the metamorphic internal zone of wopmay orogen early proterozoic canada 30 km of structural relief in a composite section based on plunge projection in wopmay orogen an early proterozoic volcano sedimentary marginal prism and its underlying archean structural basement deformed together during the 1885 ma calderian orogeny are exposed in oblique view as a result of regional cross folding a composite cross section displaying over 30 km of structural relief provides a framework in which to examine the geologic relationships in the metamorphic internal zone of the orogen a sole thrust separates archean basement and a thin mantle of autochthonous early proterozoic strata from the eastward transported calderian allochthon the allochthon is characterized by a transition from external thrust fold belt geometries in the east to polyphase isoclinal recumbent folds and thrusts in the west the center of the zone contains the hepburn batholith which has been been transported along with the deforming hot allochthon over the exposed basement both basement and allochthon are deformed by late large scale upright folds the metamorphic internal zone is interpreted to record the closing of a rift basin during which the basin fill was shortened thickened to over 30 km metamorphosed and thrust eastward while still hot over the relatively cool archean craton the basement presently exposed beneath the sole thrust remained relatively nondeformed during thrusting but underwent large scale folding due to continued compression during uplift of the internal zone

the nature and distribution of upper cenozoic ignimbrite centres in the central andes abstract a wide variety of upper cenozoic ignimbrite centres have been identified and studied on landsat imagery of the central andes some have been investigated in the field lower miocene ignimbrite centres are distributed along an arc which in north chile diverges markedly from the line of the present trench and parallels an arc of lower miocene plutons in the eastern cordillera of bolivia much of the ignimbrite erupted since the late miocene has been erupted around the faulted margins of the altiplano basin of western bolivia particularly where the western and eastern cordilleras converge typical examples of ignimbrite centres are described some centres take the form of ignimbrite shield volcanoes other ignimbrite sheets are more widely dispersed the degree of dispersal probably relates to the height of the eruption column partial or total caldera formation sometimes took place and a few of these calderas have resurgent centres late stage lava extrusions are common and some of these are located on caldera ring fractures

the microstructures of perthitic alkali feldspars revealed by hydrofluoric acid etching etching of alkali feldspar cleavage fragments with hydrofluoric acid vapor followed by study of the surfaces by scanning electron microscopy sem is a simple and rapid technique for characterizing the microtextures of crypto and microperthites this technique has a number of advantages over conventional transmission electron microscopy tem including ease of sample preparation and the large areas of crystals which can be imaged alkali feldspars studied by the method can yield important information on the cooling history of igneous and metamorphic rocks fluid feldspar interactions and the morphology and microstructures of albite exsolution lamellae some of these applications are illustrated by examples of etched crypto and micro perthites from the klokken layered syenite south greenland and the shap granite north west england

strain driven disordering of low microcline to low sanidine during partial phase separation in microperthites microperthitic feldspar crystals containing low microcline in a braid intergrowth often have distinctive microtextures including coarse semi to in coherent grain boundary pleated rims and fine coherent intracrystalline ab and or rich pleats lee et al 1997 the coarser pleated rims are generally separated from the braid microtexture in the crystal interior by a coherent to semi coherent transitional zone partial phase separation has occurred in the transitional zone in step with that in the ab and or rich pleats at the grain boundaries such that ab rich lamellar film micro antiperthite alternates along 010 with more or rich lamellar film microperthite the microtextures and phases are those expected for the respective local bulk compositions lamellar microtextures contain tweed orthoclase whereas low microcline is the only k feldspar in the fine coherent pleats and braid microperthite we propose that the small coherent pleats developed from the braid microtexture by interaction of the spontaneous coherency strains with discontinuities within or at the surface of the crystal and that their initial spacing is guided by that of the braid microperthite we infer that the transitional zone formed by straightening of the zig zag braid microtexture above the pleat heads during coarsening and partial phase separation we further infer that the resulting coherency shear strains induced a reversal of the k feldspar phase transformation involving si al disordering of low microcline into low sanidine now tweed orthoclase although the crystal was at a t within the hydrostatic t stability of microcline

erosional control on the geometry and kinematics of thrust belt development in the central andes long term erosion rates in the central andes may have influenced not only mean elevation and relief but also the regional geometry and kinematic history of the orogenic belt a drastic along strike erosional gradient exists in the modern central andes from a high erosion region directly north of the 17 5 s bend in the andes to a low erosion region south of the bend this gradient has existed since 10 15 ma based on fission track analyses of middle miocene to holocene denudation and qualitative evaluations of the preservation potential of middle late tertiary volcanic edifices and synorogenic sediment global positioning system velocity data indicate conflicting patterns of active surface shortening north and south of the 17 5 s bend whereas present day shortening in the thrust belt north of the bend is distributed over much of the width of the belt south of the bend it is concentrated near the eastern frontal margin structural data suggest a similar kinematic situation during late miocene to holocene shortening an out of sequence chronology of thrusting in the narrow 200 km wide thrust belt to the north versus a forward breaking sequence of thrusting in the relatively wide 350 km wide thrust belt to the south the long term internal deformation and limited width of the thrust belt north of the bend are attributed to prolonged subcritical thrust wedge conditions induced by rapid erosion rates since 10 15 ma such conditions inhibit thrust front advance and favor distributed deformation within the thrust belt interior in the thrust belt south of the bend a progressive eastward migration of thrusting is interpreted to be the result of long term critical thrust wedge conditions promoted by extremely low rates of denudation since middle miocene time

coarsening kinetics of coexisting peristerite and film microperthite over 104 to 105 years alkali feldspar crystals from the klokken syenite pluton south greenland have a coarse patch perthite exsolution texture on the scale of hundreds of micrometers produced by dissolution reprecipitation reactions at 500 c we have discovered that ab rich patches ab92an7or1 subsequently exsolved by spinodal decomposition to give peristerite with a periodicity of 17 nm this is the first time a peristerite has been characterized by tem within a perthite and the first peristerite to be found outside regional metamorphic rocks and pegmatites the coexisting or rich patches ab16or84 an0 3 unmixed over the same temperature interval to yield coherent film microperthite with periodicities 1 m because the peristerite and film perthite from a single hand specimen from the klokken intrusion have shared exactly the same cooling history they provide a novel opportunity to test the utility of diffusion coefficients obtained in the laboratory for prediction of coarsening rates over 105 yr time scales in the light of a calculated cooling trajectory for the pluton we find good agreement between observed and predicted periodicities for the film perthite which began to exsolve by coherent nucleation at the coherent solvus for ordered feldspars at 420 c 32 000 yr after crystal growth and took a further 13 000 39 000 yr to coarsen to a periodicity of 1 m peristerite exsolution began at the conditional spinodal at 450 c but on the basis of the best existing experimental data would have taken 520 000 4 600 000 yr to coarsen incompatible with the cooling history we speculate on changes to these best parameter values to describe nasi caal interdiffusion that would lead to peristerite coarsening rates compatible with the cooling history of the klokken intrusion we find that an interdiffusion expression batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document mathit d nasi caal 9 times 10 12 exp 220 000 mathit rt end document where d and the pre exponential term a are in units of m2 s and the activation energy e a is in j mol leads to coarsening rates compatible with the experimental data the cooling path of the klokken intrusion and the coarsening rate of the coexisting film perthite the new interdiffusion parameters are consistent with fine scale peristerite intergrowths in low grade metamorphic rocks and with the existence of the coarsest known peristerite in the light of our discovery it seems likely that peristerite is commonplace in albitic plagioclase in granitic rocks

microbial carbonates the geological record of the calcified bacterial algal mats and biofilms summary deposits produced by microbial growth and metabolism have been important components of carbonate sediments since the archaean geologically best known in seas and lakes microbial carbonates are also important at the present day in fluviatile spring cave and soil environments the principal organisms involved are bacteria particularly cyanobacteria small algae and fungi that participate in the growth of microbial biofilms and mats grain trapping is locally important but the key process is precipitation producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation various metabolic processes such as photosynthetic uptake of co2 and or hco3 by cyanobacteria and ammonification denitrification and sulphate reduction by other bacteria can increase alkalinity and stimulate carbonate precipitation extracellular polymeric substances widely produced by microbes for attachment and protection are important in providing nucleation sites and facilitating sediment trapping microbial carbonate microfabrics are heterogeneous they commonly incorporate trapped particles and in situ algae and invertebrates and crystals form around bacterial cells but the main component is dense clotted or peloidal micrite resulting from calcification of bacterial cells sheaths and biofilm and from phytoplankton stimulated whiting nucleation interpretation of these texturally convergent and often inscrutable fabrics is a challenge conspicuous accumulations are large domes and columns with laminated stromatolite clotted thrombolite and other macrofabrics which may be either agglutinated or mainly composed of calcified or spar encrusted microbes stromatolite lamination appears to be primary but clotted thrombolite fabrics can be primary or secondary microbial precipitation also contributes to hot spring travertine cold spring mound calcrete cave crust and coated grain deposits as well as influencing carbonate cementation recrystallization and replacement microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water and thus relies uniquely on a combination of biotic and abiotic factors this overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas and in the past by temporal variation in abundance of marine microbial carbonates patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry stromatolites appeared at 3450 ma and were generally diverse and abundant from 2800 to 1000 ma inception of a proterozoic decline variously identified at 2000 1000 and 675 ma has been attributed to eukaryote competition and or reduced lithification thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the palaeozoic and reappeared in the late devonian microbial carbonates retained importance through much of the mesozoic became scarcer in marine environments in the cenozoic but locally re emerged as large agglutinated domes possibly reflecting increased algal involvement and thick micritic reef crusts in the late neogene famous modern examples at shark bay and lee stocking island are composite coarse agglutinated domes and columns with complex bacterial algal mats occurring in environments that are both stressed and current swept products of mat evolution ecological refugia sites of enhanced early lithification or all three

kiruna type deposits their origin and relationship to intermediate subvolcanic plutons in the great bear magmatic zone northwest canada magnetite apatite actinolite rocks similar to those at kiruna sweden and the st francois mountains missouri are associated with seven epizonal plutons that intrude andesitic stratovolcanoes of a broadly folded early proterozoic continental magmatic arc located in the northwestern canadian shield the plutons mainly medium grained quartz monzonite monzodiorite diorite iugs sheets and laccoliths 5 to 25 km in diameter are spatially temporally and compositionally related to host andesitic stratovolcanoes they were emplaced at 2 to 3 km depth are compositionally heterogeneous metasomatically altered and had plagioclase as the liquidus phase in addition they have kilometer wide alteration halos comprising an inner zone of nearly complete wall rock albitization an intermediate zone of magnetite apatite actinolite veins pods breccias and wall rock replacement and an outer zone of disseminated sulfides one pluton balachey has a halo that is more or less completely outside the body but another rainy lake has a halo that is partly superimposed over it separation of iron phosphate and silicate melts by liquid immiscibility is not supported by 1 the gradual replacement of plagioclase by chessboard albite toward the roof of the rainy lake pluton 2 the chemical trends in the rainy lake intrusion 3 the mineralogical zoning low temperature mineralogy and replacement textures of the magnetite apatite actinolite and 4 the relationship of the magnetite apatite actinolite veins and bodies to the plutons however all available data are compatible with a deuteric origin it is concluded that high temperature low initial water content and shallow level emplacement by intermediate plutons are necessary for the development of kiruna type deposits

