplaced into a concordant zone of dilation during the late upright folding of the gneiss mass about horizontal axes earlier tight folds were probably recumbent and south verging and like the gneissosity generated in a ductile shear regime with subhorizontal glide planes these observations have important implications for archean tectonics especially the relative horizontal displacement of large greenstone masses potentially allochthonous greenstone belts

pattern of sedimentary infilling of fossil mammal traps formed in pseudokarst at cerro de los batallones madrid basin central spain fossil mammal sites of late miocene age ca 9 ma occur in hourglass shaped non interconnected cavities up to 15 m deep hosted in mudstone mostly sepiolite chert and carbonate bedrock in cerro de los batallones this paper provides a model for the sedimentary infilling of the cavities which functioned as traps for vertebrate faunas and contain one of the richest and best preserved neogene mammal assemblages of the iberian peninsula generation of the mammal bearing cavities started with the solution of underlying evaporites which resulted in fissures that were subsequently enlarged by subsurface piping a process rarely preserved in the ancient sedimentary record the system of subterranean cavities evolved into a pseudokarst landscape resulting in doline like shafts reaching the ancient land surface the sedimentary infilling of the cavities comprises both clastic and carbonate lithofacies that were investigated by outcrop observation standard and scanning electron microscope petrography mineralogical analysis and stable isotope geochemistry gravel and breccia talus deposits clast and mud supported gravel pebbly sandstone and mudstone are common detrital infill deposits mostly derived by overflow erosion of bedrock the deposits containing the mammal bones are marls and occur both in subsurface cavities and doline like depressions in the underground cavities marlstone was mainly of clastic origin and accumulated in ponds scattered over the floor of the cavity in contrast marlstone deposits in the surface dolines formed mostly as a result of biochemical carbonate deposition in small shallow lakes subjected to fluctuation of the water level the 18o and 13c carbonate values indicate different origins for the two kinds of marls during the final phases of pipe infill the doline marlstone sealed the mammal sites usually off lapping the adjacent bedrock

insights from quartz cathodoluminescence zoning into crystallization of the vinalhaven granite coastal maine the vinalhaven intrusive complex provides field and petrographic evidence for multiple replenishments of mafic and silicic magmas mingling and limited mixing and rejuvenation of granite quartz in granitic rocks preserves a record of those processes in the form of cathodoluminescence cl zoning which is related to concentration of titanium and to temperature of crystallization using the new titaniq ti in quartz geothermometer injection of mafic melts into partly crystalline vinalhaven granite resulted in partial quartz resorption followed by higher temperature growth from h2o undersaturated melt this is shown by steep rimward increases in cl intensity and ti content across discordant boundaries that truncate older growth zones quartz zoning in granite affected by mafic magmas displays large rimward jumps in ti content whereas quartz in granitic feeders and in granite far from mafic rocks typically displays broad rims with decreasing ti contents consistent with slow cooling without thermal disruptions due to mafic recharge

crustal assimilation during turbulent magma ascent ata new isotopic evidence from the mull tertiary lava succession n w scotland assimilation of crustal rocks with concomitant fractional crystallisation afc is a well documented phenomenon in many igneous suites but geochemical evidence from the tertiary mull lava succession suggests that in these magmas crustal contamination occurred by a distinctly different mechanism lavas from the lower half of the mull plateau group mpg can be divided into two broad sub types high 8 mgo basalts with elevated ba and k and lower mgo 8 basaltic hawaiites with lower ba and k the lower crust and most of the upper crust beneath mull is probably of lewisian age the sr nd and pb isotope compositions of local lewisian crustal samples yield the following ranges 87sr 86sr 0 71002 0 72348 143nd 144nd 0 51045 0 51058 and 206pb 204pb 14 0 14 6 ten lavas have also been analysed and yield the following ranges 87sr 86sr 0 7028 0 7042 143nd 144nd 0 51214 0 51230 and 206pb 204pb 15 1 17 9 however within this range it is predominantly the more primitive mafic compositions with elevated mg ba and k that show the lowest nd and pb and the highest sr isotope values modelling of these isotopic results in conjunction with major and trace element data show that 1 contamination by lewisian lower crustal material does occur 2 that the process involved was not one of assimilation with concomitant fractional crystallisation afc the proposed contamination process is one whereby the hottest most mgo rich magmas have assimilated acidic partial melts of lewisian lower crust during turbulent ascent ata through thin poorly connected dyke and sill like magma chambers the chemical composition of the contaminated lavas can be modelled successfully through addition of 5 acidic lewisian crust to an uncontaminated lava in contrast the more evolved magmas which probably fractionated at sub crustal levels were either not hot enough to molt significant amounts of crust or did not ascend turbulently because of their higher viscosity and so are less contaminated with crust

