principles of early development of karst conduits under natural and man made conditions revealed by mathematical analysis of numerical models numerical models of the enlargement of primary fissures in limestone by calcite aggressive water show a complex behavior if the lengths of the fractures are large and hydraulic heads are low as is the case in nature dissolution rates at the exit of the channel determine its development by causing a slow increase of water flow which after a long gestation time by positive feedback accelerates dramatically within a short time span mathematical analysis of simplified approximations yields an analytical expression for the breakthrough time when this happens in excellent agreement with the results of a numerical model this expression quantifies the geometrical hydraulic and chemical parameters determining such karst processes if the lengths of the enlarging channels are small but hydraulic heads are high as is the case for artificial hydraulic structures such as dams it is the widening at the entrance of the flow path which determines the enlargement of the conduit within the lifetime of the dam this can cause serious water losses this can also be explained by mathematical analysis of simplified approximations which yield an analytical threshold condition from which the safety of a dam can be judged thus in both cases the dynamic processes of karstification are revealed to gain a deeper understanding of the early development of karst systems as a further important result one finds that minimum conditions below which karstification cannot develop do not exist

late mesozoic bimodal volcanic rocks in the jinniu basin middle lower yangtze river belt yrb east china age petrogenesis and tectonic implications abstract late mesozoic intrusive and volcanic rocks are widespread in the southeast hubei province middle lower yangtze river belt yrb east china detailed in situ zircon u pb and hf isotope elemental and sr nd pb isotopic data are presented in this paper for late mesozoic volcanic rocks from the jinniu basin yrb aiming to constrain their age petrogenesis and tectonic implications the jinniu volcanic rocks show a bimodal distribution in composition with dominant rhyolite and dacite and subordinate basalt and basaltic andesite new shrimp and la icpms zircon u pb ages indicate that the volcanic rocks of three formations in the jinniu basin were erupted at quite a short age range of about 5 ma during the early cretaceous 130 125 ma the mafic rocks are moderately enriched in large ion lithophile elements lile e g ba th u and pb and light rare earth elements lree and are characterized by negative nb ta and ti anomalies and relatively high tio2 0 72 2 06 and nb 9 20 26 5 ppm contents these analyses indicate that the geochemical characteristics of the mafic rocks in the jinniu basin are similar to worldwide phanerozoic nb enriched basalt and andesites neba new in situ zircon u pb ages and field geological relationships demonstrate that neba in the southeast hubei province are not spatially or temporally associated with high silica adakitic rocks but were most likely derived from an enriched lithospheric mantle with assimilation of minor crustal materials and then fractional crystallization during the evolution of the magma overall the felsic rocks in the jinniu basin have geochemical characteristics and sr nd pb signatures and in situ zircon hf isotopic compositions similar to those of the mafic rocks compared with the mafic rocks the felsic rocks are characterized by enriched and variable concentrations of lile and ree e g ba 33 3 1372 ppm y 11 4 33 6 ppm ybc 5 07 18 7 and negative eu anomalies eu 0 22 0 98 as well as a wide range of radiogenic nd pb isotopic values with end t 10 2 to 2 4 206pb 204pb i 17 659 18 705 207pb 204pb i 15 478 15 663 and 208pb 204pb i 37 654 38 935 and in situ zircon hf compositions of ehf t 12 7 to 1 8 these features indicate that the genesis of felsic magma in the jinniu basin is consistent with extensive fractional crystallization and large amounts of crustal contamination from an evolved mafic magma sio2 55 the bimodal volcanic rocks in this study provide convincing evidence that early cretaceous volcanic rocks in the yrb developed in a back arc extensional tectonic regime