routine characterization and interpretation of complex alkali feldspar intergrowths almost all alkali feldspar crystals contain a rich inventory of exsolution twin and domain microtextures that form subsequent to crystal growth and provide a record of the thermal history of the crystal and often of its involvement in replacement reactions sometimes multiple microtextures strongly influence the subsequent behavior of feldspars at low temperatures during diagenesis and weathering they are central to the retention or exchange of trace elements and of radiogenic and stable isotopes this review is aimed at petrologists and geochemists who wish to use alkali feldspar microtextures to solve geological problems or who need to understand how microtextures influence a particular process we suggest a systematic approach that employs methods available in most well founded laboratories the crystallographic relationships of complex feldspar intergrowths were established by the 1970s mainly using single crystal x ray diffraction but such methods give limited information on the spatial relationships of the different elements of the microtexture or of the mode and chronology of their formation which require the use of microscopy we suggest a combination of techniques with a range of spatial resolution and strongly recommend the use of orientated sections sections cut parallel to the perfect 001 and 010 cleavages are the easiest to locate and most informative techniques described are light microscopy scanning electron microscopy using both backscattered and secondary electrons including the use of surfaces etched in the laboratory electron probe microanalysis and analysis by energy dispersive spectrometry in a scanning electron microscope transmission electron microscopy we discuss the use of cathodoluminescence as an auxiliary technique but do not recommend electron backscattered diffraction for feldspar work we review recent publications that provide examples of the need for great care and attention to pre existing work in microtextural studies and suggest several topics for future work

textured organic surfaces associated with the ediacara biota in south australia abstract the ediacaran period takes its name from the fossils of the ediacara biota which represent the first appearance of large and diverse assemblages of organisms in the fossil record although the global record of these distinctive body fossils is now well known a previously unrecognized megascopic organic record of textured organic surfaces tos occurs in the ediacara biota however tos is also a feature over a wider range of paleoenvironmental settings where body fossils are unknown in ediacaran siliciclastic successions that have been studied in australia namibia and western north america paleoecological analysis of successive bedding planes of strata from the late ediacaran rawnsley quartzite in the flinders ranges of south australia reveals that tos represent the most common organic features in bedding surface assemblages of the ediacara biota the tos consist of preserved patterned assemblages of textured organic mats fibers and simple tubular body fossils complex ediacara body fossils while striking for their distinctive body plans and dominating some of the beds are relatively minor components of combined overall surface area many elements of tos have previously been miss diagnosed as trace fossils which are in practice limited to two or at most three morphotypes that indicate the presence of bilateria although tos represent a simpler grade of organismic construction than discrete and more complex ediacara body fossils they were preserved in a similar manner marked variability in all components of the biota between successive surfaces suggests that ediacara ecologies fluctuated at short intervals despite an apparently consistent sedimentary regime

phosphatization of soft tissue in experiments and fossils soft tissues phosphatized in laboratory experiments closely resemble fossil phosphatized soft tissues indicating that similar processes were involved the smaller the aggregations of calcium phosphate particles precipitated the greater the fidelity of morphological preservation the highest fidelity occurs where the bacteria themselves are not replicated even though precipitation is bacterially induced while extensive phosphatization of larger carcasses however may necessitate the build up of concentrations in the sediment beforehand this is not the case for phosphatization of small quantities of soft tissue mineralization of soft tissue in the laboratory is not instant but may take several weeks or even months if decay is inhibited the precipitation of associated calcium carbonate is controlled by shifts in ph in response to the decay process

acid mine drainage at cerro rico de potosi i unabated high strength discharges reflect a five century legacy of mining intensive mining and processing of ag sn pb and zn ores have occurred in various locations within and around the city of potosi bolivia since 1545 surface and subsurface waters stream sediments and soils are contaminated with various ecotoxic metals in the headwaters of the economically vital yet highly impacted upper rio pilcomayo watershed previous studies have documented downstream trace metal contamination however not addressed specific sources the amd discharges identified in this study help link downstream pollution to primary origins the majority of amd would be considered high strength due to metal concentrations and acidity orders of magnitude greater than typical amd discharges from both operating and abandoned portals as well as tailings related deposits displayed a high degree of heterogeneity with total metal concentrations ranging from 0 11 to 7480 0 022 to 889 0 0006 to 65 3 0 001 to 310 0 12 to 72 100 0 3 to 402 0 012 to 34 8 and 0 24 to 19 600 mg l of al as cd cu fe mn pb and zn respectively net acidity and ph ranged from 10 to 246 000 mg l as caco3 equivalent and 0 90 6 94 standard units respectively data were gathered during two sampling events centered around the most extreme periods of the dry and wet seasons of one water year loadings to local streams were marginally greater for most metals in the wet season if observed loadings are historically representative cerro rico amd has contributed thousands of tonnes of ecotoxic metals to the upper rio pilcomayo over the last five centuries metal and hydrogen ion concentrations in the majority of amd sampled were several orders of magnitude above discharge limits set by the bolivian government yet no action has historically or contemporarily been taken

nanotunnels and pull aparts defects of exsolution lamellae in alkali feldspars we have studied defects associated with flat lens shaped perthitic albite lamellae in alkali feldspars using sem and tem in orthoclase phenocrysts from the shap granite cumbria nw england bulk composition or 70 20 ab 29 05 an 0 85 no dislocations were found even in optically fresh parts of grains instead dissolution inferred to be localized on edge dislocation loops has created tiny nanotunnels typically t effect of the 1 an in the feldspars is taken into account for commencement of coherent exsolution the orientation of the pericline twins was fixed shortly after coherent exsolution began previous work has indicated that edge dislocations would start to form during cooling at 400 c so that nanotunnels form at still lower t dry heating experiments were carried out to establish the stability of the defects and the homogenization behavior of the exsolution lamellae na k exchange is rapid on heating above the coherent solvus and chemical homogenization of lamellae is complete after 24 h at 700 c in contrast nanotunnels persist for 148 h at 1000 c and 5748 h at 700 c below the coherent solvus exsolution lamellae thin on heating leaving nanotunnels stranded in the orthoclase matrix microtextures related to si al ordering patterns in the framework such as albite twins are not eliminated forming ghost like lamellar strain patterns in chemically homogeneous feldspar the presence of nanotunnels in optically fresh alkali feldspars shows that not only granites but also granulite facies rocks have been pervasively affected by fluids at low t both nanotunnels and pull aparts have important implications for feldspar reactivity in the upper crust for 18 o exchange and for transport of 40 ar both in nature and in laboratory step heating

feldspar fluid interactions in braid microperthites pleated rims and vein microperthites braid microperthitic alkali feldspars in the klokken south greenland and coldwell ontario syenite intrusions have bulk compositional variations along grain boundaries called pleated rims these together with vein microperthites in aplites which cross cut the syenites have been investigated by sem and tem we distinguish two main types of pleated rims arched and parallel sided consisting of alternating ab and or rich areas on 001 which are 0 5 300 m in length normal to 010 and 0 2 20 m in width along 010 the smallest pleats which occur on intracrystalline boundaries in klokken feldspars are fully coherent and composed of low albite and low microcline above the heads of some of the coarser pleats braid microperthite grades into a film crypto and micro perthite and antiperthite microtexture called a transitional zone containing roughly planar lamellae of low albite and tweed orthoclase during pleat development local alternating volumes form in which the proportions of the phases differ phase separation and the morphology of the intergrowths changes from braided to straight in response to this change in local bulk composition straightening is also accompanied by transformation of low microcline to tweed orthoclase the coarsest pleats which occur along grain boundaries in feldspars from the coldwell syenite are semi or in coherent and have a thick coherent and semicoherent transitional zone coarsening of pleats and development of the transitional zone has been facilitated by diffusion of water into grain interiors in many cases pleated rims have suffered deuteric alteration by dissolution reprecipitation processes through the action of a water rich fluid from the grain boundary in which tweed orthoclase was transformed into irregular microcline and micropores developed vein microperthites in aplites from klokken and by extension the vein microperthites almost universal in most alkali granites are interpreted to have formed by propagation of pleat heads across entire crystals during pervasive interaction with water

eight phase alkali feldspars low temperature cryptoperthite peristerite and multiple replacement reactions in the klokken intrusion eight feldspar phases have been distinguished within individual alkali feldspar primocrysts in laminated syenite members of the layered syenite series of the klokken intrusion the processes leading to the formation of the first four phases have been described previously the feldspars crystallized as homogeneous sodian sanidine and exsolved by spinodal decomposition between 750 and 600 c depending on bulk composition to give fully coherent strain controlled braid cryptoperthites with sub m periodicities below 500 c in the microcline field these underwent a process of partial mutual replacement in a deuteric fluid producing coarse up to mm scale turbid incoherent patch perthites we here describe exsolution and replacement processes that occurred after patch perthite formation both or and ab rich patches underwent a new phase of coherent exsolution by volume diffusion or rich patches began to exsolve albite lamellae by coherent nucleation in the range 460 340 c depending on patch composition leading to film perthite with 1 m periodicities below 300 c misfit dislocation loops formed which were subsequently enlarged to nanotunnels ab rich patches bulk composition ab91or1an8 in one sample exsolved giving peristerite with one strong modulation with a periodicity of 17 nm and a pervasive tweed microtexture the ab rich patches formed with metastable disorder below the peristerite solvus and intersected the peristerite conditional spinodal at 450 c this is the first time peristerite has been imaged using tem within any perthite and the first time peristerite has been found in a relatively rapidly cooled geological environment the lamellar periodicities of film perthite and peristerite are consistent with experimentally determined diffusion coefficients and a calculated cooling history of the intrusion all the preceding textures were in places affected by a phase of replacement correlating with regions of extreme optical turbidity we term this material ultra porous late feldspar uplf it is composed predominantly of regions of microporous very or rich feldspar mean ab2 5or97 4an0 1 associated with very pure porous albite ab97 0or1 6an1 4 implying replacement below 170 90 c depending on degree of order in tem uplf has complex irregular diffraction contrast similar to that previously associated with low temperature albitization and diagenetic overgrowths replacement by uplf seems to have been piecemeal in character ghost like textural pseudomorphs of both braid and film parents occur formation of patch perthite film perthite and peristerite occurred 104 105 year after emplacement but there are no microtextural constraints on the age of uplf formation

thermal history of a bishop tuff section as determined from the width of cryptoperthite lamellae the lamellar spacings in the sanidine cryptoperthites bulk composition or 66 4 ab 32 3 an 1 3 from the bishop tuff near long valley california have been determined using transmission electron microscopy the lamellar spacings show a sharp maximum at about 43 m below the topographically highest sample and a weak minimum about 4 m above this this pattern indicates that two flows compose the section the gradational boundary between the flows is about 37 m below the highest sample heat flow calculations applied to the pattern of lamellar spacings indicate that these flows were emplaced within 1 5 yr of each other the result illustrates the power of this technique in deciphering the geologic history of igneous rocks

fault reconstructions using aeromagnetic data in the great bear magmatic zone northwest territories canada the great bear magmatic zone located in wopmay orogen is a 1 875 1 84 ga belt 450 km long by 100 km wide of volcanic and allied plutonic rocks interpreted as a paleoproterozoic magmatic arc the