microstructural evidence for magma confluence and reusage of magma pathways implications for magma hybridization karakoram shear zone in nw india in the karakoram shear zone ladakh nw india miocene leucogranitic dykes form an extensive varied and complex network linking an anatectic terrane exposed in the pangong range with leucogranites of the karakoram batholith mineral paragenesis of the heterogeneous anatectic source rocks suggests melting has resulted from water influx into rocks at upper amphibolite facies conditions and microstructures suggest anatexis was contemporaneous with shearing the network is characterized by continuous and interconnected dykes with only rare cross cutting relationships forming swarms and chaotic injection complexes where magmatic rocks cover up to 50 of the outcrop area despite this volume of magma the system did not lose continuity suggesting that it did not flow en masse and that the magma network was not all liquid simultaneously leucogranites in this network including leucosomes in migmatites carry an isotopic signature intermediate between the two main anatectic rocks in the source suggesting efficient homogenization of the magmatic products here we describe a number of microscopic features of these magmatic rocks which suggests that several pulses of magma used the same pathways giving rise to textural and chemical disequilibrium features these include i narrow tortuous corridors of fine grained minerals cutting across or lining the boundaries of larger grains interpreted to be remnants of magma filled cracks cutting across a pre existing magmatic rock ii corrosion of early formed grains at the contact with fine grained material iii compositional zoning of early formed plagioclase and k feldspar grains and quartz overgrowths documented by cathodoluminescence imaging iv incipient development of rapakivi and anti rapakivi textures and iv different crystallographic preferred orientation of early formed quartz and fine grained quartz mapping of the fine grained corridors interpreted to represent late melt channels reveal an interlinked network broadly following the s c fabric defined by pre existing magmatic grains we conclude that early formed dykes provided a pathway exploited intermittently or continuously by new magma batches new influxes of magma opened narrow channels and migrated through a microscopic network following predominantly grain boundaries along an s c fabric related to syn magmatic shearing a mixed isotopic signature resulted not from the mixing of magmas but from the micro scale interaction between new magma batches and previously crystallized magmatic rocks through local equilibration

decadal volcanic deformation in the central andes volcanic zone revealed by insar time series 1 decadal trends of volcanic deformation in the central andes volcanic zone cvz are identified with interferometric synthetic aperture radar insar stacks and time series velocity maps covering an area 19 s 27 s and 66 w 69 w we combine over 750 ers and envisat interferograms from two descending and three ascending tracks these tracks cover 100 000 km 2 and span 1992 2011 our analysis extends observations at cerro blanco uturuncu and lazufre volcanic centers and uncovers two previously undocumented deformation centers cerro overo in northern chile and putana volcano in southwest bolivia cerro overo exhibits a transition from steady 0 4 cm yr deflation to 0 5 cm yr inflation over several years putana volcano underwent a short lived episode of uplift between 13 september 2009 and 31 january 2010 with a maximum uplift of 4 0 cm cerro blanco continues 1 0 cm yr deflation since 1995 uplift at lazufre began between 1997 and 2000 and has gradually accelerated to 3 5 cm yr since 2005 uturuncu volcano continues 1 0 cm yr monotonic uplift since 1992 and shows evidence for a broad moat of subsidence surrounding the uplifting region four of the nine deformation events in the cvz are not obviously associated with a particular volcanic edifice furthermore there is significant spatial and temporal variability of these deformation events within a small geographic area

c slip in quartz from subsolidus deformed granite abstract the slip behaviour of quartz from a granite and granitic veins during subsolidus high temperature conditions 700 800 c of deformation is studied by lattice preferred orientation lpo and microstructural techniques lpos with a point maximum of c axes parallel to the shear direction which is close to the stretching lineation are presented and interpreted with the aid of light and electron microscopy in terms of c as the dominant slip direction during plastic deformation microscopy of such samples reveals the presence of 0001 tilt boundaries and free 0001 burgers vector dislocations in edge orientations a change in lpo in a decreasing temperature environment from c axes parallel to the shear direction to c axes normal to the shear direction documents a fabric transition from the rare c slip to the more common a slip fabric it is concluded that conditions of high temperature and partial pressure of water are required to initiate dominant c slip during deformation at geological strain rates

influence of melt induced mechanical anisotropy on the magnetic fabrics and rheology of deforming migmatites central vosges france in the central vosges france we describe and model the structural relationships existing between orthogneiss domains and their surrounding viscous migmatites these domains show a structural and anisotropy of magnetic susceptibility ams zonation associated with an increasing amount of melt from the core to their margins a pre extension oblate to plane strain fabric is preserved in the core of the orthogneiss domains where the granite veins were emplaced along a dilated mechanical anisotropy the internal margin of the orthogneiss bodies exhibits development of a prolate fabric and active buckling of an orthogneiss granite multilayer the external margin of the orthogneiss bodies is characterized by oblate fabrics resulting from a layer perpendicular shortening of a melt orthogneiss multilayer pinch and swell structures and extensional kink bands along which granite veins have been injected the surrounding metasedimentary migmatites show a similar structural history but the degree of fabric resetting is significantly higher this systematic fabric and structural succession is due to contrasting rheological evolution between orthogneiss and metasedimentary migmatites during progressive melting and continuously evolving degree of mechanical anisotropy in the stronger orthogneiss numerical modeling confirms the role of the relationship between initial mechanical anisotropy and superimposed deformation overprints on finite ams fabrics in migmatites

sombrero uplift above the altiplano puna magma body evidence of a ballooning mid crustal diapir the altiplano puna ultralow velocity zone in the central andes south america is the largest active magma body in earth s continental crust space geodetic observations reported an uplift in the altiplano puna proper at a rate of 10 mm year however the nature of the inferred inflation source has been uncertain we present data showing that the uplift has persisted at a nearly constant rate over the past two decades and is surrounded by a broad zone of subsidence we show that the ongoing uplift and peripheral subsidence may result from a large mid crustal diapir fed by partial melt from the altiplano puna magma body