petrogenesis of coarse grained intrusives from tahiti nui and raiatea society islands french polynesia this study is based on a set of coarse grained igneous rocks collected from two zoned plutons located in the central part of tahiti nui and raiatea the ahititera pluton central depression of tahiti nui comprises a great diversity of rocks ranging from ultrabasic to felsic in composition it shows a concentric zonation with nepheline free rocks in its periphery and nepheline bearing rocks in its central part the faaroa pluton central depression of raiatea is entirely mafic and includes only gabbros and theralites the two plutons have variable nd sr isotopic signatures especially the ahititera rocks which are subdivided into three groups based on their mineralogy geochemistry and isotope composition the isotopic variability probably reflects local heterogeneities in the society mantle plume petrographic and isotopic data have been used to define two magmatic suites in ahititera identifiable from their degree of si undersaturation the evolution of the mildly si undersaturated suite is controlled by simple fractional crystallization whereas the strongly si undersaturated suite requires additional h2o influx the third isotopic group includes only theralites the rare earth element ree compositions of the mafic rocks from both plutons do not correlate with their isotopic signature the ree patterns of the most si undersaturated rocks are systematically characterized by steeper slopes such features are also observed in lavas from seamounts located within the present day hotspot area it appears that ree concentrations in society lavas and intrusives are probably mainly governed by variable degrees of partial melting of a garnet free mantle source and are independent of their isotopic signature

modelling karst denudation on a synthetic landscape this contribution presents the results of a numerical study of karst denudation on limestone plateaux the landscape evolution model used incorporates not only long range fluvial and short range hill slope processes but also large scale chemical dissolution of limestone surfaces the relative efficiencies of fluvial and chemical processes are of equal importance to the landscape evolution of a plateau dropping to sea level along an escarpment while fluvial processes have an impact confined mostly to river channels the karst denudation process is more uniform removing material also from the plateau surface the combined effect of both processes results in a landscape evolution almost twice as effective as the purely erosional evolution of an insoluble landscape

plutonic rocks of intermediate composition on gran canaria the missing link of the bimodal volcanic rock suite analysis of abundant plutonic fragments in the voluminous miocene rhyolitic basaltic composite ignimbrite p1 the initial felsic cooling unit overlying the basaltic shield on gran canaria helps to solve the problem of bimodal volcanism syenogabbros dominate the plutonic suite and provide evidence that magmas of intermediate composition formed abundantly at depth but did not erupt the entire suite of plutonic fragments comprises 1 pyroxenitic to gabbroic cumulates that directly influenced the liquid line of descent of p1 magmas 2 gabbro syenogabbro syenodiorite and quartz syenite fragments showing moderate degrees of hydrothermal alteration chloritization partial melting and 3 diverse xenoliths differing significantly in bulk rock composition and texture from the erupted p1 magmas the compositions of plutonic series 2 overlap with both bulk rock and mineral compositions of the erupted magmas whereas series 1 rocks show lower alkali and silica contents compositional variations among the plutonic rocks are compatible with fractional crystallization as the major petrogenetic process locally modified by magma mixing and selective element contamination variations in trace element concentrations of the plutonic rocks however are inferred to be the result of evolved interstitial melts penetrating into or draining out of crystalline mushes during slow intratelluric solidification the presence of f rich amphibole suggests that crystalline mushes were invaded by f bearing fluids overall compositional similarity and the fact that selective contamination effects are similar support the interpretation that the series 2 plutonic fragments represent solidified portions of the magmatic system that ultimately produced the erupted p1 magmas this system extended from the zone of underplating at the mantle crust boundary through the lower and into the upper crust of gran canaria and was roughly vertically zoned in composition as shown by geobarometric calculations

origin and morphology of limestone caves limestone caves form along ground water paths of greatest discharge and solutional aggressiveness flow routes that acquire increasing discharge accelerate in growth while others languish with negligible growth as discharge increases a maximum rate of wall retreat is approached typically about 0 01 0 1 cm yr determined by chemical kinetics but nearly unaffected by further increase in discharge the time required to reach the maximum rate is nearly independent of kinetics and varies directly with flow distance and temperature and inversely with initial fracture width discharge gradient and pco2 most caves require 104 105 yr to reach traversable size their patterns depend on the mode of ground water recharge sinkhole recharge forms branching caves with tributaries that join downstream as higher order passages maze caves form where 1 steep gradients and great undersaturation allow many alternate paths to enlarge at similar rates or 2 discharge or renewal of undersaturation is uniform along many alternate routes flood water can form angular networks in fractured rock anastomotic mazes along low angle partings or sponge work where intergranular pores are dominant diffuse recharge also forms networks and spongework often aided by mixing of chemically different waters ramiform caves with sequential outward branches are formed mainly by rising thermal or h2s rich water dissolution rates in cooling water increase with discharge co2 content temperature and thermal gradient but only at thermal gradients of more than 0 01 c m can normal ground water co2 form caves without the aid of hypogenic acids or mixing artesian flow has no inherent tendency to form maze caves geologic structure and stratigraphy influence cave orientation and extent but alone they do not determine branch work versus maze character