volcanic evolution of the island of tenerife canary islands in the light of new k ar data new age determinations from tenerife together with those previously published 93 in all provide a fairly comprehensive picture of the volcanic evolution of the island the oldest volcanic series with ages starting in the late miocene are formed mainly by basalts with some trachytes and phonolites which appear in anaga teno and roque del conde massifs in anaga ne three volcanic cycles occurred one older than 6 5 ma a second one between 6 5 and 4 5 ma with a possible gap between 5 4 and 4 8 ma and a late cycle around 3 6 ma in teno nw after some undated units the activity took place between 6 7 and 4 5 ma with two main series separated by a possible pause between 6 2 and 5 6 ma in the zone of roque del conde s the ages are scattered between 11 6 and 3 5 ma between 3 3 and 1 9 ma the whole island underwent a period of volcanic quiescence and erosion the large canadas volcano made up of basalts trachytes and phonolites was built essentially between 1 9 and 0 2 ma to the ne of this central volcano linking it with anaga is a chain of basaltic emission centers with a peak of activity around 0 8 ma the canadas caldera had several collapse phases associated with large ignimbrite emissions there were at least an older phase more than 1 ma old on the western part of the volcano and a younger one less than 0 6 ma old in the eastern side the two large valleys of guimar and la orotava were formed by large landslides less than 0 8 ma ago and probably before 0 6 ma ago the present canadas caldera was formed by another landslide less than 0 2 ma ago this caldera was later filled by the huge teide volcano which has been active even in historic times during the same period a series of small volcanoes erupted at scattered locations throughout the island the average eruptive rate in tenerife was 0 3 km3 ka with relatively small variations for the different eruptive periods this island and la gomera represent a model of growth by discontinuous pulses of volcanic activity separated by gaps often coinciding with episodes of destruction of the edifices and sometimes extended for several million years the neighbouring gran canaria on the other hand had an initial rapid shield building phase during which more than 90 of the island was built and a series of smaller pulses at a much later period a comparison between these three central islands indicates that the previously postulated westward displacement in time of a gap in the volcanic activity is valid only as a first approximation several gaps are present on each island overlapping in time and not clearly supporting either of the models proposed to explain the evolution of the canaries

exceptional fossil record distribution of soft tissue preservation through the phanerozoic preservation of soft bodied fossil biotas konservat lagerstaten that preserve traces of volatile nonmineralized tissues readily degraded by bacteria are not evenly spaced through geologic time when compared to outcrop area exceptional faunas appear to be over represented in the cambrian and jurassic these concentrations in time correspond to particular environments indicating that controls on the distribution of exceptional faunas may have operated on a global scale the reduction in the number of exceptional faunas after the cambrian may reflect the evolution and diversification of deep bio turbators specific conditions favoring stagnation and episodic burial were required to ensure preservation in younger rocks

basinal and shelf sedimentation in relation to the archaean proterozoic boundary abstract in most countries the archaean proterozoic boundary has been designated as an unconformity between the oldest shelf type quartz arenites and a basement consisting of greenstone belts intruded by granitic rocks or high grade gneisses the quartz arenites have been characterized as shallow water shelf deposits and sediments in the greenstone belts as resedimented basinal wackes of volcanogenic origin radiometric ages for rocks of the canadian shield led to initial designation of the archaean proterozoic boundary as thus defined at about 2 5 ga subsequent age determinations have clearly established that a number of quartz arenites originally assigned to the proterozoic are 2 7 2 94 ga old e g witwatersrand and pongola supergroups of south africa to accommodate these old quartz arenites some have suggested that the arch aean proterozoic boundary should be regarded as diachronous embracing a span of time about equivalent to that of the phanerozoic however quartz arenites also are abundant in 3 2 3 5 ga old greenstone belts and high grade metamorphic terranes of south africa western australia and the u s a likewise large regions of basinal volcanogenic sediments occur in post 2 5 ga terranes this greatly expands the diachroneity of geological units that were initially regarded as either uniquely archaean or proterozoic we therefore recommend that chronometric and chronostratigraphic sense be retained for the archaean proterozoic boundary divested of the requirement for all quartz arenites to be proterozoic according to this analysis of the precambrian record shelf sediments have increased in abundance relative to basinal sediments through time but at different rates in different areas this change is attributable to progressive cratonization of shield areas a process that was strongly diachronous on a worldwide scale

glide twinning and pseudotwinning in peristerite twin morphology and propagation optically visible albite glide twins in a peristerite an9or1 6 identified from their tapering shape and relationship to grain boundaries were studied by transmission electron microscopy near the tips in sections a the microstructure consists of small 400 nm long lensshaped albite twins centred exclusively on the oligoclase lamellae the lenses extend partly outwards into the two adjacent low albite lamellae and induce strong inhomogeneous strain where the lenses are closer together they form depending on the sense of shear nearly linear left or right stepping en echelon arrays with overlap of the strain fields slightly farther in from the tip the twin domains coalesce to form continuous pinch and swell lamellae being always thicker in the oligoclase because of si al order only elastic glide pseudotwins are possible in low albite in oligoclase glide pseudotwins may be mechanically stable metastable relative to si al order and may deviate only slightly from true twins pseudotwins develop first in the oligoclase propagate dynamically by jumping across the intervening albite lamellae extend lengthways and thicken sideways and finally coalesce they are stabilized by diffusion controlled inversion of si al order giving rise to true twins described in a companion paper

time specific aspects of facies state of the art examples and possible causes abstract the term time specific facies tsf was introduced to the scientific community by the late otto h walliser to refer to unique facies typical of particular narrow intervals some of which were related to bioevents in some senses however the concept was recognized much earlier and is even engrained in the very names of some geologic periods the concept of time specific facies is expanded slightly herein to include distinctive or unique regional to global characteristics of the sedimentary record that characterize particular intervals of geologic time the recognition of time specific and widespread processes in the sedimentary record is a critical step in unraveling the interplay between processes of differing scale these range from very short term but widespread facies that overlap with event deposits to general facies types that may persist for intervals up to 10s of millions of years in duration this paper briefly summarizes the history of development of the tsf concept provides examples to illustrate a few of the key aspects of time specific facies and offers a few tentative explanations for this phenomenon among the factors that control tsfs abrupt changes in redox conditions and early diagenetic mineralization sedimentary condensation often associated with abrupt sea level change altered climate and paleoceanography and biotic evolution and extinction seem to be most critical and permit a preliminary genetic classification of tsfs inevitably however many tsfs reflect multiple effects and some remain largely unexplained

origin of cryptoperthites in the bishop tuff and their bearing in its thermal history the bishop tuff is a large rhyolitic ash flow which erupted from long valley caldera in eastern california and contains abundant sanidine cryptoperthite phenocrysts which can be used to constrain the time between the two major cooling units the cryptoperthites bulk composition or66ab33an1 have exsolved in two principal stages followed by a third minor stage the earliest exsolution which produced patches of relatively wide lamellae 100 800 nm without wedge shaped ends wse occurred by coherent nucleation and was probably related to a high flux of water associated with fumarolic activity in this area the spacing of these lamellae shows no correlation with depth or thermal history the second major stage of exsolution occurred by spinodal decomposition which produced lamellae with wse having spa ein g s of from 80 to 170 nm and the spacing of these lamellae varies with depth in a pattern consistent with the two known cooling units heat flow calculations indicate that this pattern of lamellar spacings is consistent with an interval of about 1 0 2 0 years between the eruption of these two cooling units

understanding sibumasu in the context of ribbon continents abstract well described ribbon continent collision belts and modern examples with 2d seismic coverage particularly zealandia provide analogues for the permo triassic development of sibumasu two key parameters for the ribbon continents are their width and how rifting developed which in turn impact the structural metamorphic igneous and stratigraphic development of ribbon continents in ways that make them significantly different from the collisional development of major continents key characteristics of the tectonics of sibumasu are 1 a tendency towards basin inversion during the late triassic and possibly early jurassic and mixed thick skinned and thin skinned style deformation 2 extensional collapse around 215 ma is an important feature of sibumasu particularly in northern thailand and probably other areas too 3 slab delamination and extension related decompression melting were probably important mechanisms for generating granites in addition to crustal thickening 4 metamorphism is relatively high temperature lower pressure

40ar39ar analysis of perthite microtextures and fluid inclusions in alkali feldspars from the klokken syenite south greenland laser probe andin vacuo crushing have been used to apply the40ar39ar technique to microtexturally well characterised pristine and turbid alkali feldspar from the 1166 ma klokken syenite individual crystals are several millimetres in size but sem and tem studies reveal complex microstructures which define a range of sub grain sizes over three orders of magnitude from 100 m in pristine areas with fine scale cryptoperthite down to 200 nm in turbid areas with coarsened perthites and numerous micropores laser probe40ar39ar measurements of turbid feldspar gave low apparent ages indicating significant40ar loss 40 average which can be accounted for by sustained heating of the small sub grains 1 m for 1166 ma at low temperature 150 c pristine feldspar gives high apparent ages indicating the presence of excess40ar this component was readily lost from the feldspar by laboratory heating at low temperature 600 c and was also released by crushing which indicates that it was released from fluid inclusions during crushing release of excess40ar is correlated with cl release and the40ar cl ratio of the fluid is similar to that of mantle fluids turbid feldspar also released a fluid during crushing but the40ar cl ratio was lower probably as a result of evolution of the original fluid during boiling anomalously high apparent ages are often a feature of low temperature ar release in stepped heating age spectra of feldspars the presence of excess40ar in fluid inclusions may substantially affect the lower temperature release in feldspar stepped heating spectra the character of the microtextures in the host feldspar provides information on the temperature of fluid trapping because inclusions formed above the coherent alkali feldspar solvus have little or no effect on the exsolution textures whereas fluids trapped at low temperatures are associated with major structural rearrangements leading to development of patch perthite

argon loss by alkali feldspars abstract ar loss is generally thought to preclude the use of alkali feldspars for dating old rocks by the k ar method and has been thought to occur via structural features such as the boundaries of perthite lamellae or twins we have done a 40 ar 39 ar step heating study of a suite of alkali feldspars from the klokken syenite intrusion u pb age 1166 4 ma which have been fully characterized by transmission electron microscopy all are low albite low microcline intergrowths but some are non turbid strain controlled cryptoperthites while others are turbid coarse microperthites geological and textural evidence suggests that the turbidity developed by fluid rock interactions during the initial cooling of the pluton in the stability field of low microcline 5 a after crystallization the finest cryptoperthites have numerous fully coherent perthite lamellar boundaries lamellar periodicity 40 nm and twin composition planes the have retained radiogenic 40 ar and yield total degassing ages as high as 1125 16 ma and maximum ages on the age spectra of 1162 16 ma the coarse turbid microperthites with patchy intergrowths and twins on the mm scale gave total degassing ages as low as 650 ma maximum age 750 ma argon leakage occurs least in feldspars with the most lamellar interfaces but in proportion to the degree of turbidity which is thought to be caused by large numbers of micropores strain controlled intergrowths in perthites allow the recognition of volumes of alkali feldspar crystals which have not interacted with deuteric fluids these retain argon and can give reliable k ar ages in old rocks saddle shaped age spectra appear to result from the preferential degassing at 1000 c of part of the feldspar structure which has lost more of its argon in nature the laboratory degassing does not occur in a manner analogous to that which has occurred naturally

the nature of potassium feldspar exsolution microtextures and development of dislocations as a function of composition in perthitic alkali feldspars transmission electron microscope data on the morphology of exsolution lamellae the nature of the potassium feldspar and the development of dislocations at lamellar interfaces in coherent cryptoperthites and fine microperthites are reviewed dislocations have been reported previously in only two crystals and periodic dislocations noted in only one an or rich microperthite periodic dislocations spacing 100 150 nm are here described from a ternary mesoperthite or26 ab52 an22 small crystallites 30 nm of other phases have sometimes nucleated on the dislocations the 020 lattice fringes of the feldspar phases have been imaged the difference in 020 spacings can be almost entirely accommodated by the regular dislocations so that the boundaries may be termed nearlyperfectly semicoherent

