an 40ar 39ar thermochronology of the ofoten troms region implications for terrane amalgamation and extensional collapse of the northern scandinavian caledonides fifteen 40ar 39ar cooling ages are reported for metamorphic hornblende and muscovite from far traveled terranes constituting the ofoten nappe stack of northern norway eight cooling ages on hornblende range from 425 to 394 ma and seven muscovite ages from the same or nearby outcrops as the hornblendes range from 400 to 373 ma these data are compared with 40ar 39ar ages from over a large part of the northern caledonides to evaluate regional mineral cooling patterns results indicate that 1 scandian silurian devonian metamorphism was predominant 2 most of the nappes investigated contain some vestige of pre scandian tectonism and or metamorphism 3 hornblende and muscovite cooling ages are progressively younger to the west and south which suggests a hinged to the east mineral cooling pattern and 4 a late out of sequence thrust is the only disruption of this cooling pattern synmetamorphic amalgamation of the nappes resulted from scandian a type subduction the hinged to the east mineral cooling pattern implies isostatic adjustment and exhumation of the footwall of a west dipping crustal scale extensional fault located somewhere west of the present norwegian coast during late synorogenic gravitational collapse the late out of sequence fault formed contemporaneously with uplift in the hinterland implying a kinematic and temporal connection with east directed contractional faulting in the foreland

geobarometry geothermometry and late archean geotherms from the granulite facies terrain of south india geobarometry and geothermometry of garnet bearing chamockitic and metapelitic assemblages from the high grade terrain from south india indicate three fields of equilibration high pressure granulites p 8 3 pm 1 0 kb t 760 pm 40 circ c are restricted to the charnockites and mafic granulites of the north slopes of the nilgiris these place a minimum constraint of 30 km on the maximum thickness of late archean crust medium pressure granulites p 6 4 pm 1 0 kb t 735 pm 40 circ c characterize the charnockites of the central nilgiris and the madras granulites to the east low pressure granulites p 5 0 pm 1 0 kb t 700 pm 20 circ c characterize the charnockites and metapelites south of the bhavani shear zone although migmatites from the kodaikanal massif may reflect somewhat higher temperatures all three granulite fields lie on a metamorphic geotherm piezothermic array which is strongly convex to the temperature axis and characteristic of convective heat transfer

glacier velocity variability due to rain induced sliding and cavity formation abstract the largest accelerations of glaciers and ice sheets are caused by changes in basal slip here we examine glacier speed and rain induced accelerations using a near continuous 26 month long gnss time series from a large maritime glacier tasman glacier new zealand during periods of high rain rate we observe short term increases in 24 hour speeds to up to 15 times background speed speeds calculated over 3 hour intervals increase to up to 36 times background speed acceleration events correspond with times when bed separation also increases rapidly indicating that the acceleration is associated with the growth of water filled cavities at the bed glacier speeds then decrease prior to the reduction in bed separation indicating cavity growth not cavity extent controls the acceleration the short term accelerations are superimposed on longer term periods of enhanced velocity that persist for days to weeks and decay at similar rates to bed separation estimates and proglacial lake levels a power law relationship between observed rain rate and speed exists at the glacier front and exhibits no apparent upper bound overall we estimate that rain induced accelerations account for 11 14 of tasman glacier s displacement during the observation period the rain rate velocity relationship and the rainfall record since 2001 indicate that rain induced speed up events result in interannual variability in glacier displacement of 2 13

fracture propagation to the base of the greenland ice sheet during supraglacial lake drainage surface meltwater that reaches the base of an ice sheet creates a mechanism for the rapid response of ice flow to climate change the process whereby such a pathway is created through thick cold ice has not however been previously observed we describe the rapid 2 hours drainage of a large supraglacial lake down 980 meters through to the bed of the greenland ice sheet initiated by water driven fracture propagation evolving into moulin flow drainage coincided with increased seismicity transient acceleration ice sheet uplift and horizontal displacement subsidence and deceleration occurred over the subsequent 24 hours the short lived dynamic response suggests that an efficient drainage system dispersed the meltwater subglacially the integrated effect of multiple lake drainages could explain the observed net regional summer ice speedup