experimental determination of the compositional limits of immiscibility in the system cacl2h2oco2 at high temperatures and pressures using synthetic fluid inclusions abstract using the synthetic fluid inclusion method the temperature pressure and compositional limits of fluid immiscibility in the binary system cacl 2 h 2 o have been determined at 600 and 700 c at pressures between 1 and 2 kbar the limits of the immiscible regions were determined by observation of two different inclusion types in the samples and by measurements of the depression of ice melting temperatures the region of immiscibility in this system extends to much higher pressures than that of the nacl h 2 o system with similar compositional ranges a similar study was done on the cacl 2 h 2 o co 2 system at 500 600 and 700 c at pressures of 1 1 5 2 and 3 kbar in the ternary system the presence of two different inclusion types and the measurement of clathrate melting temperatures were used to delineate the compositional regions of immiscibility the presence of co 2 extends the regions of immiscibility shown in the binary system study to much higher pressures tie lines in the two phase regions were determined by measuring the homogenization temperatures of co 2 vapor and liquid phases and the volume ratios of inclusion bubble to total inclusion in vapor rich type inclusions the locations of these tie lines indicate that the cacl 2 is heavily partitioned towards the more liquid rich phase the experimental results show that with even relatively small amounts of a divalent salt such as cacl 2 immiscibility can exist to very high temperatures and pressures from these results immiscibility should be a very common phenomenon in geologic processes ranging from sedimentary to quite high grade metamorphic environments and that it should have a very important effect on reactions between hydrothermal fluids and rocks and the resulting mineral assemblages

discovery of a trachyte ignimbrite sequence at hualālai hawaii ignimbrites are common in many intraplate ocean islands but have been missing from the known geological record in hawaii during a recent field campaign the remnants of a trachytic ignimbrite sequence have been discovered at hual lai volcano fortuitously preserved from subsequent basaltic lava flow cover we provide a preliminary description of these deposits as well as bulk and glass chemical analyses to determine their potential relationship with other nearby trachytes from pu u wa awa a pww and pu u anahulu pa the results suggest that these ignimbrites are from neither pww nor pa but instead may relate to trachytes that are found as maar wallrock blocks some 20 km distant therefore despite being rare overall in hawaii the ignimbrites and more generally trachytes were probably widespread around hual lai compared to other intraplate ocean islands the combination of a fast moving plate high magma supply and eruption rates underneath hawaiian volcanoes may explain the scarcity of ignimbrites preserved at the surface their presence at hual lai could reflect unusual conditions of edifice stress during the transition from shield to post shield volcanism

karst aquifer evolution in fractured porous rocks the evolution of flow in a fractured porous karst aquifer is studied by means of the finite element method on a two dimensional mesh of irregularly spaced nodal points flow within the karst aquifer is driven by surface recharge from the entire region simulating a precipitation pattern and is directed toward an entrenched river as a base level during the early phase of karstification both the permeable rock matrix modeled as triangular elements and fractures within the rock matrix modeled as linear elements carry the flow as the fractures are enlarged with time by chemical dissolution within the system calcite carbon dioxide water flow becomes more confined to the fractures this selective enlargement of fractures increases the fracture conductivity by several orders of magnitude during the early phase of karstification thus flow characteristics change from more homogeneous pore controlled flow to strongly heterogeneous fracture controlled flow we study several scenarios for pure limestone aquifers mixed sandstone limestone aquifers and various surface recharge conditions as well as the effect of faulting on the aquifer evolution our results are sensitive to initial fracture width faulting of the region and recharge rate

the gold flat tuff nevada insights into the evolution of peralkaline silicic magmas the gold flat tuff is the youngest 9 15 ma ash flow sheet erupted from the black mountain volcanic centre southwest nevada usa this paper explores some aspects of the very complex nature of the tuff s magmatic plumbing system the main body of the deposit is a mixed magma product comprising pantelleritic and comenditic melts derived from independently evolving reservoirs and antecrysts and enclaves derived from a range of basic to intermediate sources metre scale cognate xenoliths point to the presence of alkali feldspar accumulation zones the pantellerite contains phenocrysts of fluorite and chevkinite ce the inferred intermediate magma component contains perrierite ce phenocrysts the pantellerite has unusually high contents of f 2 2 wt f cl 2 9 wt and zro 2 1 04 wt the high halogen contents may have influenced the evolution of the strongly peralkaline magma the crystallization conditions are poorly constrained but those for the pantelleritic magma may have been close to water saturation 4 wt melt water at temperatures 740 c and fo 2 around fmq