development of microporosity diffusion channels and deuteric coarsening in perthitic alkali feldspars turbidity is an almost universal feature of alkali feldspars in plutonic rocks and has been investigated by us in alkali feldspars from the klokken syenite using sem and tem it is caused by the presence of myriads of tubular micro inclusions either fluid filled micropores or sites of previous fluid inclusions and is associated with coarsening of microperthite and development of sub grains micropores are abundant in coarsened areas in which porosities may reach 4 5 but are almost absent from uncoarsened pristine braind microperthite areas the coarsening is patchy and involves a scale increase of up to 103 without change in the composition of the phases low albite and low microcline or in the bulk composition of the crystal it occurs abruptly along an irregular front within individual crystals which retain their original shapes the coherent braid microperthite gives way across the front to an irregular semi coherent film perthite over a few m and then to a highly coarsened irregular patch perthite containing numerous small sub grains on scales of a few hundred nm in both phases the coarsening and micropore formation occured at a t 400 450 c and it is inferred to have been driven by the release of coherent strain energy low angle grain boundary migration being favoured by a fluid the patchy nature of the coarsening and the absence of a relationship with initial grain boundaries suggest that the fluid was of local origin possibly arising in part through exsolution of water from the feldspar the sub grain texture and microporosity modify profoundly the permeability of the rock and greatly enhance the subsequent reactivity of the feldspars

peristerite exsolution in metamorphic plagioclase from the lepontine alps an analytical and transmission electron microscope study transmission and analytical electron microscopy were used to examine relationships between microstructures and compositions in greenschist and amphibolite facies metamorphic plagioclase albite and oligoclase from the lepontine alps switzerland and italy two kinds of exsolution microstructures related to the peristerite miscibility gap an1 25 were observed lamellae in bulk compositions ranging from a few mole percent anorthite to an10 15 and tweeds in bulk compositions from almost pure albite to an15 18 lamellae are typically 15 to 35 nm thick individual lamellae in crystallographically homogeneous or tweedy areas commonly have highly irregular spacings or end at dislocations or subgrain boundaries suggesting formation by heterogeneous nucleation tweeds are characteristically diffuse and probably formed by spinodal decomposition many tweeds have one exsolution direction that is consistently sharper or coarser than the other tweeds and lamellae may be intergrown in patches with irregular curving boundaries some of which define narrow stripes several micrometers long exsolution directions in these tweeds are approximately parallel and perpendicular to the lamellae the perpendicular direction is usually less diffuse or more regular than the parallel direction and may continue between widely spaced individuals in patches of lamellae tweeds in these intergrowths appear to be slightly more sodic than adjacent areas with lamellae except near fractures and in areas with high dislocation densities differences in microstructures within a single grain almost invariably reflect differences in composition microstructural variability within single grains and among different grains from the same hand sample was so large that it was impossible to identify systematic relationships between microstructures and metamorphic grade

the pallacos of cerro rico de potosi bolivia a new deposit type pallacos are a new kind of ore deposit and are described here in detail for the first time they are found on the flanks of the cerro rico de potosi bolivia the world s largest silver deposit fundamentally pallacos are gravels whose clasts contain sufficient disseminated silver so as to be bulk minable the silver bearing clasts originated from the summit of cerro rico and are composed of dacite porphyry or air fall tuffs or arkosic sands of the caracoles formation that have been altered to vuggy silica or jasperoid analysis of facies bedforms depositional textures and clast sizes and percentages suggests that there are two types of pallacos each with a different depositional mechanism one kind the huacajchi type formed from multiple water transported debris flows the other kind the potosina type is interpreted as catastrophic landslides metal zoning clast alteration style and silver grade vary between the two pallaco types

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence structures at calderas may form as a result of precursory tumescence subsidence due withdrawal of magmatic support resurgence and regional tectonism structural reactivation and overprinting are common to explore which types of structures may derive directly from subsidence without other factors evidence is reviewed from pits caused by the melting of buried ice blocks mining subsidence scaled subsidence models and from over 50 calderas this review suggests that complex patterns of peripheral deformation with multiple ring and arcuate fractures both inside and outside caldera rims topographic embayments arcuate graben and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history downsag is a feature of many calderas and it does not indicate subsidence on an inward dipping ring fault as has been inferred previously where magmatic inflation is absent or slight initial arcuate faults formed during collapse are likely to be multiple and dip outwards to vertical associated downsag causes the peripheries of calderas undergo radial centripetal extension and this accounts for some of the complex peripheral fractures arcuate crevasses graben and some topographic moats the structural boundary of a caldera defined here as the outermost limits of subsidence and related deformation including downsag commonly lies outside ring faults and outside the embayed topographic wall it is likely to be funnel shaped i e inward dipping even though ring and arcuate fractures within it may dip outward inward dipping arcuate normal faults at shallow levels and steep inward dipping contacts between a caldera s fill and walls may both occur at a caldera that has initially subsided on outward dipping ring faults they arise due to peripheral surficial extension gravitational spreading and scarp collapse topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag respectively these processes may occur in addition to surficial degradation of oversteep ring fault scarps

erosion and tectonics at the margins of continental plateaus the topography across the eastern margin of the central andean plateau north of 18 s beni region bears a strong resemblance to the topography of the southern margin of the tibetan plateau nepal himalaya with both regions featuring a steep frontal slope and high peaks at the plateau edge in contrast the topography of the eastern margin of the central andean plateau south of 18 s pilcomayo region tapers toward the foreland more gently and has no line of high peaks at the margin both the himalayan and the beni regions have been the sites for large amounts of denudation and in both regions geologic evidence suggests that erosion has been sufficiently vigorous for the physiographic plateau margin to have retreated toward the plateau interior during the neogene we hypothesize that the steep frontal slope and high peaks of the beni region and himalayan front largely reflect the high orographic precipitation and high erosion rates occurring in these regions and that the more gentle topography of the semiarid pilcomayo region reflects a tectonic landform only slightly modified by erosion we propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope eroding back the plateau while enhancing or maintaining the steep long wavelength slope a numerical model coupling orographic precipitation erosion and tectonic uplift demonstrates the plausibility of this hypothesis the erosional efflux in both the beni and nepal himalaya have been considerable and simple mass balance calculations for the himalaya suggest that during the neogene the erosional mass efflux has generally outpaced the tectonic mass influx this contrasts with the apparent prior domination of tectonic influx and may reflect a decrease in the rate of tectonic addition during the same period and or increased late cenozoic erosion rates

a silurian soft bodied biota a new silurian llandoverian biota from wisconsin with a significant soft bodied and lightly sclerotized component is dominated by arthropods and worms the fauna includes the earliest well preserved xiphosure a possible marine uniramian three new arthropods of uncertain affinity and possibly the first paleozoic leech this may be only the second locality to yield a conodont animal lack of a normal shelly fauna suggests an unusual environment the discovery adds significantly to the few such exceptionally preserved faunas known from lower paleozoic rocks

peristeritic plagioclase in north sea hydrocarbon reservoir rocks implications for diagenesis provenance and stratigraphic correlation reservoir sandstones from well 30 16 7 in the upper jurassic humber group uk north sea contain plagioclase feldspars with a range of chemical compositions microtextures and origins the chemical composition of feldspar grains in samples from three different depths in the core ranges from 0 to 18 mol an two of the samples have a well defined gap in the compositional range between an 4 and an 9 this gap reflects the absence of plagioclase feldspars with those compositions in the ultimate source area of the humber group sandstones examination of an bearing feldspars on either side of the gap by tem shows that they have lamellar and or tweed exsolution microtextures that are characteristic of plagioclase feldspars having bulk compositions within the peristerite miscibility gap this is the first time that peristerites have been found in a sedimentary rock and they suggest a low grade metamorphic provenance for the plagioclase population compositional gaps have been commonly observed in ab rich plagioclase feldspars from metamorphic rocks although the precise reasons why feldspars do not form within the gap remain poorly understood in addition to the an bearing peristerites 30 16 7 contains a varied suite of an poor feldspars the microtexture of some of these grains is consistent with a low grade metamorphic provenance whereas others are inferred to be diagenetic in origin and to have formed by albitization of detrital alkali or plagioclase feldspar stratigraphic differences in the presence and size of the compositional gap within the humber group suggests that plagioclase feldspars may be a powerful new tool for high resolution stratigraphic correlation and may also be very sensitive indicators of changes in provenance

role of microbial mats in the fossilization of soft tissues it has been speculated that microbial mats are an important agent in the fossilization of soft tissues particularly when apatite ca 5 co 3 po 4 3 oh f is involved this has been tested by chemical analyses of the jurassic limestones of cerin france where phosphatized soft tissues are abundant and are associated with unequivocal microbial mats the sedimentary distribution of p k and fe following deposition was controlled by the presence of the mats p concentrations in the mats may approach 2 5 times those elsewhere in the sediment the highest p concentrations correlate with the occurrence of phosphatized soft tissues this is the first analytical evidence to demonstrate a fundamental role for microbial mats in the preservation of soft bodied fossils

trace element discrimination of arc slab failure and a type granitic rocks abstract although widely used classification schemes for granitic rocks have until now failed to separate slab failure from arc magmatism in collisional orogens our previous work showed that cretaceous cordilleran type batholiths are composed of a pre collisional belt of mesozonal to epizonal arc plutons commonly emplaced into their own cover and a post accretion tract of what we interpret as slab failure plutons emplaced into the tectonically thickened collisional hinterland histograms based on new geochemical compilations of pre and post collisional igneous rocks were employed to derive trace element criterion to distinguish between these two plutonic types using rocks over the sio2 range of 55 70 wt and aluminum saturation index of 20 nb y 0 4 ta yb 0 3 la yb 10 gd yb 2 and sm yb 2 5 also we found that a type magmas can be distinguished from other post collisional igneous rocks based on ta yb 6 ppm and nb y 60 ppm these trace element differences between our pre collisional arc and post collisional slab failure plus a type groups allowed us to build composite discrimination diagrams for destructive margin igneous rocks although most s type granites are also post collisional they should not be plotted on our diagrams for some of their geochemical features overlap those of our slab failure group unlike our arc and slab failure groups a type granites are formed in diverse tectonic settings such that their classification is more related to geochemical signature and magma sources than to tectonic setting lastly we show how our discrimination diagrams delineate examples of slab failure magmatism through time and suggest links with porphyry cu au deposits

intracaldera structure and megabreccias at dorobu caldera northeastern honshu japan in the pliocene dorobu caldera 13 by 6 km in diameter on northeastern honshu japan a number of occurrences of a distinctive megabreccia block facies is found within voluminous ash flow tuffs the dorobu ash flow tuffs a and b dorobu tuff a is believed to be the deposit of the initial caldera forming eruption an isopach map of tuff b and orientation of narrow dikes indicate that an internal large depression occurred inside the present caldera margin this and locations of post caldera conduits and near vent facies of dorobu tuff c suggest that the caldera has a nested collapse structure and possible conduits of the tuff b are within the internal depression annular collapses that sequentially moved towards the caldera center occurred during eruption of tuffs a and b the overall structure of dorobu caldera is nested and it possibly has step like floor as described elsewhere from piecemeal calderas megabreccia block facies consists of polyhedral massive blocks composed of miocene to pliocene country rocks with large diameters up to several hundreds meters the block facies as a whole does not consist of sheets or piles of debris as described in the san juan mountains and stillwater range usa but as isolated blocks set in the matrix of dorobu tuffs a and b in previous studies of caldera development the mode of emplacement of caldera collapse breccias has been considered to be by rock fall or rock slide during the caldera forming eruption a paleo magnetic study of the block facies shows two significant characteristics predominant variations of the magnetic directions of blocks indicate that they were emplaced with rotations caused by rock slide rock fall or a toppling motion in contrast magnetized directions of one megabreccia block almost coincide with that of the caldera country rock suggesting that this block must have kept its original magnetic direction our findings suggest that it is not a rock slide deposit derived from caldera walls but rather a segment of subsided caldera floor the mode of emplacement of the megabreccia block facies at dorobu caldera is mostly similar to the other megabreccias however additionally it is necessary to distinguish segments of caldera floor from rolled or toppled megabreccia blocks