the transition from magmatic to high temperature solid state deformation implications from the mount stuart batholith washington criteria for syntectonic emplacement of plutons are commonly ambiguous the strongest single criterion is probably the preservation of a continuous transition from submagmatic to high temperature solidstate deformation this transition is not commonly documented an exception is the mount stuart batholith which displays a variety of syntectonic structures within its sheared northeastern margin including the following 1 s c fabrics that range from submagmatic to high temperature solid state 2 solid state foliation and lineation defined by amphibolite facies assemblages that overprint and are parallel to magmatic foliation and lineation 3 folds of magmatic and solid state foliation that have hinge lines parallel to magmatic and solid state lineation and to equivalent structures in wall rocks 4 dikes that were folded and boudinaged over a wide range of rheological states with boudin necks locally filled by tonalite 5 pegmatite dikes and mineralized joints that are sub perpendicular to magmatic and solid state lineation and 6 submagmatic and high temperature solidstate ductile shear zones we suggest that many plutons emplaced during regional deformation do not preserve evidence for syntectonic deformation because of the transitory nature of the submagmatic state and the obscuring effects of postemplacement deformation syntectonic features are most likely preserved in plutons cooled at slow to moderate rates or in plutons deformed at high strain rates during emplacement the optimum conditions for preservation may occur in plutons emplaced along fault zones in the mid crust such as the mount stuart batholith and in intrusions at deeper levels that were rapidly exhumed and or intruded during the waning stages of regional deformation

a micromorphological view through a namaqualand termitaria heuweltjie a mima like mound heuweltjies occur throughout namaqualand and the western and southern cape coasts broad low relief termitaria of the harvester termite microhodotermes viator which show different vegetation patterns to the surrounding soils distinguishable even in satellite images there is a direct relationship between soil hardpan occurrence and the heuweltjies characteristically in the semi arid areas the hardpans grade outwards from a central sepiolitic petrocalcic horizon laterally through petro sepiolitic palygorskitic variable in the degree of cementation to the petroduric horizon on the edges in a landscape in which these hardpans are otherwise absent the aim of the study was to investigate the genesis of these hardpans associated with the heuweltjie mound with a particular focus on the build up of calcite and sepiolite the micromorphological study examined a transect through a representative sepiolite bearing heuweltjie on the coast near the olifants river mouth and revealed evidence of termite activity in the central petro calcic part of the mound this concentration of calcite and mg rich clay in the centre can be explained by termite foraging as the regionally characteristic ca and mg rich foliage is moved into the centre of the mound facilitating the precipitation of calcite and sepiolite as bacterial decomposition and subsequent leaching modify the soil solution in the mound the increase in coarse fine ratio from the centre outward showed that accumulation of calcite and sepiolite displaced the original sand matrix by a greater degree than the silica accumulation consistent with the topographically raised surface in the centre limpid yellow low birefringent nodules some with pseudo negative uniaxial interference figures showed a fibrous nature under esem and their low ca and molar mg si ratios of 0 64 to 0 68 esem edx were consistent with sepiolite the presence of the sepiolite with its mg rich composition and hydrophilic character together with organic acids generated from the vegetation collected by the termites is considered to be an explanation for the formation of ooids radial calcite crystals associated with a core of sepiolite colourless non calcareous pseudo uniaxial negative spherulites were present in fresh termite excrement from an active heuweltjie near stellenbosch these were either produced in the termite gut itself or had been ingested along with herbivore dung by the termites the presence of faecal spherulites in termite excrement is significant since it shows that faecal spherulites can be distributed over a wider area than that directly associated with mammalian herbivores

overlapping of pedogenesis and meteoric diagenesis in distal alluvial and shallow lacustrine deposits in the madrid miocene basin spain the miocene distal alluvial fan and palustrine deposits of the madrid basin paracuellos de jarama area were examined to establish the sequence of its pedogenic diagenetic processes and the main factors controlling them in this area the diagenetic processes operated not only on carbonates as commonly studied but also in high magnesium clays and opaline cherts this paper provides a dynamic model for saline alkaline lake margins that complements the existing freshwater palustrine model three sections ber pel and upc containing limestones sepiolite dolostones and opaline cherts were examined these represent the entire transition from alluvial fan deposit to lake environment the alluvial fan deposits pel section show the most complex sequence of processes after a weak edaphic imprint the primary mudstone and siltstones are affected by silicification opaline levels formed mainly by selective replacement of sepiolites and later dolomitization several types of dolomite are present rounded crystals spherules and globular bodies being the most characteristic in the ponds situated at the foot of the alluvial fans or in interfan areas sepiolite precipitated within free water bodies ber section and an intense pedogenesis led to the formation of palustrine sepiolite deposits later the intense silicification of the sepiolite produced lenticular opaline levels that were locally transformed to quartz by aging in the shallow lakes upc section palustrine limestones and sepiolite were deposited the carbonates are partially affected by silicification the sepiolite becoming completely silicified the opaline levels clearly preserve the structure of the limestones and sepiolite host rocks all the opaline levels show local aging processes the silicification processes were different in the three settings due to the interplay of groundwater with sedimentary environments and facies in the distal alluvial fan sediments of the pel section the initial host rock affected by silicification is not preserved due to the later dolomitization that erased both it and its textures the silicification environment was therefore not easy to determine in the ber and upc section the main silicification took place in groundwater influenced environments but evidence was seen of cements and silicifications in vadose environments in all study settings the overlapping of pedogenic vadose and groundwater processes was controlled by changes in the position of the water table four stages were concluded to explain the transformation from the primary deposits to the later dolomitization process and the formation of vadose cements the majority of the silicifications seen occurred as responses to changes in the water table position linked to the aggradation of this closed basin in all the studied environments the sepiolite deposits were preferentially silicified their fibrous structure and high absorption capacity helping to retain interstitial fluids moreover sepiolite and opal are stable under relatively similar geochemical conditions in arid environments the dissolution of the sepiolite caused an increase in silica in the interstitial fluids favouring silicification and ruling out the need of a biogenic source the replacement of limestones by silica is usually driven by variations in ph and salinity which are very common in surface environments such as shallow lakes and soils