early development of karst systems 2 turbulent flow a simulation model developed to explore patterns of fracture enlargement within incipient limestone karst aquifers has been extended to turbulent flow in contrast to the highly selective passage enlargement that occurs early in cave network development under laminar flow the transition to turbulent flow results in more general passage enlargement leading to maze networks when initial fractures are large and hydraulic gradients are high these results support previously published hypotheses for the development of maze patterns including formation within structural settings that have created initially large fractures or within flow systems periodically inundated by flooding maze development is also favored under turbulent flow when passages are entirely water filled and where the groundwater flow system is long lived by contrast branched patterns are most common when passages become free surface subterranean streams because depression of the piezometric surface along main passages downcutting along main passages and possible infilling with sediment of side passages limit the sharing of discharge among interconnected fractures or bedding planes that promote maze development

the role of dissolution kinetics in the development of karst aquifers in limestone a model simulation of karst evolution to model the development of karst aquifers from primary fissures in limestone rock a numerical model of solutional widening of such fractures by calcite agressive water is suggested the geological setting determines relevant geometrical parameters i e length of the fracture its initial width and the hydraulic gradient driving water from the input to the output to simulate the solutional widening as it proceeds in time the solution rates must be known as a function of concentration c of dissolved calcite they are given as a first order kinetic rate law for c leq k cdot c eq as f 1 alpha 0 c c eq for c k cdot c eq close to equilibrium a fourth order rate law becomes dominant f 4 beta cdot c c eq 4 the parameters alpha 0 beta and k depend on the chemistry of the carbonate system and rock chemistry the saturation concentration c eq with respect to calcite depends on the initial co 2 concentration of the inflowing solution

early development of karst aquifers on percolation networks of fractures in limestone we have modeled flow and dissolution processes within percolation networks representing stochastic primary fracture systems in limestone at the beginning of karstification flow is evenly distributed on all fractures available as the system develops by dissolutional widening of the fractures preferred flow pathways evolve which attract more and more flow until at breakthrough the total flow rate increases dramatically these breakthrough times have been investigated with regard to their dependence on the initial fracture aperture width the dimension of the evolving karst aquifer hydraulic gradients connectivity of the percolation network and the chemical parameters such as higher order rate constants and exponents and equilibrium concentration they exhibit a behavior similar to that of breakthrough times for one dimensional conduits dreybrodt 1996 our results show that the structure of the evolving karst aquifer is determined by the initial geological setting however the final conduits are selected from competing pathways with potential breakthrough times close to each other whereby the details of the distribution of undersaturation with respect to calcite play an important role

groundwater systems in bare and covered karst aquifers evidence from tracer tests hydrochemistry and groundwater ages the east guilin region contains a karst dominated hydrological system along the lijiang river the two main topographic characteristics of the basin are peak cluster depression and peak forest despite the fact that these areas represent adjacent units they have different groundwater movement patterns this study describes the groundwater and solute movement in the different hydrogeological sub regions via several techniques our results indicated that the topographic boundary between the peak cluster depression and the peak forest is clear however a transition zone exists between these topographic zones and it can be determined in terms of groundwater movement we employed several methods that are widely considered to be effective tracer tests were conducted in the transition zone hydrochemistry techniques were used in the peak forest and groundwater age dating based on cfcs was employed throughout the study area the main conduits could be found in the transition zone but the groundwater flow was much slower in the transition zone than in the mountain peak cluster area minor conduits also accounted for a high proportion of the total flow in the transition zone the solute migration within the plain which was determined by analyzing the nitrates was controlled by mixing and distance effects the maximum nitrate concentration was limited at the local scale the nitrate concentration gradient at the regional scale was not related to the groundwater movement indicating that the groundwater recharged in a dispersed manner and discharged at discrete locations along the river the age dating revealed that the groundwater was older in the plain than in the bare mountain zone this was due to the strong mixing of young and old water which was the result of the characteristics of the karst media in the aquifer our investigation of the groundwater system in a bare covered karst aquifer provides data for decision making in effective groundwater management