graben structure in the las canadas edifice tenerife canary islands implications for active degassing and insights on the caldera formation abstract a graben structure has been identified at the western area of the las canadas caldera wall here referred as the los azulejos graben this graben is 1 km wide and is bounded by two major normal faults trending ne sw the los azulejos fault and the ucanca fault the graben was active for at least 0 5 ma from the end of the ucanca fm to the end of the guajara fm and before the collapse of the las canadas edifice that formed the western caldera a right lateral transtension regime operated in the graben as suggested by small fault orientations and kinematics the prolongation of the ne rift zone of tenerife to the canadas edifice is the most likely volcano tectonic scenario for the graben in this context inflation of phonolitic shallow magma chambers may have produced reverse faults and reactivation of normal faults an intense and widespread hydrothermal alteration here called azulejos type occurred mainly before the graben formation while a fault related hydrothermal alteration occurred during and after the graben diffuse carbon dioxide and hydrogen degassing in and around the las canadas caldera show relatively enriched values along a ne sw trend suggesting that faults in the los azulejos graben act as a pathway for deep seated gases to the surface diffuse degassing and hydrothermalism indicate that the graben area has been a zone of intense fluid circulation during the evolution of the las canadas edifice

duration of magmatic hydrothermal and supergene activity at cerro rico de potosi bolivia new high precision 40ar 39ar dating of sanidine and biotite from two rhyolitic domes and an ignimbrite combined with existing fission track data and a hydrothermal sericite age suggests that the world class ag deposit at cerro rico was emplaced during a protracted period of magma related hydrothermal activity beginning at 13 77 0 03 ma and continuing for at least 0 2 m y this may have been sustained by a large single injection or repeated injections of fractionated ag enriched magma into a high level magma chamber k ar dating of alunite indicates that supergene oxidation had begun by about 13 5 ma soon after dome emplacement and progressed semicontinuously for at least 7 5 m y this oxidation while not leading to significant enrichment has significantly enhanced the economic viability of the disseminated part of the orebody

cryptoperthites and cooling rate in a layered syenite pluton a chemical and tem study alkali feldspars in the klokken layered syenite south greenland are optically either coarse turbid deuteric patch perthites or clear unaltered fine braid microor cryptoperthites irregular clear volumes can be found in most turbid grains electron microscopy shows that all clear crystals or volumes are cryptoperthites in which the periodicity of the exsolution lamellae increases systematically with distance from the roof of the intrusion from 40 nm to 300nm they are composed of low albite and diagonally associated maximum microcline a secondary coarsening is responsible for optically visible braid microperthites the layered series consists of interleaved granular syenites and coarser grained laminated syenites with mineral layering microprobe analyses show a downward decrease in ca and ba in the granular syenite feldspars consistent with chilling from the roof inwards most laminated syenite feldspars contain 1 an increasing to 5 in mafic layers the periodicity of the exsolution lamellae in the laminated syenite feldspars is consistently up to twice that in adjacent granular syenites periodicity is not influenced by ab or or an content but high an feldspars have continuous zig zag intergrowths while others are more lozengeshaped lamellar periodicities largely reflect cooling rate in the pluton cooling times in the exsolution interval were probably about 103 104 years at the top of the series whereas existing experimental data suggest that the periodicities would be reached in a few days or years the periodicities at the top of the granular series are only slightly greater than experimentally determined initial wavelengths for spinodal decomposition suggesting that little or no coarsening occurred despite the long cooling times coarsening did occur lower in the pile framework ordering and twinning must slow or stop coarsening the difference in periodicity between the granular and laminated syenite series may be caused by differences in amount of water incorporated in the feldspar structure from the magma the exsolution of this water at low temperatures could be the cause of the deuteric unmixed patch perthites small volumes of cryptoperthite which have escaped deuteric unmixing may be common in igneous rocks in general and may be useful relative indicators of cooling rate

absence of silicic volcanism on mars implications for crustal composition and volatile abundance on earth eruptions of silicic magmas with volumes of 1 100 km3 are generally associated with stratovolcanoes or composite cones whereas larger 100 1000 km3 ignimbrite deposits are erupted from resurgent calderas and related structures small volumes of silicic rocks are generated by differentiation within basic magma chambers associated with stratovolcanoes but large silicic deposits are largely confined to areas where the granitic crust is thicker than 25 km as mantle derived magmas rise through the thick granitic crust fractional crystallization and assimilation of hydrous crustal rocks concentrate volatiles and sio2 in the magma yielding explosive silicic eruptions on mars under similar chemical conditions ignimbrite eruptions could take place over a broader range of physical conditions than on earth but no large volume ignimbrite deposits can be identified positively explosive basic to ultrabasic eruptions may be responsible for small cinder conelike features on mars and perhaps for the ancient flank scoured paterae such eruptions may have been driven by near surface water ice magma interactions or volatile rich magmas the paterae represent a unique style of volcanism that stopped 2 b y ago we suggest that the volatiles associated with these features were derived from mantle sources and that the cessation of patera formation may coincide with the termination of the period of maximum planetary degassing volcanism since 2 b y ago has been dominantly effusive except for minor explosive ash events such as those which blanketed part of the summit of hecates tholus we propose that such eruptions were driven by volatiles generated by differentiation of a magma chamber similar to icelandic explosive eruptions the apparent lack of large scale silicic volcanism on mars is circumstantial evidence that mars is deficient in volatiles and lacks a granitic crust

collapse mechanism of the paleogene sakurae cauldron sw japan the paleogene sakurae cauldron of sw japan is characterized by a nested structure with a polygonal outline 21 13 km2 including a circular collapsed part 5 km in diameter total thickness of the caldera infill amounts to 2 000 m the lower member of the infill consists mainly of felsic crystal tuff and lesser intercalated andesitic lava flows whereas the upper member is composed of high grade ignimbrite capped with a large rhyolitic lava dome these members represent the first and second stage eruptions respectively faults bounding the cauldron rim comprise intersecting radial and concentric faults producing the polygonal outline of this cauldron the primary collapse of this cauldron thus occurred as a polygonal caldera basin where products of the first stage eruption accumulated in contrast the inner collapse part is defined by a ring fracture system this sector subsided concurrently with accumulation of the high grade ignimbrite of the second stage eruption this inner circular collapse thus represents syn eruptional subsidence concurrent with the climactic eruption magma drainage during the first stage probably induced outward dipping ring fractures in the chamber roof opening of the ring fractures following subsidence of the central bell jar block caused rapid evacuation of magma as voluminous pumice flows even though magma pressure may have decreased to some degree

eramosa lagerstatte exceptionally preserved soft bodied biotas with shallow marine shelly and bioturbating organisms silurian ontario canada the middle silurian eramosa lagerstatte of ontario canada preserves taxonomically and taphonomically diverse biotas including articulated conodont skeletons and heterostracan fish annelids and arthropods with soft body parts and a diverse marine flora soft tissues are preserved as calcium phosphate and carbon films the latter possibly stabilized by early diagenetic sulfurization it is significant that the biotas also include a decalcified autochthonous shelly marine fauna and trace fossils this association of exceptionally preserved and more typical fossils distinguishes the eramosa from other silurian shallow marine lagerstatten such as the waukesha lagerstatte and suggests that the eramosa is not the product of exceptional preservation in an atypical environment a bias claimed for many post cambrian lagerstatten the eramosa lagerstatte may provide a more reliable balanced measure of what has been lost from the silurian fossil record

la pacana caldera and the atana ignimbrite a major ash flow and resurgent caldera complex in the andes of northern chile the recently discovered la pacana caldera 60 35 km is the largest caldera yet described in south america this resurgent caldera of pliocene age developed in a continental platemargin environment in a major province of ignimbrite volcanism in the central andes of northern chile at about 23 s latitude collapse of la pacana caldera was initiated by the eruption of about 900 km3 of the rhyodacitic atana ignimbrite the atana ignimbrite was erupted from a composite ring fracture system and formed at least four major ash flow tuff units that are separated locally by thin air fall and surge deposits all four sheets were emplaced in rapid succession about 4 1 0 4 ma ago caldera collapse was followed closely by resurgent doming of the caldera floor accompanied by early postcaldera eruptions of dacitic to rhyolitic lava domes along the ring fractures the resurgent dome is an elongated asymmetrical uplift 48 5 12 km which is broken by a complex system of normal faults locally forming a narrow discontinuous apical graben later postcaldera eruptions produced large andesitic and dacitic stratocones along the caldera margins and dacitic domes on the resurgent dome beginning about 3 5 ma ago and persisting into the quaternary hydrothermally altered rocks occur in the eroded cores of precaldera and postcaldera stratovolcanoes and along fractures in the resurgent dome but no ore deposits are known a few warm springs located in salars within the caldera moat appear to be vestiges of the caldera geothermal system

chondrites a trace fossil indicator of anoxia in sediments the trace fossil chondrites a highly branched burrow system of unknown endobenthic deposit feeders occurs in all types of sediment including those deposited under anaerobic conditions in some cases such as the jurassic posidonienschiefer formation of germany chondrites occurs in black laminated carbonaceous sediment that was deposited in chemically reducing conditions in other cases such as numerous oxic clastic and carbonate units throughout the geologic column chondrites typically represents the last trace fossil in a biotutbation sequence this indicates that the burrow system was produced deep within the sediment in the anaerobic zone below the surficial oxidized zone that was characterized by freely circulating and oxidizing pore waters

a burgess shale like fauna from the lower cambrian of north greenland the cambrian explosion refers to the major adaptive radiation of the metazoans during the earliest phanerozoic an event that is best known by the abrupt appearance of hard parts near to the precambrian cambrian boundary 550 myr bp 1 deciphering this important evolutionary episode depends largely on the study of these skeletal remains2 3 together with trace fossils that include burrows and trackways4 5 many of these latter fossils evidently represent the activities of soft bodied organisms such as worms and lightly sclerotized arthropods whose exact identity is seldom known not least because they can be fossilized only in exceptional circumstances6 however documenting the soft bodied representatives of early metazoan communities cannot be neglected if they accounted for a substantial fraction of these early biotas7 here we report a new soft bodied fauna from the lower cambrian of north greenland this assemblage has highly significant similarities with the younger burgess shale and provides additional evidence for the geological longevity of this faunal type and its widespread distribution in deeper waters of cambrian oceans

experimental study of caldera formation scaled experiments have been carried out on caldera collapse mechanisms using silicone as analogue magma and dry sand as analogue rock experiments were carried out in two and three dimensions using a range of roof aspect ratios thickness width 0 2 to 4 5 appropriate for caldera collapse they reveal a general mechanism of collapse only weakly dependent on the shape of the reservoir for low roof aspect ratios 1 subsidence starts by flexure of the roof and the formation of outward dipping reverse ring faults which in turn trigger formation of peripheral inward dipping normal ring faults the subsidence always occurs asymmetrically in cross section the reverse faults delimit a coherent piston bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets the surface depression consists of a nondeformed area piston surrounded by an annular extensional zone tilted strata for high aspect ratios 1 multiple reverse faults break up the roof into large pieces and subsidence occurred as a series of nested wedges 2 d or cones 3 d the extensional zone dominates the surface depression in the case where preexisting regional faults do not play a major role the collapse mechanics of calderas probably depends strongly on the roof aspect ratio calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults the annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant

buried or brined eurypterids and evaporites in the silurian appalachian basin abstract eurypterid bearing deposits from the late silurian appalachian basin are often interpreted as having been deposited under hypersaline conditions these interpretations are based on the close association of abundant eurypterid remains with evaporite deposits and structures such as salt hoppers in the salina and bertie groups to determine whether this association reflects life habitat or is the result of taphonomic or diagenetic processes the co occurrence of eurypterids and salt hoppers in the upper silurian appalachian basin was examined at several stratigraphic scales a survey of eurypterid remains from the prolific ellicott creek breccia member of the fiddler s green formation bertie group found that 2 of the 479 specimens surveyed are crosscut by salt hoppers or incipient halite structures in a regional survey displacive salt hoppers occurred in the same bed as eurypterid remains in 37 19 51 of all eurypterid bearing units in these units salt hoppers were typically the only structures found intimately associated with eurypterids sometimes crosscutting them the disruptive nature of the hoppers in the ellicott creek breccia for example suggests that they formed within the sediment rather than at the air water interface and that organic remains might have acted as nucleation points for developing halite crystals to explain these associations we present a depositional model in which displacive salt hoppers formed within nacl saturated groundwater as a result of surface evaporation in the vadose zone during regressive phases and only after eurypterid remains were buried in this scenario the intimate association of eurypterids and salt hoppers in these deposits reflects early stage diagenetic overprinting rather than conditions during life our model largely refutes the hypothesis that eurypterids were halotolerant organisms based on their co occurrence with salt hoppers we conclude that eurypterids preserved in upper silurian carbonate ramp deposits were primarily denizens of more normal marine or hyposaline subtidal settings

alteration facies linkages among iron oxide copper gold iron oxide apatite and affiliated deposits in the great bear magmatic zone northwest territories canada high temperature metasomatism driven by ascent of voluminous saline fluid columns in the upper crust plays a major role in the genesis of iron oxide alkali alteration ore systems but fundamental questions remain on genetic linkages among iron oxide copper gold iocg iron oxide apatite ioa albitite hosted uranium and skarn deposits that they produce excellent surface exposures of such systems in the great bear magmatic zone of northernwestern canada record the depth to paleosurface prograde evolution of iron oxide alkali alteration facies and mineralization across the belt albitite corridors that are tens of kilometers in length record the earliest reactions between highly saline fluids and host rocks along fault zones and subvolcanic intrusions pervasive albitization partitioned metals from the host rocks into the ascending fluid column leaving behind structurally weakened corridors of porous albitite these corridors were cut replaced and overprinted by amphibole and magnetite bearing calcic iron alteration assemblages in extreme cases the discharge of calcium iron and specialized metals formed iron oxide apatite deposits vanadium rare earth elements while recharging the outgoing fluids in sodium potassium and base and precious metals as temperatures declined and fluid chemistry evolved through fluid rock reactions the formation of potassic iron alteration assemblages breccias and sulfides resulted in magnetite and hematite group iocg mineralization within carbonate units skarns formed prior to are replaced by and evolved to calcic iron alteration facies skarns can locally host base metal mineralization tectonically uplifted albitite breccias replaced by potassic iron alteration assemblages became a preferential host for uranium mineralization the results of this study also illustrate that permutations and cyclical build up of alteration products can arise from a combination of faulting differential uplift and renewed magmatism framed within an alteration facies deposit model alteration zones and mineral occurrences play a pivotal role in predicting the mineral potential of iron oxide and alkali altered systems at district to deposit scales

exsolution and coarsening mechanisms and kinetics in an ordered cryptoperthite series cryptoperthites from the klokken layered syenite intrusion were examined by tem to determine the role of exsolution ordering and twinning in the development of the coherent microtextures during slow cooling the stratigraphic position of the samples in the layered series giving an independent variable in determining their evolution both periodicity primary and secondary and morphology change with distance from the top of the series most samples contain low microcline in the diagonal association

zoned ternary feldspars in the klokken intrusion exsolution microtextures and mechanisms the microtextures developed during relatively slow cooling as a function of bulk composition in zoned ternary feldspars from syenodiorites and syenites in the klokken intrusion described in the preceding paper were determined by tem and their origin and evolution deduced the feldspars normally have a plagioclase core and an alkali feldspar rim cores become smaller and rims larger and the an content of both decrease with distance from the contact of the intrusion the following microtextural sequence was observed the inner plagioclase cores are homogeneous oligoclase andesine with albite growth twins only but are crypto antiperthitic towards the outer core at first small platelets of low sanidine a few nanometres thick and up to 10 nm long occur sporadically only on albite twin composition planes with further increase in bulk or they are homogeneously distributed in the plagioclase thicker through going plates in platelet free areas are found which induce albite twins in the surrounding plagioclase the microtextures in the rims are regular cryptomesoperthitic with 601 lenses or lamellae depending on the bulk or content of low sanidine in albite twinned low oligoclase andesine albite and pericline twins in plagioclase in an m twin relationship together with lenticular low sanidine were found in only one small area the overall diffraction symmetry of the mesoperthites is monoclinic showing that exsolution started in a monoclinic feldspar whereas that of the antiperthites is triclinic the intermediate zone between the core and rim is more complex and microtextures vary over distances of a few micrometres the cryptomesoperthites are very regular where or rich and probably arose by spinodal decomposition the platelets in the outer cores arose by heterogeneous nucleation on twin composition planes and by homogeneous nucleation elsewhere near the intermediate zone they coarsened to give larger plates which induced albite twins in the plagioclase because of the zoning microtextures that were initiated in areas of given composition can propagate laterally into zones of different composition a diagram is given showing the relationship between ternary bulk composition and the microtexture developed in coherent perthitic alkali feldspars and plagioclases from slowly cooled rocks

the kari kari caldera bolivia and the cerro rico stock abstract the rocks comprising the kari kari massif southeast of the city of potosi bolivia consist entirely of welded ignimbrites it is argued that the massif constitutes the resurgent centre of a 20 m y old resurgent caldera plutonic rocks are exposed in the south but volcanic rocks of the caldera rim are exposed in the north and indicate a shallower erosion level there the volcanic rocks consist of a coarse moat deposit consisting of angular fragments of basement material and juvenile clasts overlain by an extensive garnet bearing ignimbrite a plant fossil bearing lacustrine deposit was laid down in a lake within the caldera the cerro rico stock noted for its silver tin mineralisation may be a late intrusion along the caldera ring fractures

secular variation in greenstone sequence development emphasizing superior province canada abstract evidence from the superior province of the canadian shield suggests the existence of several distinct lithostratigraphic associations in greenstone belts 1 quartz arenite and carbonate bearing sequences 2 mafic ultramafic volcanic sequences 3 mafic to felsic volcanic cycles and 4 timiskaming type sequences of fluviatile sediments and calc alkaline to alkaline volcanics recently recognized quartz arenite bearing sequences in the sachigo and wabigoon subprovinces are from base to top quartz arenite stromatolite bearing carbonate oxide facies iron formation and komatiite or tholeiite primary structures in the quartz arenites indicate shallow water deposition and represent mature sedimentation granitoid provenance of these sediments is postulated based upon petrography of coarse arenites and clasts of associated conglomerates the few dated superior province quartz arenite bearing sequences are 2 85 ga the shallow water origin of the primary structures and biota and the areal extent of the quartz arenite and carbonate sequences invite comparison with platform sediments at passive margins platform sequences unconformably overlie 3 ga granitoid rocks and older mafic volcanics where original lower contacts are preserved in two localities mafic volcanic sequences contain sheet flows iron formations and deep water argillites based on physical volcanology and sedimentology these sequences are compared to modern deep water mafic plain and hawaiian volcanism they generally underlie mafic to felsic volcanic cycles and overlie platform sequences mafic sequences are 2740 2700 ma in age mafic to felsic volcanic cycles consist of deep water tholeiitic flows overlain by shallow water to subaerial intermediate to felsic pyroclastics and flows the facies style of volcanism petrography and geochemistry of the mafic to felsic cycles most closely resemble the characteristics of crust in continental magmatic arc systems this type of lithostratigraphic association varies in age from 2775 to 2700 ma in the superior province with minor exceptions timiskaming type sequences consist of fluviatile conglomerate immature arenites and wackes with subaqueous to subaerial volcanics of variable chemistry longitudinal and lateral variation within fault bounded basins is extreme and the sequences have generally been affected by only late stages of deformation the combination of rock types facies distribution the fault bounded nature of the repositories and the locus of sedimentation along zones of transcurrent displacement suggest timiskaming sequences are best compared with modern pull apart basins superior province timiskaming sequences are within the superior province platform sequences are the earliest postorogenic lithostratigraphic unit in greenstone belts followed by mafic plain sequences arc volcanics and pull apart basins we suggest that platform sequences based upon limited evidence are preceded by an orogeny and that pull apart basins represent a subsequent orogeny the lithostratigraphic units above have distinct metallogenic associations 1 platform sequences based upon compilation of several shields may contain detrital au and u and oxide facies iron formations associated felsic volcanics contain no recognized volcanogenic cu zn deposits in the superior province the following metallogenic associations are noted 2 mafic plain sequences contain komatiite flow related ni deposits cu ni and platinum group deposits are associated with flows and coeval intrusions 3 arc volcanic sequences contain volcanogenic cu zn sulphide deposits and scarce deposits of porphyry type cu mo and 4 timiskaming sequences and related plutonism are related to lode au

paleobiology and taphonomy of exceptionally preserved organisms from the waukesha biota silurian wisconsin usa abstract the silurian llandovery telychian waukesha lagerstatte in the brandon bridge formation of wisconsin usa is preserved in a dolomitic plattenkalk representing deposition in a shallow water sediment trap on a carbonate platform renewed study of the deposit demonstrates biodiversity far richer than previously reported and includes both biomineralizing and non biomineralizing organisms at present remains of at least 12 animal phyla are known in addition to algae and microorganisms of these remains of cnidarians brachiopods mollusks and echinoderms were previously unknown from this deposit the macrobiota is dominated by trilobites graptolites conulariids non biomineralizing arthropods some of which were previously unreported and some worms palaeoscolecids and annelids new abundant evidence of microbial mats and associated microbial structures indicates a thriving microorganismal community evidence including remnants of preserved microbes on macrofossils demonstrates that microbial processes played a leading role in both burial and preservation of non biomineralizing tissues this process referred to as microbial entombment is a biologically mediated sedimentary process by which organic remains are entombed within a microbial mat

feldspar microtextures and multistage thermal history of syenites from the coldwell complex ontario optical and tem transmission electron microscopy observations of perthites from augite syenites in the coldwell complex ontario reveal a complex set of microtextures that outline a multistage thermal history regular microtextures linear or braid texture straincontrolled coherent intergrowths show a progressive evolution from the margin of the intrusion inwards with lamellar spacings in the range 40 100 nm the textures evolve in a manner similar to those for the klokken intrusion and reflect differences in cooling rates and bulk composition superimposed upon the regular microtexture are 10 m scale compositional fluctuations which we call ripples the boundary relationships and bulk composition of ripples which are themselves ab rich and or rich linear coherent cryptoperthites suggest that they formed by coarsening during a phase of high temperature 530 c fluid feldspar interaction this was followed by a return to coherent exsolution in which fluid was not involved coarse irregular patch microperthite cross cuts all other microtextures these final deuteric intergrowths are believed to result from a further low temperature 380 c fluid feldspar interaction and are associated with subgrain formation and the presence of micropores the outermost syenite sample against a gabbro ring structure has distinctive modified microtextures indicating that the gabbro is at least in part a later intrusion our findings show that tem work on alkali feldspar microtextures can identify discrete thermal events in the cooling history of igneous plutons and illustrates the potential of such microtextures for establishing the relative ages of intrusive rocks