intensity of quartz cathodoluminescence and trace element content in quartz from the porphyry copper deposit at butte montana textures of hydrothermal quartz revealed by cathodoluminescence using a scanning electron microscope sem cl refl ect the physical and chemical environment of quartz formation variations in intensity of sem cl can be used to distinguish among quartz from superimposed mineralization events in a single vein in this study we present a technique to quantify the cathodoluminescent intensity of quartz within individual and among multiple samples to relate luminescence intensity to specific mineralizing events this technique has been applied to plutonic quartz and three generations of hydrothermal veins at the porphyry copper deposit in butte montana analyzed veins include early quartz molybdenite veins with potassic alteration pyrite quartz veins with sericitic alteration and main stage veins with intense sericitic alteration cl intensity of quartz is diagnostic of each mineralizing event and can be used to fingerprint quartz and its fluid inclusions isotopes trace elements etc from specific mineralizing episodes furthermore cl intensity increases proportional to temperature of quartz formation such that plutonic quartz from the butte quartz monzonite bqm that crystallized at temperatures near 750 c luminesces with the highest intensity whereas quartz that precipitated at 250 c in main stage veins luminesces with the least intensity trace element analyses via electron microprobe and laser ablation icp ms indicate that plutonic quartz and each generation of hydrothermal quartz from butte is dominated by characteristic trace amounts of al p ti and fe thus in addition to cl intensity each generation of quartz can be distinguished based on its unique trace element content aluminum is generally the most abundant element in all generations of quartz typically between 50 and 200 ppm but low temperature main stage quartz containing 400 to 3600 ppm al is enriched by an order of magnitude relative to all other quartz generations phosphorous is present in abundances between 25 and 75 ppm and p concentrations in quartz show little variation among quartz generations iron is the least abundant of these elements in most quartz types and is slightly enriched in cl dark quartz in pyrite quartz veins with sericitic alteration titanium is directly correlated with both temperature of quartz precipitation and intensity of quartz luminescence such that bqm quartz contains hundreds of ppm ti whereas main stage quartz contains less than 10 ppm ti our results suggest that ti concentration in quartz is controlled by temperature of quartz precipitation and that increased ti concentrations in quartz may be responsible for increased cl intensities

origin of the gypsum rich silica nodules moghra formation northwest qattara depression western desert egypt abstract gypsum rich silica nodules appear in two shale horizons of the moghra formation early miocene northwestern qattara depression western desert egypt these nodules are gray to milky white in colour mostly botroidal and rose like in shape and range in diameter from 2 to 7 5 cm the silica nodule bearing shale is composed mainly of smectite with a little minor kaolinite the silica nodules consist mainly of quartz and are composed of gypsum free matrix and gypsum rich megacrystalline quartz the matrix consists of microflamboyant quartz less than 36 m in diameter and chalcedony the megacrystalline quartz occurs as lenticular and prismatic forms length 90 250 m width 30 90 m the microprobe petrographic and sem examinations confirmed the occurrence of gypsum relics diameter 2 16 m within the megacrystalline quartz the chalcedony and mosaic microcrystalline quartz occurs as pore lining and pore filling cements the structure of the silica nodules begins with quartzine in its outer rim then gypsum free microcrystalline quartz in the middle part and ends with gypsum rich lenticular to prismatic megaquartz in the center field study petrographic examination and microprobe analysis reveal that the silica nodules were formed by silicification of precursor gypsum nodules deposited in a marginal sabkha environment under an arid climate the silicification selectively affected the gypsum nodules rather than the surrounding shale and occurred both through gypsum replacement and void filling transformation of isopachous chalcedony into mosaic microcrystalline quartz also occurred the texture of the silica minerals reflects the different physico chemical conditions under which they crystallized spherical nodules grew chiefly by the diffusive supply of the silica and elongated ones grew by pore water advection the integrated effect of climate ph salinity crack systems within the sediment and oscillation in the groundwater level and its chemical composition contributed to the formation of the nodules

getting granite dikes out of the source region whether a dike can propagate far from a magma reservoir depends upon the competition between the rate at which propagation widens the dike and the rate at which freezing constricts the aperture available for magma flow various formulations are developed for a viscous fluid at temperature t sub m intruding a growing crack in an elastic solid the initial solid temperature equals t sub m at the source and decreases linearly with distance from the source if t sub m is the unique freezing temperature of the fluid dike growth is initially self similar and an essentially exact solution is obtained if t sub m is above the solidus temperature the solution is approximate but is designed to overestimate the distance the dike may propagate the ability of a dike to survive thermally depends primarily upon a single parameter that is a measure of the ratio of the dike frozen margin thickness to elastic thickness perhaps more intuitively one may define a minimum distance from the essentially solid reservoir wall to the point at which the host rock temperature drops below the solidus necessary for dikes to propagate far into subsolidus rock it is concluded that for reasonable material properties and source conditions most basalt dikes will have little difficulty leaving the source region but most rhyolite dikes will be halted by freezing soon after the magma encounters rock at temperatures below the magma solidus while these results can explain why granitic dikes are common near granitic plutons but rare elsewhere the potentially large variation in magmatic systems makes it premature to rule out the possibility that most granites are transported through the crust in dikes nonetheless these results highlight difficulties with such proposals and suggest that it may also be premature to rule out the possibility that most granite plutons ascend as more equidimensional bodies