fluid compositions and p t conditions of vein type uranium mineralization in the beaverlodge uranium district northern saskatchewan canada the beaverlodge district in northern saskatchewan is known for vein type uranium mineralization most of the uranium deposits are spatially related to major structures and hosted by ca 3 2 1 9 ga granitic rocks and albitite derived from them and by ca 2 33 ga murmac bay group amphibolite all of which are unconformably overlain locally by deformed but unmetamorphosed redbeds of the ca 1 82 ga martin group and by the flat lying ca 1 75 1 5 ga athabasca group the uranium mineralization is mainly hosted in fault rocks breccias and carbonate quartz albite veins referred to as breccia style and vein style mineralization respectively with the latter being the focus of this study most of the mineralized veins occur in the basement rocks although some crosscut the martin group this study examines the field petrographic fluid inclusion and c o isotope characteristics of mineralized and non mineralized veins from 19 deposits occurrences as well as from the martin group with an aim to better understand the mineralizing environment and processes the coexistence of liquid dominated l v vapour dominated v l and vapour only v fluid inclusions within individual fluid inclusion assemblages fias in the veins suggests fluid immiscibility and heterogeneous trapping the l v inclusions with the lowest homogenization temperatures th within individual fias are interpreted to represent homogeneous trapping of the liquid phase which yield th values from 78 to 330 c mainly 100 to 250 c and salinities from 0 2 to 30 8 wt nacl equivalent mass spectrometric analysis of bulk fluid inclusions shows that the volatiles are dominated by h2o average 97 2 mol with minor amounts of co2 ch4 h2 o2 n2 ar and he fluid pressures were estimated to be 200 bars based on the inference of fluid immiscibility fluid temperatures of 100 to 250 c and low concentrations of non aqueous volatiles 3 mol the 18ovpdb and 13cvpdb of carbonate minerals associated with mineralization range from 20 5 to 8 9 and 10 1 to 0 9 respectively the 18ovsmow values of the parent fluids calculated using the th values range from 9 6 to 17 0 with the majority from 0 to 5 0 o isotopes of paired equilibrium quartz and calcite analyzed by secondary ion mass spectrometry sims yield temperatures from 161 to 248 c which are consistent with the fluid inclusion data the new fluid inclusion and stable isotope data are inconsistent with a metamorphic or magmatic hydrothermal model as proposed in some previous studies for breccia style and vein style mineralization but rather support a model in which the vein type uranium mineralization took place at relatively low temperature 100 to 250 c and shallow 2 km conditions with fluid pressure fluctuating between hydrostatic and sub hydrostatic regimes possibly related to episodic faulting the mineralizing fluids were mainly sourced from the martin lake basin and uraninite was precipitated as a result of mixing between this basin derived fluid and fluids carrying reducing agents fe2 ch4 derived from the basement although fluid rock reactions and fluid immiscibility may have also played a role

permian triassic red stained albitized profiles in the granitic basement of ne spain evidence for deep alteration related to the triassic palaeosurface extensive areas of the variscan granitic basement in ne spain display profiles of red stained albitized facies characterized by albitization of ca plagioclase chloritization of biotite and microclinization of orthoclase along with the alteration of igneous quartz to secondary cl dark quartz these profiles have a geopetal structure beneath the triassic unconformity with a very intense and pervasive alteration in the upper part that progressively decreases with depth to 150 200 m where the alteration is restricted to the walls of fractures the red albitized facies contains secondary maghemite and hematite that indicate oxidizing conditions dating of microclinized orthoclase and secondary monazite that have formed in the red stained albitized facies yielded k ar and u th pbtotal ages of 240 and 250 ma respectively suggesting that the alteration developed during the permian triassic period the geopetal disposition of the red albitized profile with respect to the triassic unconformity its large regional extent and the fracture controlled alteration in the lower part of the profile indicate groundwater interaction the 18o values of albitized plagioclase 11 microclinized orthoclase 13 and secondary cl dark quartz 12 suggest that the alteration temperature was about 55 c this low temperature suggests that the alteration occurred during interaction of the granitic rocks with na rich fluids below a surficial weathering mantle on the permian triassic palaeosurface the latter is possibly related to triassic evaporitic environments in long lasting stable landscapes in which na rich solutions infiltrated deep regional groundwaters