folded cauldrons of the early proterozoic labine group northwestern canadian shield great bear magmatic zone 1 875 1 84 ga is a linear belt 100 km wide by 800 km long of little metamorphosed volcanic rocks and allied plutons of calc alkaline affinity the entire zone was folded shortly after magmatism and sections many kilometers thick through several cauldron complexes are exposed eruptions of rhyolite led to collapse of black bear cauldron in which 1 5 km of tuff ponded the cauldron then became the locus of fluvial and lacustrine sedimentation followed by augite plagioclase porphyritic andesite volcanism both of which also accumulated within the cauldron ash flow eruptions of dacite caused collapse of clut cauldron which was accompanied by landsliding and avalanching of the cauldron walls abundant andesitic and intrusive debris is intercalated with the propylitized intracauldron facies tuff adjacent to the walls and some blocks are as large as 1 km across clut cauldron also became the site for fluvio lacustrine sedimentation after collapse and was likely resurgent resurgence was probably related to the emplacement of a quartz monzonite pluton which occupies the core of the cauldron cornwall cauldron which is exposed entirely in cross section contains 1 2 km of intracauldron facies tuff overlain by a complex of dacite lava flows the cauldron was intruded by a granodiorite monzogranite pluton probably responsible for resurgence data from the area suggest the following 1 the similarity of early proterozoic cauldrons to cenozoic examples indicates that cauldron collapse is a process that has occurred since 1 9 ga 2 batholiths beneath ash flow tuff fields are probably composite bodies made up of many individual plutons 3 most of the plutons are sheetlike in cross section and 4 cauldron collapse begins as soon as ash flow eruptions and relief on the cauldron margin becomes large early in the subsidence history

continental intra arc depressions a nonextensional model for their origin with a proterozoic example from wopmay orogen the 1 875 1 86 ga great bear magmatic zone is a linear belt 100 km wide by 800 km long of low tio 2 high al 2 o 3 volcanic rocks ranging continuously from basalt to rhyolite cut by allied hornblende biotite bearing plutonic rocks comprehensive study of the facies volcanic styles chemistry and petrography indicates that it is similar to cenozoic continental arcs preservation of high level volcanic rocks suggests that the zone never had an unusually thick crust and therefore is similar to some intra arc depressions such as the mesozoic paleogene depression of chile common in younger continental arcs these long linear depressions are the sites for voluminous pyroclastic volcanism contain sections tens of kilometres thick and are zones of subsidence whose surface remains close to sea level they originate when the mass of basaltic andesite arriving at the base of the crust equals the mass of vitric ash removed from the areas by high level atmospheric transport during ash flow eruptions

tectonic evolution of the emergent part of a volcanic ocean island lanzarote canary islands abstract a reconstruction of the structural evolution of lanzarote confirms the existence of two principal stages of construction of the island separated by a main erosional event the first pre erosional is the shield stage miocene pliocene in age which is divided in three stratigraphic groups where different formations can be distinguished using unconformities mappable over the whole area of outcrop in the second post erosional stage developed during the quaternary nine stratigraphic levels are tentatively identified and are grouped in two phases of volcanic activity shields phase and pyroclastic cones phase data from 580 photogeologic lineaments as well as field data from 204 fault planes 195 with sense of movement 273 dykes and 45 volcanic alignments are presented the tectonic style of the deformations is brittle and polyphasic strike slip faults are dominant but normal and reverse faults are also present both the so called atlantic and african directions are well developed in trends of strike slip faults and volcanic indicators at least two deformation phases can be identified from the geometrical analysis of faults and from the stress tensor calculated using lineations on fault planes the reconstructed stress field responsible for the measured deformations changed orientation during the evolution of the island of lanzarote it is suggested that a permutation between principal stress axes 1 and 3 occurred subparallel to the length of the island between the pre erosional and post erosional stages

the geochemical characteristics of granitoids in contrasting arcs and comments on magma sources geochemical variations in modern subduction related igneous suites with respect to arc maturity in time and space are illustrated using data for both volcanic suites basalt andesite and dacite and intrusive granitoid suites diorite tonalite granodiorite and granite from circum pacific arcs using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites a subduction zone enrichment of lithospheric mantle locally coupled with crustal assimilation allied with fractional crystallization afc in zones of thickened crust all of which yield magmas with enhanced concentrations of the large ion lithophile lil elements k rb th u lrees etc b with increasing distance from the active trench contributions from within plate sub continental lithosphere producing mantle derived magmas with enhanced levels of high field strength hfs elements among which nb ta hf and y are particularly distinctive thus even for the evolved granitoids of intrusive arc series ratios of hfs lil elements not significantly affected by crystal fractionation such as ta nb k rb la may throw some light on the origin of mafic intermediate precursor magmas in terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites 1 primitive calcic arc granitoids with low lil and hfs element abundances 2 normal calc alkaline continental arc granitoids with enhanced lil element abundances and low hfs lil ratios 3 mature alkali calcic arc granitoids with high levels of lil and hfs elements and higher hfs lil ratios 4 back arc anorogenic alkaline granitoids with the highest levels of hfs elements

plagioclase relations in pelites central menderes massif turkey i the peristerite gap with coexisting kyanite in pelites of the central menderes massif albite and oligoclase with only slight chemical zoning coexist in apparent textural equilibrium in the garnet zone staurolite zone and staurolite kyanite transition zone the metamorphic temperature range is estimated as approximately 440 550 c from the hodges spear calibration of the garnet biotite geo thermometer or 440 500 c ganguly saxena calibration while oligoclase composition at the peristerite gap changes from an22 to an14 albite also becomes more sodic an1 5 an0 6 the slope of the albite limb is thus the reverse of that reported in other areas and may not be a true equilibrium feature occurrence of kyanite at temperatures below the crest of the gap is due to low water activity in the presence of graphite ah2o is estimated at approximately 0 1 0 2 from the na content of muscovite coexisting with albite kyanite quartz

cooling process of a granitic body deduced from the extents of exsolution and deuteric sub solidus reactions case study of the okueyama granitic body kyushu japan abstract the variation in cooling processes with depth in a magma body is evaluated quantitatively by analysis of the extent of exsolution coarsening and deuteric coarsening as sub solidus reactions this method is applied to evaluation of the okueyama granitic body of central kyushu japan exsolution coarsening has produced microperthite textures in this body while deuteric coarsening has resulted in patchperthite myrmekite and reaction rims respectively through measurement of six textural parameters including the width and spacing of lamellae and the thickness of myrmekite and reaction rims the extent of these sub solidus reactions is shown to increase systematically with depth in the granite body indicating that the okueyama cooled gradually from the roof the hornblende plagioclase and ternary feldspar thermometers indicate temperature a range of 710 to 620 c for volume diffusion associated with exsolution coarsening while deuteric coarsening is found to have occurred at temperatures below 500 c on the basis of the ternary feldspar thermometer the cooling period corresponding to exsolution coarsening is estimated using a one dimensional heat transfer model yielding periods of 820 y at the roof and 1390 1890 y at the base of the exposure 1000 m below the roof depending on total depth of the original magma body

paleomagnetic evidence for the timing of collapse and resurgence of the lake city caldera san juan mountains colorado rocks of the 23 1 m y old lake city caldera consist of the compositionally zoned sunshine peak tuff postcollapse intracaldera lava flows and resurgent quartz syenite intrusions declinations of reversely magnetized i 45 to 75 sunshine peak tuff change from easterly d 93 130 throughout most of the tuff to southerly d 195 207 within the late eruptive phases the postcollapse lava flows are also reversely magnetized and their emplacement appears to have spanned resurgence of the caldera in contrast the uppermost levels of the resurgent intrusion record two distinct components of magnetization a reversed component isolated by thermal demagnetization typically at temperatures greater than 400 c and a normal component defined by thermal demagnetization at lower temperatures a baked contact test implies that emplacement and initial cooling of the intrusion occurred during a time of reversed polarity and that continued cooling at lower blocking temperatures took place during normal polarity assuming that the reversed directions of the extrusive and intrusive rocks record a single period of reversed polarity as suggested by the paleomagnetic and radiometric age data the time for caldera development from eruption of the ash flow tuffs to emplacement of the resurgent intrusion was less than about 300 000 years on the basis of best estimates of durations of reversed polarities in the early miocene the estimated 300 000 year time span for the development of the lake city caldera is very close in duration to the radiometrically determined time spans of much younger pleistocene calderas having similar dimensions and similar volumes of erupted material

early history of the michigan basin subsidence and appalachian tectonics geometries of cambrian to silurian stratigraphc sequences in the michigan basin record discrete episodes of basin centered subsidence separated by periods of regional tilting backstripping reveals irregular subsidence rates that argue against a simple thermal contraction model depositional facies architecture also reflects episodic subsidence patterns basin centered facies tracts dominating during subsidence reactivations these three lines of evidence indicate that subsidence cessations and reactivations characterize the early history of the michigan basin periods of episodic subsidence correlate temporally with orogenic events in the appalachians suggesting that reactivation of basin subsidence is related to tectonic activity we propose that appalachian orogenic activity caused the episodic subsidence of the michigan basin possibly through weakening of the lower crust and reactivation of a preexisting upper crustal isostatic imbalance

the role of decay and mineralization in the preservation of soft bodied fossils abstract fossil deposits that preserve soft bodied organisms provide critical evidence of the history of life usually only more decay resistant materials e g cuticles survive as organic remains as a result of selective preservation and subsequent diagenesis to more resistant biopolymers permineralization the permeation of tissues by mineralizing fluids may preserve remarkable detail particularly of plants however evidence of more labile tissues e g muscle normally requires the replication of their morphology by rapid in situ growth of minerals i e authigenic mineralization this process relies on the steep geochemical gradients generated by decay microbes the minerals involved and the level of detail preserved which may be subcellular depend on a number of factors including the nature of microbial activity and amount of decay availability of ions and the type of organism that is fossilized understanding these controls is essential to determining the conditions that favor exce

nd isotopic evidence from wopmay orogen for 2 0 2 4 ga crust in western north america abstract nd isotopic data from the 1 9 ga wopmay orogen n w t canada strongly suggest the involvement of crust in the range of 2 0 2 4 ga remnants of three compositionally diverse magmatic arcs 1940 1902 ma 1890 1880 ma and 1875 1840 ma are preserved in wopmay orogen each of the three arcs originated west of the limit of exposed archean crust nineteen samples of plutonic and volcanic rocks from the three arcs yield a narrow range of initial e nd from 1 8 to 0 3 there is no apparent correlation between e nd and geographic distribution nor with whole rock chemistry sio 2 54 75 two basalt samples from west of the archean have initial e nd s between 2 and 3 two 1 87 ga granites which intrude through archean crust have much lower initial e nd s of 5 9 and 8 6 the data from the three magmatic arcs are best interpreted in terms of a source region which had a very limited range of e nd at 1 9 ga which we interpret as 2 0 2 4 ga crust presumably hottah terrane basement wopmay fault zone is therefore the surface manifestation of a suture between 2 0 2 4 ga hottah terrane and the archean slave craton there is a growing body of data which suggests that much of western north america may be underlain by 2 0 2 4 ga crust geochronologic and isotopic data from other precambrian cratons also suggest that 2 0 2 4 ga crust may be much more widespread than previously thought models of crustal growth which interpret 1 9 1 7 ga crust as mixtures of archean crust and proterozoic depleted mantle may be overestimating the amount of juvenile material added to the crust in this time period