structural complexity resulting from pervasive ductile deformation in the karakoram shear zone ladakh nw india 1 the nw se trending 800 km long dextral karakoram shear zone bounds southwest tibet we investigate an area immediately nw of the pangong lake where midcrustal rocks sections of the shear zone are exposed here the dominant shear zone is characterized by dextral mylonitic rocks sheared at amphibolite facies and partly retrogressed to greenschist facies these rocks define a kilometric se plunging inclined to recumbent fold in the footwall of a north directed oblique dextral thrust that exhumed anatectic rocks of the pangong range argon cooling ages reveal that the pangong range cooled from 700 c to 300 c between 18 15 and 10 ma contemporaneously with overthrusting toward the se closer to the pangong lake this kilometric fold is rotated from its nw se trend to an e w trend and the sense of shear changes to sinistral with a broadly north directed thrusting component the documented structures contradict predictions of normal movement in this area derived from considerations of rigid block behavior of the crust and support the interpretation that the system records dextral transpression thrusting and exhumation of the deeper and hot rocks exposed in the pangong range folding of the thrust footwall and the development of a sinistral e w trending shear zone are all compatible with transpression caused by a northward push by a rigid indenter such as the ladakh batholith

synthetic aperture radar interferometry to measure earth s surface topography and its deformation synthetic aperture radar interferometry insar from earth orbiting spacecraft provides a new tool to map global topography and deformation of the earth s surface radar images taken from slightly different viewing directions allow the construction of digital elevation models of meter scale accuracy these data sets aid in the analysis and interpretation of tectonic and volcanic landscapes if the earth s surface deformed between two radar image acquisitions a map of the surface dis placement with tens of meters resolution and subcentimeter accuracy can be con structed this review gives a basic overview of insar for earth scientists and presents a selection of geologic applications that demonstrate the unique capabilities of insar for mapping the topography and deformation of the earth

cherts with moganite in continental mg clay deposits an example of false magadi type cherts madrid basin spain abstract during the miocene the madrid basin was a closed basin in which a succession of alluvial and lake deposits were replaced by different types of crusts mainly calcretes dolocretes and silcretes in a geographically restricted area to the south mg clay deposits were precipitated in ponds on the sand flats a discontinuous nodular and stratiform mg rich chert appears at the top of the mg clay deposits and displays visual mineralogical and petrological characteristics that are very different from the other silcretes of the madrid basin as revealed by x ray diffraction the cherts consist mainly of quartz and moganite which are commonly found in magadi type cherts the petrological characteristics reveal that the chert consists of a groundmass crossed by several networks of quartz veins the groundmass is formed by mosaic quartz with different sizes of crystals usually less than 20 m and length slow chalcedony the presence of gridwork extinction patterns the differentiation of silica flakes and the preservation of the micromorphological characteristics of the mg clay deposits indicate an early replacement by quartz and moganite the quartz networks show veins of different shapes rectilinear irregular branched and v shaped sizes from a few microns to several millimeters and textures mainly megaquartz microquartz and length slow chalcedony the v shaped veins and some irregular and branched veins are considered to be a consequence of the silicification of cracks in the host rocks the other veins represent silica filling cracks that formed by shrinkage during the various stages of chert formation many macroscopic attributes e g some kinds of cracks mineralogical features the presence of moganite and petrological characteristics e g rectilinear or gridwork orientation of quartz crystals are similar to those of magadi type cherts in spite of this the origin and the environment of formation of this mg rich chert which crystallized by silicification of mg clays during deposition of silcrete are very different from those of the typical pleistocene magadi cherts of african lakes a pronounced shrinkage during silicification in a zone just below the water table and the presence of the moganite are the two factors that caused these mg rich cherts to appear similar to magadi type cherts

miocene silcretes in argillaceous playa deposits madrid basin spain petrological and geochemical features four regressive sequences are present in the opaline rocks and related deposits of the miocene intermediate unit of the madrid basin the sequences consist of silty mudstones and argillaceous opals separated by transitional facies the silty mudstone consists mainly of dioctahedral smectites whereas the argillaceous opal is principally opal ct and variable amounts of sepiolite in the transitional facies lamina of dioctahedral smectite co exist with neoformed opal ct and sepiolite petrological and geochemical features major trace and ree elements indicate that the opaline levels and the transitional facies are related and are a consequence of silcrete formation in an argillaceous playa deposit the isocon method was used to calculate changes in element concentration associated with silcrete formation the geochemical data suggest silicification in an arid environment the silcrete profile occurs four times possibly as a result of highstand lowstand fluctuations of the lake level caused by climatic changes structures and cements in the silcretes indicate that although silicification may have commenced at the top of a groundwater table it continued in the unsaturated zone above the water table

seeing is believing the lava lamp on my son s bureau gives him a vantage point to ponder the colorful blobs rising and falling as their temperature and density change geologists are not so privileged some of earth s most impressive geologic features such as the massive granitic plutons of california s sierra nevada mountains are direct consequences of magma transport but we cannot directly observe these transport processes on page 250 of this issue fialko and pearse 1 use satellite data and computational modeling to infer the transport process in one location they conclude that a magmatic diapir a roughly spheroidal mass of partially molten material is rising beneath a portion of the altiplano plateau in the central andes

the influence of grain boundary fluids on the microstructure of quartz feldspar mylonites quartz rich domains in pre alpine water deficient amphibolite facies 510 580 c 250 450 mpa pegmatite mylonites from mont mary mm western alps preserve a fine dynamically recrystallized grain size without significant annealing despite the high synkinematic temperatures and subsequent static greenschist facies alpine overprint the microstructure is dramatically different from more typical water rich amphibolite facies mylonites such as from the simplon fault zone in the central alps where the recrystallized grain size is on the millimetre scale the difference reflects the dominant strain induced recrystallization mechanism 1 progressive subgrain rotation and grain boundary bulging for the dry mm examples and 2 fast grain boundary migration for the wet simplon examples the grain boundary microstructure imaged with sem is also very different with most grain boundaries in the dry mm samples lacking porosity whereas grain boundaries in the wet samples are decorated by a multitude of pores quartz grain boundaries from both wet and dry samples are locally coated by thin 100 s of nanometres possibly amorphous silica films despite the differences in microstructure the crystallographic preferred orientations cpos of quartz rich domains from both areas are very similar water deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism s but apparently have little influence on the intracrystalline slip systems as reflected in the cpos strong c axis y maxima in mm mylonites both k feldspar and plagioclase an33 38 dynamically recrystallize consistent with the inferred metamorphic conditions under water deficient conditions mid to lower crustal rocks can deform heterogeneously under transitional ductile brittle conditions at high differential stress for mm ca 300 500 mpa as estimated for dry mohr coulomb failure and preserve this high stress microstructure because of the low mobility of dry grain boundaries