early development of karst systems 1 preferential flow path enlargement under laminar flow modeling of flow and solutional processes within networks of interconnected conduits in limestone aquifers indicates that enlargement occurs very selectively during the early stages of karst aquifer development under laminar flow if initial flow paths are uniform in size almost all enlargement occurs along a single set of connected conduits that lie along a direct path between recharge and discharge locations and are aligned along the hydraulic gradient with a sufficiently large variation in initial aperture widths enlargement occurs along the flow path offering the least resistance to flow but since flow rates in laminar flow are proportional to the fourth power of diameter but only linearly proportional to hydraulic gradient the preferentially enlarged set of fractures may follow an indirect path results disfavor earlier suggestions that nonselective cave patterns result from artesian flows at least under laminar flow conditions and that all passages should be competitive until the onset of turbulent flow

karst aquifer evolution in fractured rocks we study the large scale evolution and flow in a fractured karst aquifer by means of a newly developed numerical method a karst aquifer is discretized into a set of irregularly spaced nodal points which are connected to their set of natural neighbors to simulate a network of interconnected conduits in two dimensions the conduits are allowed to enlarge by solutional widening the geometric flexibility of this method along with a simplified model for the dissolution kinetics within the system water carbon dioxide calcite enables us to study both laminar and turbulent flow in a karst aquifer during its early phase of evolution a sensitivity analysis is conducted for parameters such as conduit diameter hydraulic pressure differences and recharge conditions along the surface of the aquifer and shows that passage evolution depends strongly on the recharge condition and the amount of water available under fixed hydraulic head boundary conditions an early single passage system develops under laminar conditions and is transformed into a maze like passage system after the onset of turbulence fixed recharge boundary conditions are more likely to result in a branchwork like passage system although the addition of distributed recharge may lead to a maze like system of secondary passages

karst massif susceptibility from rock matrix fracture and conduit porosities a case study of the sierra de las nieves malaga spain the potential contamination of a karst massif from surface sources can be evaluated and represented in a susceptibility map in the case of a carbonate aquifer the susceptibility assessment must take into account their very complex and heterogeneous nature the complexity originates in the presence of three types of porosity matrix rock fracture and conduit porosity this paper presents a method for karst susceptibility mapping by estimating the three porosity types and evaluates their integration in a single susceptibility index matrix rock porosity is measured in the laboratory from samples collected in the field and is well correlated with lithology fracture porosity is estimated from fracture mapping and field measurements of secondary fracture porosity geostatistical methods are used to obtain continuous fields of rock matrix porosity and fracture porosity conduit porosity is calculated from a power model fitted to speleological cave mapping data however because of the scarcity and sampling bias of conduit data probabilistic models are conjectured a fourth porosity factor evaluated is the detritic filling of karst depressions the integration of the different porosities in a single susceptibility factor gives a quantitative map that is reclassified to provide a qualitative easy to interpret susceptibility index map of the karst system porosity estimation may also be of interest in recharge estimation and mathematical modelling of flow and transport in karst systems the case study used to illustrate this approach is the sierra de las nieves karstified rock mass a high relief mediterranean karst in the province of malaga in southern spain

laboratory and numerical simulations of spatio temporal variability of water exchange between the fissures and conduits in a karstic aquifer abstract fissures and conduits are main void of karstic aquifer karst aquifers are highly vulnerable to contamination as water exchange between fissure system and conduit system greatly influences the karstification of limestone terrains heterogeneity of karstic aquifer and spring flow regime a two dimensional tank model was constructed and a corresponding numerical model modflow 2005 conduit flow process mode 1 cfpm1 was developed to explore the effects of rainfall intensity and intrinsic structure of karst system hydraulic conductivity of fissures and conduit diameter on conduit matrix water exchange nine different scenarios were designed to quantitatively simulate the spatial and temporal variability of water exchange between the fissures and conduits the study reproduced the area of fissure water recharging to conduit recharge zone the area of conduit water discharging to fissure water discharge zone and the area of fissure water moving laterally transport zone in both physical model and numerical model the results from rainfall scenarios show a relatively stable ratio of exchange zone recharge zone and discharge zone with increasing rainfall intensity indicating that conduit matrix water exchange is not sensitive to rainfall intensity and conduit water discharging to fissure water is just a short term phenomenon it is also found that an increase in hydraulic conductivity of fissure system enhances extensiveness of water transport but restrains conduit matrix water exchange however the enlargement of conduit can prompt the extent of conduit matrix water exchange a positive feedback between the extent of conduit matrix water exchanges and karstification can be confirmed these findings might be essential for better understanding groundwater flow dynamics in karst aquifers