magmatic record of neoarchean arc polarity reversal from the dengfeng segment of the central orogenic belt north china craton abstract the dengfeng segment of the archean central orogenic belt in the north china craton ncc includes circa 2 66 2 5 ga metavolcanics metasedimentary and plutonic rocks all units are intruded by 2 5 2 4 ga mafic dykes and a suite of coeval circa 2 5 2 45 ga potassic granites and leucogranitic veins mafic dykes are characterized by mixed alkaline and calc alkaline affinities enriched lree and negative nb and zr anomalies indicating a lithospheric mantle source region potassic granitic dykes exhibit highly fractionated lree patterns and negative hfse anomalies and positive zircon ehf t values which are similar to those of ttg gneisses in the area leucogranitic veins cutting across the diorites are characterized by high contents of sio2 and k2o lree enriched patterns positive eu and sr anomalies negative nb and ti anomalies we suggest that the granitic dykes and leucogranitic veins were derived from partial melting of thickened older crusts combined with other reported 2 5 ga mafic dykes and potassic granites from the ncc we propose that they demonstrate an ancient example of arc polarity reversal and provide a record of the formation of a new plate boundary in the archean the underplating of the mafic dyke source beneath the newly accreted eastern block may have generated the widespread 2 5 2 47 ht lp metamorphism with anticlockwise p t paths in the eastern block

charnockite and granite formation and influx of co 2 at kabbaldurga the discovery at kabbaldurga quarry karnataka south india1 3 of patches of charnockite apparently in an arrested state of development has refocused attention on the mechanism of charnockite formation in particular the relative roles of co2 and h2o during high grade metamorphism and charnockite development can be studied in such an area during an influx of a co2 rich volatile phase h2o may be liberated as a result of the reduction in the state of hydration by the breakdown of hydrous minerals4 the p t conditions in which this occurs are important because with an increase in xh2o conditions which may allow anatexis could be initiated in advance of such a co2 rich phase indeed weaver5 has suggested that the k rich acid charnockites found at pallavaram madras6 developed due to changes in volatile composition before the onset of charnockite conditions the exposure at kabbaldurga quarry is interpreted here as displaying evidence that such a process has taken place

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

uranium migration and retention during weathering of a granitic waste rock pile abstract this study investigates the post mining evolution of s type granitic waste rocks around a former uranium mine vieilles sagnes haute vienne nw massif central france this mine was operated between 1957 and 1965 in the la crouzille former world class uranium mining district and is representative of intra granitic vein type deposits 50 years after mine closure and the construction and subsequent re vegetation of the granitic waste rock pile we evaluate the environmental evolution of the rock pile including rock alteration neo formation of u bearing phases during weathering and u migration vertical trenches have been excavated through the rock pile down to an underlying paleo soil allowing the investigation of the vertical differentiation of the rock pile and its influence on water pathways weathering processes and u migration and retention arenization dominantly drives liberation of u by dissolution of uraninite inclusions in the most alterable granitic minerals i e k feldspar and biotite retention of u in the matrix at the base of the waste rock pile and in the underlying paleo soil most likely occurs by precipitation of nano uranyl phosphates or a combination of co precipitation and adsorption reactions of u onto fe oxy hydroxides and or clay minerals even though u migration was observed u is retained in stable secondary mineral phases provided the current conditions will not be modified