silica alteration zones in the barberton greenstone belt a window into subseafloor processes 3 5 3 3 ga ago abstract silicification of volcanic rocks ranging from komatiitic to dacitic in composition is a common phenomenon of the 3 5 to 3 2 ga old volcano sedimentary succession of the barberton greenstone belt silica enrichment occurs in zones several tens of metres thick at the top of volcanic sequences that are capped by sedimentary chert horizons sio2 contents increase upsection from the original igneous value up to 92 silicification is associated with a depletion of most elements commonly mobile during water rock interaction while k2o rb and ba are enriched in these zones some transition metals sc v and cr were immobile whereas ni and co and some other metals cu zn were mobile and are depleted the 18o values of volcanic rocks show a positive linear relationship with silica content and range from 9 0 to 17 3 the element depletion enrichment patterns and oxygen isotope data indicate low temperature c 100 150 c hydrothermal processes for the origin of the alteration zones the loss of most elements coupled with the addition of sio2 as well as ree systematics indicate high water rock ratios and a fluid ree composition typical of archaean seawater that had a 18o value of 0 hydrothermal activity is attributed to heat derived from cooling of volcanic rocks and a high regional heat flow in an oceanic plateau like setting and resulted in the establishment of shallow subseafloor convection cells diffuse venting of hydrothermal fluids over broad areas of the ancient ocean floor provided an ideal environment for both seafloor and subsurface biota 3 5 ga ago

exploding xenoliths and the absence of elephants graveyards in granite batholiths abstract biotite monzogranite near the eastern margin of the late devonian south mountain batholith at portuguese cove nova scotia surrounds a large 20 m 25 m anisotropic decimetre scale interbedded psammite and semipelite xenolith of the metamorphic regional greenschist thermal cordierite hornfels country rocks of the cambro ordovician meguma group the xenolith has two distinct types of surfaces against the monzogranite i one rounded diffuse and apparently old surface that had prolonged contact with the granite magma and ii two straight sharp and apparently new surfaces that had more recently come into contact with the granite magma more than 375 thin sheets width n 224 0 1 0 5 cm n 66 0 5 1 0 cm n 51 1 0 5 0 cm n 30 5 0 10 0 cm n 3 10 cm n 2 ranging in composition from monzogranite to pegmatite to coarse quartz sericite cut the xenolith parallel to bedding producing a cumulative volume expansion of 14 in the xenolith aspect ratios in some of the thin sheets exceed 1000 1 several sheets cut the xenolith perpendicular to bedding producing a volume expansion of

rise of the andes the surface uplift of mountain belts is generally assumed to reflect progressive shortening and crustal thickening leading to their gradual rise recent studies of the andes indicate that their elevation remained relatively stable for long periods tens of millions of years separated by rapid 1 to 4 million years changes of 1 5 kilometers or more periodic punctuated surface uplift of mountain belts probably reflects the rapid removal of unstable dense lower lithosphere after long term thickening of the crust and lithospheric mantle

uturuncu volcano bolivia volcanic unrest due to mid crustal magma intrusion uturuncu volcano sw bolivia is a dormant stratovolcano 85 km3 dominated by dacitic lava domes and flows 39ar 40ar ages show that the volcano was active between 890 ka and 271 ka with the lavas becoming younger and less extensive at higher elevations there are current signs of unrest between 1992 and 2006 geodetic satellite measurements record an ongoing 70 km deformation field with a central uplift rate of 1 to 2 cm yr deformation indicates volume changes of 400 x 108 m3 over 14 years an average of 1 m3 s 10 2 km3 yr the deformation is attributed to magma intrusion into the altiplano puna regional crustal magma body deformation models indicate a source at depths of 17 to 30 km beneath current local relief in a reconnaissance survey persistent seismic activity mean of 2 6 earthquakes per hour with a maximum of 14 per hour was recorded at about 4 km depth below the center of the uplift 4 km sw of the volcano s summit the seismic events have a normal b value 1 04 and activity is attributed to brittle deformation in the elastic crust above the active deep magma intrusion the porphyritic dacite lavas 64 68 sio2 have a plagioclase orthopyroxene biotite magnetite ilmenite assemblage and commonly contain juvenile silicic andesite inclusions cognate norite nodules and crustal xenoliths temperature estimates are in the range 805 to 872 c for the dacites and about 980 c for the silicic andesites the dacite magmas formed by fractional crystallization of andesite forming norite cumulates and involving partial melting of crust compositions and zoning patterns of orthopyroxene and plagioclase phenocrysts indicate that compositional variation in the dacites is caused by magma mixing with the silicic andesite reversely zoned orthopyroxene phenocrysts in the andesitic end member are explained by changing oxidation states during crystallization fe3 fe2 ratios from orthopyroxene crystals and fe3 in plagioclase provide evidence for a relatively reduced melt that subsequently ascended degassed and became more oxidized as a consequence of degassing the geophysical and petrological observations suggest that dacite magma is being intruded into the altiplano puna regional crustal magma body at 17 km or more depth consistent with deformation models in the late pleistocene dacitic and andesitic magmas ascended from the regional crustal magma body to a shallow magma system at a few kilometers depth where they crystallized and mingled together the current unrest together with geophysical anomalies and 270 ka of dormancy indicate that the magmatic system is in a prolonged period of intrusion such circumstances might eventually lead to eruption of large volumes of intruded magma with potential for caldera formation