the age and thermal history of cerro rico de potosi bolivia cerro rico de potosi bolivia is the world s largest silver deposit and has been mined since the sixteenth century for silver and for tin and zinc during the twentieth century together with by product copper and lead the deposit consists primarily of veins that cut an altered igneous body that we interpret to be a dacitic volcanic dome and its underlying tuff ring and explosion breccia the deposit is compositionally and thermally zoned having a core of cassiterite wolframite bismuthinite and arsenopyrite surrounded by a peripheral lower temperature mineral assemblage consisting principally of sphalerite galena lead sulfosalt and silver minerals the low temperature assemblage also was superimposed on the high temperature assemblage in response to cooling of the main hydrothermal system both the dacite dome and the ore fluids were derived from a larger magmatic hydrothermal source at depth the dome was repeatedly fractured by recurrent movement on the fault system that guided its initial emplacement the dome was extruded at 13 8 0 2 ma 2 based on u th pb dating of zircon mineralization and alteration occurred within about 0 3 my of dome emplacement as indicated by a 40ar 39ar date of 13 76 0 10ma 1 for sericite from the pervasive quartz sericite pyrite alteration associated with the main stage high temperature mineralization the last thermal event able to reset zircon fission tracks occurred no later than 12 5 1 1 ma 1 as indicated by fission tract dating minor sericite and magmatic steam alunite veins were episodically formed around 11 ma and between 8 3 and 5 7 ma the younger episodes occurring at the time of extensional fracturing at cerro rico and wide spread volcanism in the adjacent los frailes volcanic field none of these younger events appear to be significant thermal mineralizing events the exceptionally flat thermal release pattern of 39ar from sericite and the results of the fission tract dating of zircon show that none of the younger events was hot enough and lasted long enough to cause significant loss of ar or annealing of zircon fission tracks u th pb dating of zircon cores indicates a precambrian progenitor for some zircons and ree analyses of dated samples of hydrothermally altered dacite show the presence of a prominent positive eu anomaly which constrains interpretations of the origin and evolution of the magmatic hydrothermal system

sequence development on a sediment starved low accommodation epeiric carbonate ramp silurian wabash platform usa mid continent during icehouse to greenhouse transition abstract twelve cored wells through the silurian section of the wabash platform indiana u s a and 2 non cored basinal wells with cuttings were studied to define the sequence stratigraphy of a sediment starved ramp within a low latitude epeiric sea during the latest ordovician icehouse to silurian greenhouse transition the wabash platform approximately 200 000 km2 area was bounded to the north by the michigan basin to the east by the appalachian foreland basin and passed to the southwest into the vincennes basin which was open to the ocean facies developed include platform fringing buildups stromatactis wackestone core stromatoporoid skeletal wackestone caps and crinoidal rudstone packstone flank facies and non buildup crinoid grainstone packstone sheets shoals skeletal wackestone variably cherty between fair weather and storm wave base deep lagoon and deep ramp and carbonate mudstone clean to argillaceous sub storm wave base deep lagoon and deep ramp basin the eleven sequences 2 m to 30 m thick and 1 to 4 m y durations average 2 5 m y generally can be correlated to global sea level cycles the lower three sequences are disconformity bounded reflecting moderate ice sheets but the others are relatively conformable the most easily mapped regional surfaces are the transgressive surfaces because the correlative conformities are cryptic the wide well spacing prevented mapping of lowstand system tracts the transgressive systems tracts are upward deepening upward fining carbonate units that become more argillaceous and silty upward the highstand systems tracts are upward coarsening carbonate mudstone to wackestone packstone and rare grainstone barrier bank complexes and isolated buildups aggraded during deposition of the upper 4 sequences to form a discontinuous raised rim 40 m relief to the ramp while the interior of the platform remained sediment starved subsidence rates in the basin were very low 0 6 to 1 2 cm k y but the low platform accumulation rates 0 2 cm k y to 0 8 cm k y prevented the seafloor from building to sea level in all but the lower three sequences and later over barrier banks accumulation rates 2 cm k y parasequence development in high accommodation settings elsewhere in the silurian in north america records transition from moderate ice sheets to an ice free world this is weakly expressed on the wabash platform due to the dominantly deeper subtidal setting the remarkably low sedimentation rates resulted from the epeiric sea being isolated from the open ocean and they likely are typical of the large epeiric seas of the paleozoic

exceptionally preserved fossil assemblages through geologic time and space abstract geologic deposits containing fossils with remains of non biomineralized tissues i e konservat lagerstatten provide key insights into ancient organisms and ecosystems such deposits are not evenly distributed through geologic time or space suggesting that global phenomena play a key role in exceptional fossil preservation nonetheless establishing the influence of global phenomena requires documenting temporal and spatial trends in occurrences of exceptionally preserved fossil assemblages to this end we compiled and analyzed a dataset of 694 globally distributed exceptional fossil assemblages spanning the history of complex eukaryotic life 610 to 3 ma our analyses demonstrate that assemblages with similar ages and depositional settings commonly occur in clusters each signifying an ancient geographic region up to hundreds of kilometers in scale which repeatedly developed conditions conducive to soft tissue preservation using a novel hierarchical clustering approach we show that these clusters decrease in number and shift from open marine to transitional and non marine settings across the cambrian ordovician interval conditions conducive to exceptional preservation declined worldwide during the early paleozoic in response to transformations of near surface environments that promoted degradation of tissues and curbed authigenic mineralization potential we propose a holistic explanation relating these environmental transitions to ocean oxygenation and bioturbation which affected virtually all taphonomic pathways in addition to changes in seawater chemistry that disproportionately affected processes of soft tissue conservation after these transitions exceptional preservation rarely occurred in open marine settings excepting times of widespread oceanic anoxia when low oxygen levels set the stage with these patterns non marine cluster count is correlated with non marine rock quantity and generally decreases with age this result suggests that geologic processes which progressively destroy terrestrial rocks over time limit sampling of non marine deposits on a global scale future efforts should aim to assess the impacts of such phenomena on evolutionary and ecological patterns in the fossil record

magmatism in the huarina belt bolivia and its geotectonic implications abstract the huarina belt is a tectono stratigraphic unit extending along the western side of the eastern cordillera and southern altiplano it includes the well known bolivian tin belt both geophysical and isotopic evidence suggest that the huarina belt is located between three cratonic blocks in a major lithospheric weakness zone that has probably been inherited from the proterozoic most of the phanerozoic backarc magmatism in the bolivian andes occurred along this belt local ordovician basalts and andesites and clasts of devonian granitoids occurring in late devonian early carboniferous sedimentary units are testimonies of a scarce palaeozoic igneous activity during the mesozoic rift related magmatism resulted in peraluminous to metaluminous high k calcalkaline triassic granitoids as well as discrete cretaceous centers of alkaline and carbonatitic rocks in the cenozoic huge high k generally peraluminous ignimbrite sheets along with numerous volcanic and intrusive centers were generated along the belt shoshonitic alkaline and ultrapotassic rocks have been locally described on the basis of both geochemical and geophysical evidence a general evolution model is proposed for the huarina belt during paleozoic and mesozoic times the predominant extensional regime of the continental crust led to lithospheric thinning basin generation and production of within plate magmas during the cenozoic prevailing compressional conditions led to a gradual thickening of the lithosphere inducing delamination of its denser and less viscous root the mantle generated magmas interplayed widely with crustal melts giving rise to the dominantly peraluminous magmas

exsolution by spinodal decomposition ii perthite formation during slow cooling of anatexites from ngoronghoro tanzania perthites in slowly cooled granulite facies rocks from the ngoronghoro structure tanzania show complex microstructures reflecting several stages of exsolution and coarsening mesoperthites with an integrated bulk composition of or26 5ab71an2 5 are comprised of lamellar intergrowth of 5 to 10 m wide orthoclase rich and albite rich lamellae spindle perthites with a bulk composition of or67ab31 5an1 5 are comprised of 2 to 10 m wide and several 109s of m long albite rich spindles in an orthoclase rich host in the mesoperthite primary exsolution occurred by spinodal decomposition whereas the spindle perthite formed by a nucleation and growth mechanism the interfaces between the exsolved phases are generally incoherent unmixing and coarsening of the mesoperthite is discussed in the light of non linear uphill diffusion the microstructure evolution and successive chemical separation of the exsolved phases during cooling are modeled based on cahn hilliard theory we find that coarsening primarily occurs during the first 20 c to 30 c of cooling after the homogeneous precursor feldspar has entered the coherent spinodal the extent of coarsening and the characteristic size of the exsolved phases depend on the cooling rate over this temperature interval for geologically relevant cooling rates on the order of 5 c ma to 50 c ma the compositions of the exsolved phases develop along the solvus down to about 480 c to 530 c and are frozen in only at lower temperatures the compositions that are finally preserved in the orthoclase rich and in the albite rich phases depend on cooling rate over this temperature interval both the characteristic lamella width of the mesoperthite as well as the compositions of the exsolved phases indicate slow cooling at 5 c ma during the coarsening stage and down to about 450 c during later stages of the cooling history the albite rich phase of the mesoperthite exsolved about the peristerite gap and the orthoclase rich host of the spindle perthite exsolved a second generation of coherent albite rich spindles the incoherent phase boundaries of the primary perthite microstructures served as passageways for fluids during late stage deuteric alteration which lead to secondary coarsening in the mesoperthite and to alteration of the albite rich precipitates in the spindle perthite finally both types of perthites were replaced by coarse grained patch perthite during deuteric alteration

the volcanic history of volcan alcedo galapagos archipelago a case study of rhyolitic oceanic volcanism volcan alcedo is one of the seven western galapagos shields and is the only active galapagos volcano known to have erupted rhyolite as well as basalt the volcano stands 4 km above the sea floor and has a subaerial volume of 200 km3 nearly all of which is basalt as volcan alcedo grew it built an elongate domal shield which was partly truncated during repeated caldera collapse and partial filling episodes an outward dipping sequence of basalt flows at least 250 m thick forms the steepest to 33 flanks of the volcano and is not tilted thus a constructional origin for the steep upper flanks is favored about 1 km3 of rhyolite erupted late in the volcano s history from at least three vents and in 2 5 episodes the most explosive of these produced a tephra blanket that covers the eastern half of the volcano homogeneous rhyolitic pumice is overlain by dacite rhyolite commingled pumice with no stratigraphic break the tephra is notable for its low density and coarse grain size the calculated height of the eruption plume is 23 30 km and the intensity is estimated to have been 1 2x108 kg s rhyolitic lavas vented from the floor of the caldera and from fissures along the rim overlie the tephra of the plinian phase the age of the rhyolitic eruptions is 120 ka on the basis of k ar ages between ten and 20 basaltic lava flows are younger than the rhyolites recent faulting resulted in a moat around part of the caldera floor alcedo most resently erupted sometime between 1946 and 1960 from its southern flank alcedo maintains an active transient hydrothermal system acoustic and seismic activity in 1991 is attributed to the disruption of the hydrothermal system by a regional scale earthquake