simulation of the development of gypsum maze caves the development of gypsum maze caves under artesian conditions has been simulated the numerical model simulations show that the evolution of maze caves in this type of setting requires structural preferences such as laterally extended fissure networks in a horizon of the gypsum layer without any structural preferences vertical shafts rather than maze caves are predicted to develop the most important stage for the development of horizontal caves under artesian conditions is found to be the initial karstification period during this period the structure of the mature conduit system is established the solutional enlargement of conduits is spatially extended total dissolution rates are higher than the later ones

on the use of multiple point statistics to improve groundwater flow modeling in karst aquifers a case study from the hydrogeological experimental site of poitiers france abstract limestone aquifers often exhibit complex groundwater flow behaviors resulting from depositional heterogeneities and post lithification fracturing and karstification in this study multiple point statistics mps was applied to reproduce karst features and to improve groundwater flow modeling for this purpose mps realizations were used in a numerical flow model to simulate the responses to pumping test experiments observed at the hydrogeological experimental site of poitiers france the main flow behaviors evident in the field data were simulated particularly i the early time inflection of the drawdown signal at certain observation wells and ii the convex behavior of the drawdown curves at intermediate times in addition it was shown that the spatial structure of the karst features at various scales is critical with regard to the propagation of the depletion wave induced by pumping indeed i the spatial shape of the cone of depression is significantly affected by the karst proportion in the vicinity of the pumping well and ii early time inflection of the drawdown signal occurs only at observation wells crossing locally well developed karst features

petrogenesis of siluro devonian rhyolites of the tobique group in the northwestern appalachians northern new brunswick canada tectonic implications for the accretion history of peri gondwanan terranes along the laurentian margin

trace element indicators of crystal accumulation in silicic igneous rocks abstract if fractional crystallization coupled with some crustal assimilation afc is the dominant process for generating evolved magmas in the earth s crust large quantities of igneous cumulates must be present in the rock record however despite some reported occurrences intermediate to silicic rocks characterized by an unambiguous crystal accumulation signature have remained elusive the signature is expected to be subtle in viscous silicic magmas and sensitive criteria need to be developed to test whether these cumulates are truly rare or simply difficult to discern we propose models that use specific trace element concentrations and ratios zr ba eu eu and zr hf within a well constrained framework for the physical extraction of liquid from crystals mostly occurring between 50 and 70 vol crystals to identify silicic cumulates the results of these models indicate that crystal accumulation or melt extraction has occurred in multiple natural examples around the world both in plutonic and volcanic realms however these examples were selected because they represent conditions that are most favorable high amount of melt extraction well constrained initial liquid compositions for unambiguously identifying cumulate residue in many cases we show that the trace element signature for crystal accumulation will remain largely concealed in addition the different liquid lines of descent related to the various tectonic settings on earth dry reduced in hot spots divergent margins and wetter more oxidizing in arc environments will lead to different trace element evolution and each criterion must be applied on a case by case basis despite these limitations the undeniable but subtle presence of silicic cumulates in the mid to upper crust lends support to the idea that igneous differentiation is dominantly driven by melt extraction from partially crystalline reservoirs