greenland supraglacial lake drainages triggered by hydrologically induced basal slip a dense network of gps observations shows that rapid lake drainage events on the western greenland ice sheet are preceded by period of ice sheet uplift and or enhanced basal slip surface lakes are common during the warm season on the greenland ice sheet especially in middle to lower elevations these lakes can drain rapidly often preceded by a hydro fracturing event the mechanism behind the hydro fracturing and thus the connection between the lakes and the broader greenland hydrological system has been unclear now laura stevens and colleagues use a dense network of gps observations to show that rapid lake drainage events at their study site in western greenland were preceded by uplift events likely from water entering the basal system from neighboring moulins vertical conduits connecting the surface and bedrock the uplift created sufficient stress in the ice sheet to open the hydro fractures thus allowing the rapid lake drainage although speculative the authors suggest that the comparative lack of crevassing in upper elevations might limit the geographical spread of the uplift fracturing mechanism water driven fracture propagation beneath supraglacial lakes rapidly transports large volumes of surface meltwater to the base of the greenland ice sheet1 these drainage events drive transient ice sheet acceleration1 2 3 and establish conduits for additional surface to bed meltwater transport for the remainder of the melt season1 4 5 6 although it is well established that cracks must remain water filled to propagate to the bed7 8 9 the precise mechanisms that initiate hydro fracture events beneath lakes are unknown here we show that for a lake on the western greenland ice sheet drainage events are preceded by a 6 12 hour period of ice sheet uplift and or enhanced basal slip our observations from a dense global positioning system gps network allow us to determine the distribution of meltwater at the ice sheet bed before during and after three rapid drainages in 2011 2013 each of which generates tensile stresses that promote hydro fracture beneath the lake we hypothesize that these precursors are associated with the introduction of meltwater to the bed through neighbouring moulin systems vertical conduits connecting the surface and base of the ice sheet our results imply that as lakes form in less crevassed interior regions of the ice sheet10 11 12 13 14 where water at the bed is currently less pervasive5 14 15 16 the creation of new surface to bed conduits caused by lake draining hydro fractures may be limited

hypocenter determination in strongly heterogeneous earth models using the shortest path method the shortest path method moser 1991 for the calculation of seismic ray paths and travel times along them can be applied directly in the hypocenter location method proposed by tarantola and valette 1982 it uses the analogy between seismic rays in the earth and shortest paths in networks to construct first arrival times from one point to all other points of a three dimensional grid simultaneously in a fast robust way in earth models of arbitrary complexity doing this for all stations of a seismic array one can find the hypocenter location by minimizing the difference between the observed and the calculated travel times at the stations over the three dimensional grid the concept of probability density functions allows then for a fully nonlinear examination of the uncertainties in the hypocenter location due to uncertainties in the travel time data numerical errors in the calculated travel times and to a limited extent incomplete knowledge about the earth model the result is a three dimensional contour map of regions of equal confidence for the earthquake location the method becomes especially attractive when more than one event recorded by the same array is studied because the calculation of the travel times which is relatively the most time consuming operation has to be done only once the method is applied on the location of an event that occurred on january 18 1989 in israel

episodic propagation of a rift on the amery ice shelf east antarctica 1 we investigate ice shelf rift propagation using a combination of gps and seismic measurements near the tip of an active rift in the amery ice shelf these measurements reveal that propagation occurs in episodic bursts which were identified based on swarms of seismicity accompanied by rapid rift widening the bursts last approximately 4 hours and are separated by 10 24 days in between bursts the rift widens at a rate comparable to that of ice shelf spreading comparison of automatic weather station data and tidal amplitudes show that the propagation bursts are not directly triggered by winds or tides suggesting that rift propagation is driven by the background glaciological stress in the ice shelf we show that the ice debris that partly fills the rift may play a role in controlling the rate of propagation

the transformation of amphibolite facies gneiss to charnockite in southern karnataka and northern tamil nadu india amphibolite facies metamorphic grade gives way southward to the granulite grade in southern karnataka as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes petrologic investigations in the transitional zone south of mysore have established the following points 1 the transition is prograde amphibole bearing gneisses intimately associated with charnockite at kabbal and several similar localities are not retrogressive after charnockite as proved by patchy obliteration of their foliation by transgressive very coarse grained charnockite high fluorine content of biotite and amphibole in gneisses and high large ion lithophile element contents in gneisses and charnockites these features are in contrast to very low fluorine in retrogressive amphiboles and biotites very low large ion lithophile element contents and zonal bleaching of charnockite in clearly retrogressive areas as at bhavani sagar tamil nadu 2 metamorphic temperatures in the transitional areas were 700 800 c pressures were 5 7 kbar and h2o pressures were 0 1 0 3 times total pressures based on thermodynamic calculations using mineral analyses dense co2 rich fluid inclusions in the kabbal rocks confirm the low h2o pressures at the first appearance of orthopyroxene farther to the south in the nilgiri hills and adjacent granulite massif areas peak metamorphic temperatures were 800 900 c pressures were 7 9 kbar and water pressures were very low so that primary biotites and amphiboles those with high f contents are rare 3 the incipient granulite grade metamorphism of the transitional areas was introduced by a wave of anatexis and k metasomatism this process was arrested by drying out under heavy co2 influx charnockites so formed are hybrids of anatectic granite and metabasite of metabasite and immediately adjacent gneiss or are virtually isochemical with pre existing gneiss despite gross recrystallization to granulite mineralogy these features show that partial melting and metasomatism are attendant rather than causative in charnockite development copious co2 from a deep crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis granulite metamorphism of both neosome and paleosome followed the process is very similar to that deduced for the madras granulites by weaver 1980 the massif charnockites for the most part extremely depleted in lithophile minor elements show many evidences of having gone through the same process