the role of dynamic recrystallization in the development of lattice preferred orientations in experimentally deformed quartz aggregates abstract experiments on coaxially deformed quartz aggregates show that when dynamic recrystallization involves grain size reduction the lattice preferred orientation lpo of the recrystallized grains depends on the recrystallization mechanism at low temperatures and fast strain rates strain induced grain boundary migration recrystallization favors growth of grains oriented poorly for slip on the basal and prism planes resulting in an lpo of c axes parallel to the 1 direction at higher temperatures or slower strain rates progressive subgrain rotation produces recrystallized grains which inherit the small circle girdle lpo of their deformed host grains and which continue to re orient by slip at even higher temperatures and slower strain rates progressive subgrain rotation is accompanied by rapid grain boundary migration which does not favor grains of any particular orientation thus the lpo is again similar to that of non recrystallized grains in contrast when dynamic recrystallization involves grain growth driven by surface energy reduction a strong lpo of c axes parallel to 1 develops during grain growth but then evolves with increasing strain toward a small circle girdle the correlation of lpos with recrystallization mechanisms has important implications for the assumption of homogeneous strain in theoretical models of lpos and for interpreting natural lpos in recrystallized rocks

a reconnaissance laser raman and fourier transform infrared survey of silcretes from the kalahari desert botswana this paper documents the rst application of laser raman and fourier transform infrared vibrational spectroscopic techniques raman ft ir to the analysis of silcrete analyses are undertaken at a reconnaissance level on bulk silcrete samples of varying ages drawn from a range of geomorphological settings across the kalahari desert of botswana results suggest that raman ft ir combined with inspection of petrographic thin sections provides an effective tool for establishing the range and type of silica polymorphs present within samples analyses of raman and mid infrared spectra indicate that the majority of silcretes are dominated by quartz with subordinate moganite there is however considerable inter sample variability ranging from silcretes consisting of almost pure quartz to those containing complex admixtures of disordered microcrystalline or non crystalline silica in association with quartz and clays taken as a whole the results are supportive of a similar environment of formation for all samples with development taking place under broadly semi arid conditions raman ft ir is also shown to have a number of bene ts over other conventional techniques for silcrete analysis such as x ray diffraction firstly inspection of the oh region of the infrared spectrum of bulk samples is shown to have potential for establishing the provenance of quartz clasts within silcrete host sediments through the identi cation of high temperature conditions of crystal growth in detrital quartz grains raman and mid infrared analyses also permit clay identi cation within silcrete samples where such minerals are not readily resolvable in thin section the technique further allows the relative maturity of silcrete cements to be established particularly for recently formed silcretes these are found typically to contain less mature silica polymorphs and incorporate larger quantities of molecular water than quartz dominated silcretes comparisons of the range of silica polymorphs identi ed using raman ft ir with those established by petrographic investigation can also reveal evidence of silica paragenesis this is the case in two stratigraphically older samples from the kalahari margin in which less stable silica polymorphs appear to have transformed over time to chalcedony and microquartz whilst the whole rock approach used in this survey generates some problems of interpretation the overall results suggest that the use of micro raman and micro ft ir techniques should allow even more precise information about the environments of formation and developmental histories of silcrete pro les to be produced in the future copyright 2004 john wiley sons ltd

spatial distribution of melt bearing structures in anatectic rocks from southern brittany france implications for melt transfer at grain to orogen scale abstract in this study we present quantitative spatial information on the one and two dimensional distribution of inferred melt bearing structures in anatectic supracrustal rocks of the southern brittany migmatite belt south of the transcurrent south armorican shear zone sasz based on these data we infer the mechanism of melt extraction from partially molten crust former melt bearing structures include foliation parallel leucosomes and cross cutting granitic leucosomes that infill inter boudin partitions and extensional shear surfaces as well as discordant dykes of granite petrographic i e mineralogical and microstructural continuity of granite from structure to structure suggests that they once formed a continuous melt bearing network measurements along one dimensional line traverses perpendicular to layering of stromatic migmatite exposed in clean sub horizontal outcrop surfaces provide information about thickness and spacing distributions of foliation parallel leucosomes most leucosome thicknesses fall in the range of 1 10 mm with upper limits around 20 30 mm the number of thicker layers decreases abruptly with increasing thickness which is inconsistent with scale invariance this suggests that leucosome formation was controlled by short range melt movement along grain boundaries to form melt rich layers constrained by pre existing compositional layering spacing distributions also are not scale invariant however the large percentage of leucosomes 40 60 in these line traverses suggests that spacing distributions may be controlled in part by impingement of leucosomes making it difficult to derive genetic information from these data qualitative observation of inferred melt bearing structures in mutually perpendicular two dimensional exposures from the same outcrop reveals anisotropy of the leucosome network related to a well developed sub horizontal quartz feldspar lineation reflecting stretching associated with transcurrent movement along the sasz analysis of these two dimensional distributions using the box counting method corroborates the observed anisotropy but indicates that leucosome morphology and perhaps distribution is not scale invariant the applicability of the box counting method or of fractal analysis to understanding melt movement in migmatites is discussed in light of these results based on the anisotropy of melt bearing structures we infer that melt movement in structures now represented by layer parallel leucosomes was primarily sub horizontal these layers fed steeply dipping structures now represented by cross cutting leucosomes in particular those developed at inter boudin partitions and granite dykes the formation and orientation of these steeply dipping structures was in part controlled by far field stresses related to dextral displacement along the sasz melt extraction is inferred to have occurred along these steeply dipping structures extracted melt accumulated in plutons at higher crustal levels such as the quiberon sarzeau and guerande granites