ignimbrite correlation using whole rock geochemistry an example from the sulcis sw sardinia italy ignimbrites are useful chronological markers in the geological record at local and regional scales they also provide information on the dynamics of the eruption that produced them making their study of great importance in terms of volcanic hazard assessment however their study is usually hampered by their lateral variation and discontinuity when stratigraphic and lithologic criteria are not sufficient for correlation purposes the use of multiple complementary correlation tools may be necessary to correctly determine their areal extension volume and facies variations whole rock geochemistry is considered one of the less reliable correlation techniques due to the pyroclastic nature of these deposits and their emplacement dynamics these may introduce vertical and horizontal geochemical heterogeneity in the final deposit in addition the occurrence of zoned ignimbrites due to magma supply of changing composition is common in this work we show that if appropriately used whole rock geochemistry can be a valid and highly useful tool for ignimbrite correlation we provide an example from the study of an ignimbrite sequence containing 18 units sensu lato in the sulcis region sw sardinia italy a protocol has been developed for unit recognition based on successive simple binary diagrams where the whole rock composition of a problem sample can be plotted immobile trace elements have been preferentially used to minimize effects of element mobilization associated with alteration and weathering the diagrams provided here are designed for the sulcis but the methodology followed to develop them may be applied to other study areas

minimum hydrochemical conditions allowing limestone cave development early theoretical investigations into initiation of limestone cave development suggested that water entering small fractures approaches saturation within a short distance and thus appeared to be unable to produce much further enlargement of these fractures the answer to this problem seems to be white s 1977 kinetic trigger idea emphasizing the precipitous drop in dissolution rates as equilibrium is approached however minimum conditions of flow rate passage geometry and initial co2 concentration may exist below which the waters will so nearly achieve saturation that caves will be unable to develop modeling results indicate that under geologically reasonable pco2 and head gradient values the minimum initial aperture widths in which caves will develop is of the order of 100 m initial apertures wider than about 500 m will form passages very easily even under low head gradients there will thus be strong differential selection among initial flow paths with regard to cavern passage development

magma mixing and crustal recycling recorded in ternary feldspar from compositionally zoned peralkaline ignimbrite a gran canaria canary islands miocene ignimbrite a on gran canaria contains three compositional endmember fiamme types two rhyolites and one trachyte each of which crystallized distinct feldspar various textural and compositional criteria are interpreted as reflecting a complex scenario within the magma chamber in which the crystals formed about 25 30 of the feldspar phenocrysts contain evidence for magma mixing in the form of 1 partial to severe dissolution resorption rims 2 distinct zones of drastically different compositions and 3 overgrowth textures on formerly resorbed crystals four major types of zoning in the oligoclase to anorthoclase feldspars of ignimbrite a include a normal and a reversely zoned type and two complexly zoned types the feldspars with normal and reverse zonation show only minor compositional amplitudes between individual zones ab or 4 whereas the complexly zoned types show compositional differences between zones of up to 18 mol ab and 20 mol or and are commonly associated with an internal dissolution surface complex zoning with large compositional amplitudes and dissolution textures is taken as evidence of crystal movements within the magma and across compositional boundaries between magma batches a multiple step cycle model involving growth and transport of a crystal into another magma batch and its return to the original host magma is suggested by the data moreover feldspars from one rhyolite compositional group are found to be substantially elevated in 18o suggesting an input of a high 18o component to this rhyolite the other endmember rhyolite appears to be related to the endmember trachyte by mainly crystal fractionation of anorthoclase feldspar this observation is consistent with trace element and rare earth element concentrations for the magma endmembers and their feldspars where contamination led to a depletion in incompatible trace elements and light rare earth elements in the contaminated rhyolite and its feldspar phenocrysts we suggest that the combination of textural and compositional variation in ternary feldspar of peralkaline rhyolitic systems is well suited to reconstruct dynamic processes such as magma mixing and contamination in evolving rhyolitic magma chambers

open system processes in the genesis of silica oversaturated alkaline rocks of the rallier du baty peninsula kerguelen archipelago indian ocean abstract the rallier du baty peninsula forms the southwestern part of the kerguelen archipelago indian ocean whose magmatic activity is related to the long lived 115 ma kerguelen plume the peninsula is mostly made of alkaline rocks constituting two well defined ring complexes this paper focuses on the northern ring complex which is not yet known recent field studies have revealed seven discrete syenitic ring dykes ranging in age from 6 2 to 4 9 ma and two later volcanic systems 40ar 39ar dating of a trachytic ignimbrite linked to the dome carva volcano complex yields an age of 26 3 ka this represents the last major eruptive event on the kerguelen archipelago the volcanism is bimodal with trachybasalts and trachyandesites constituting the mafic lavas and trachytes and rhyolites constituting the felsic lavas the volume of erupted felsic magma is by far the larger and is represented by abundant pyroclastic deposits and lava flows boulders of plutonic rocks are found to the northwest of dome carva and represent intermediate rocks i e monzogabbros and monzonites that are not present at the surface basic rocks are mostly trachybasalts and trachyandesites while true basalts are scarce their mineralogy consists chiefly of plagioclase olivine diopside and oxides sieve textured plagioclase is common as well as corroded olivine and diopside phenocrysts peralkaline commenditic trachytes are the most abundant type of acid volcanic rocks they consist of abundant sanidine augite and magnetite phenocrysts and interstitial quartz aegerinic pyroxenes and na amphiboles ring dykes of quartz poor alkali feldspar syenites display the same mineralogy except hornblende is common and replaces diopside hornblende is particularly abundant in intermediate monzogabbros major and trace element variations of volcanic rocks emphasise the predominant role of fractional crystallisation with a general decrease of mgo cao p2o5 tio2 feo ba sr and ni from basic to felsic rocks however the scattering of the data from the basic rocks indicates that other processes have operated the overall evolution from trachyte to rhyolite is in agreement with the fractionation of sanidine as the major control an increase of incompatible elements from trachyte to rhyolite is observed the felsic lavas display an increase of 87sr 86sr i without any significant variations in the nd isotopic composition the genesis of the basic rocks is complex and reflects concomitant processes of fractional crystallisation mixing between different basic magmas and probable assimilation of ba rich oceanic crust major and trace element modelling confirms the possibility of producing the trachytes through continuous differentiation from a basaltic alkaline parent discrepancies observed for some trace elements can be explained by the crystallisation of amphibole at an intermediate stage of magma evolution the overall evolution from trachyte to rhyolite is thought to be controlled by crystal fractionation high 87sr 86sr i of the trachytes is interpreted to reflect interaction with an ocean derived component probably during assimilation of hydrothermally altered oceanic crust boulders of amphibole bearing monzonites and monzogabbros found to the northwest of dome carva are thought to represent intermediate magma composition that formed at depths but did not erupt

escarpment evolution on high elevation rifted margins insights derived from a surface processes model that combines diffusion advection and reaction experiments with a surface processes model of large scale 1 1000 km long term 1 100 m y erosional denudation are used to establish the controls on the evolution of a model escarpment that is related to the rifting of a continent the model describes changes in topographic form as a result of simultaneous short and long range mass transport representing hillslope diffusive processes and fluvial transport advection respectively fluvial entrainment is modeled as a first order kinetic reaction which reflects the credibility of the substrate and therefore the fluvial system is not necessarily carrying at capacity one dimensional and planform models demonstrate that the principal controls on the evolution of an initially steep model escarpment are 1 antecedent topography drainage 2 the timescale or equivalently a length scale in the fluvial entrainment reaction 3 the flexural response of the lithosphere to denudation and 4 the relative efficiencies of the short and long range transport processes when rainfall and substrate lithology are uniform a significant amount of discharge draining over the escarpment top causes it to degrade only when the top of the model escarpment coincides with a drainage divide can escarpment retreat occur for these conditions an additional requirement for retreat of a model escarpment without decline is a long reaction time scale for fluvial entrainment this corresponds to a substrate that is hard to detach by fluvial erosion and therefore to fluvial erosion that is not transport limited continuous backtilting of an escarpment due to flexural isostatic uplift in response to denudational unloading helps maintain the scarp top as a divide it is essential if the escarpment gradient is to be preserved during retreat in a uniform lithology low flexural rigidities promote steep and slowly retreating escarpments for given rainfall and substrate conditions the morphology of a retreating model escarpment is determined by the ratio of the short range diffusive and long range advective transport efficiencies a low ratio which is interpreted to correspond to a relatively arid climate and weathering limited conditions promotes steep sharp topped escarpments with straight main slopes and escarpment retreat occurs over a wide range of height scales a high ratio interpreted to correspond to a more humid temperate climate produces a convex upper slope and concave lower slope morphology and only major escarpments are predicted to preserve a high scarp gradient lithological contrasts in the model produce more complex morphologies and predict the formation of scarps crowned by an erosionally resistant caprock however resistant caprocks are not an essential requirement for model scarps to retreat we conclude that the inferred controls and model behavior are both consistent with the present day morphology of rifted continental margins and with modern conceptual models of landscape evolution