evaluating quartz crystallographic preferred orientations and the role of deformation partitioning using ebsd and fabric analyser techniques quartz crystallographic preferred orientations cpo from three distinct orthogneisses using both the electron back scatter diffraction ebsd and fabric analyser fa techniques reveal a clear trend from basal and rhomb slip for high p t conditions 670 20 degrees c 9 kbar rhomb and basal slip for medium p t 590 15 degrees c 6 kbar and a dominance of prism slip for lower p t conditions 570 degrees c 4 5 kbar the textural variations are interpreted in terms of a temperature field gradient and microscale strain partitioning controlled by a weak feldspar matrix that can locally invert the expected slip system sequences locally quartz cpus are different within one thin section and in comparison to bulk orientation measurements both ebsd and the fabric analyser techniques are ideal to determine such textural heterogeneities while the ebsd is a powerful technique to determine the full cpo measurements from similar locations inside several quartz grains from the orthogneisses and an annealed and undeformed quartzite show that the fa is an accurate tool to determine cpus of c axis orientations in uniaxial materials in a cpo focussed study the fa is a cheap alternative to ebsd as the analysis of whole thin section can be accomplished in a very short time with minimal sample preparation with the fa it is possible to evaluate the cpus of several samples quickly with an accuracy that allows identification of the main slip systems and their homogeneity

dolomite silica stromatolites in miocene lacustrine deposits from the duero basin spain the role of organotemplates in the precipitation of dolomite this research provides an ancient analogue for biologically mediated dolomite precipitation in microbial mats and biofilms and describes the involvement of highly structured extracellular polymeric secretion eps templates in dolomite nucleation the structure of eps is shown to match the hexagonal trigonal lattice geometry of dolomite which favoured the epitaxial crystallization of dolomite on the organic substrate this structure of eps also matches the arrangement of silica nanospheres in opal which further accounts for the organically templated formation of opal enabling the non replacive co existence of dolomite and silica the study is focused on a 50 m thick dolomite succession that is exposed in central areas of the tertiary duero basin and was deposited in a mudflat saline lake sedimentary complex during the middle to late miocene 9 to 15 ma in the intermediate intervals of the succession poorly indurated dolomite beds pass gradually into silica beds on the basis of sedimentological compositional geochemical and petrographic data silica and dolomite beds have been interpreted as mineralized microbial mats the silica beds formed in marginal areas of the lake in response to intense evaporative concentrations this resulted in the rapid and early precipitation of opal silicification accounted for the exceptional preservation of the microbial mat structure including biofilms filamentous and coccoid microbes and eps extracellular polymeric secretions have a layered structure each layer being composed of fibres which are arranged in accordance with a reticular pattern with frequent intersection angles at 120 and 60 therefore the structure of eps matches the lattice geometry of dolomite and the arrangement of silica nanospheres in opal additionally eps binds different elements with preference to si and mg the concurrence of suitable composition and surface lattice morphologies in the eps favoured the crystallization of dolomite on the substrate in some cases dolomite nucleation took place epicellularly on coccoid micro organisms which gave way to spheroid crystals organic surfaces enable the inorganic mineral precipitation by lowering the free energy barrier to nucleation most of the microbial mats probably developed on the lake floor under sub aqueous conditions where the decomposition of organic matter took place the subsequent formation of openly packed dolomite crystals with inter related si enriched fibrils throughout is evidence for the pre existence of fibrillar structures in the mats miocene dolomite crystals are poorly ordered and non stoichiometric with a slight ca excess up to 5 which is indicative of the low diagenetic potential the microbial dolomite has towards a more ordered and stoichiometric structure this confirms that microbial imprints can be preserved in the geological record and validates their use as biosignatures

grain boundary migration microstructures in a naturally deformed quartzite abstract in this study several grain scale microstructures are presented that are thought to demonstrate the migration direction of once mobile grain boundaries in a naturally deformed quartzite an analysis is presented of the sense of migration of the boundaries and the characteristics of the patterns of relative grain growth and shrinkage grain boundary migration seems to be correlated with the relative crystallographic orientations of neighbouring grains for the quartz quartz grain boundaries and the pattern of preferred grain growth is roughly symmetrical about the mica foliation plane

assimilation of felsic crust by basaltic magma thermal limits and extents of crustal contamination of mantle derived magmas basaltic magma is the most widely used probe of mantle chemistry and structure chemical characterization of the sources for mantle derived magma is possible only after quantitative evaluation of possible contamination by crustal wall rocks the most commonly invoked contamination mechanisms are the combined processes of crustal assimilation and magmatic fractional crystallization afc because of the energetic link between these processes predicting assimilation and crystallization progress requires modeling both heat and mass transfer in the magma country rock system using the melts software package we modeled isenthalpic heat balanced afc between two common types of basaltic magma and several types of felsic crust at 1 kbar our simulations show that during the early stages of isenthalpic afc the ratio of rates of assimilation to crystallization r may be substantially greater than one 2 0 2 7 allowing assimilation of a mass of country rock up to 5 18 of the initial magma mass with only 3 7 crystallization the second stage of afc beginning with plagioclase and or pyroxene saturation is characterized by lower r 0 5 1 0 the initial high r stage results from suppression of crystallization associated with the change in magma composition as assimilation progresses under certain conditions even small degrees of crystallization of olivine alone coupled with cooling of the magma can accommodate relatively large amounts of crustal assimilation and cause large shifts in isotopic and trace element geochemical indices with little differentiation