origin of exotic clasts in the central southern apennines clues to the cenozoic fold and thrust collisional belt in the central mediterranean area

porphyry intrusions and albitites in the bardoc kalgoorlie area western australia and their role in archean epigenetic gold mineralization minor intrusions in the menzies kambalda greenstone belt of the archean eastern goldfields province western australia range from quartz feldspar porphyry to plagioclase hornblende porphyry the porphyries display enrichment of mobile and incompatible elements k to zr and depletion of relatively compatible elements with negative nb p and ti anomalies on mid ocean ridge basalt normalized spidergrams the composition and timing of emplacement of the porphyries are consistent with a genetic relationship with spatially related granitoids porphyries occur in 30 of gold mines in the menzies kambalda belt the association appears to be largely structural since both the intrusions and the mineralizing fluids exploit zones of weaknesses such as lithological contacts and shear zones porphyries have been modified to varying degrees by hydrothermal alteration especially pervasive albitization textural evidence indicates that secondary albite and associated sodic amphibole formed late in the deformatio

the geochemistry of the la galissonniere pluton a middle proterozoic late orogenic intrusion from the eastern grenville province quebec the la galissonniere pluton is a late orogenic pluton located in the eastern grenville province of quebec the pluton contains biotite hornblende zircon titanite allanite and apatite along with secondary sericite and epidote the pluton features elevated values of nb y and f the normalized rare earth element patterns have light rare earth element concentrations of 400 500 chondrite moderate slopes 10 20 and noticeable eu anomalies trace element discriminant diagrams suggest that the pluton formed in a late orogenic environment and indicate a possible period of late precambrian crustal attenuation in the eastern grenville province

calving processes and the dynamics of calving glaciers abstract calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers in many parts of the world calving rates can increase dramatically in response to increases in velocity and or retreat of the glacier margin with important implications for sea level change despite their importance calving and related dynamic processes are poorly represented in the current generation of ice sheet models this is largely because understanding the calving problem involves several other long standing problems in glaciology combined with the difficulties and dangers of field data collection in this paper we systematically review different aspects of the calving problem and outline a new framework for representing calving processes in ice sheet models we define a hierarchy of calving processes to distinguish those that exert a fundamental control on the position of the ice margin from more localised processes responsible for individual calving events the first order control on calving is the strain rate arising from spatial variations in velocity particularly sliding speed which determines the location and depth of surface crevasses superimposed on this first order process are second order processes that can further erode the ice margin these include fracture propagation in response to local stress imbalances in the immediate vicinity of the glacier front undercutting of the glacier terminus by melting at or below the waterline and bending at the junction between grounded and buoyant parts of an ice tongue calving of projecting submerged ice feet can be regarded as a third order process because it is paced by first or second order calving above the waterline first order calving can be represented in glacier models using a calving criterion based on crevasse depth which is a function of longitudinal strain rate modelling changes in terminus position and calving rates thus reduces to the problem of determining the ice geometry and velocity distribution realistic solutions to the problem of modelling ice flow therefore depend critically on an appropriate choice of sliding law models that assume that basal velocities are controlled by basal drag can replicate much of the observed behaviour of calving glaciers with grounded termini but an important limitation is that they cannot be used to model floating glacier termini or ice shelves alternative sliding laws that parameterise drag from the glacier margins provide more flexible and robust ways of representing calving in ice sheet models such models can explain a remarkable range of observed phenomena within a simple unifying framework including downglacier increases in velocity and strain rates where basal and or lateral drag diminishes flow acceleration in response to thinning through time the tendency for glaciers to stabilise at pinning points in relatively shallow water or fjord narrowings the constraints on ice shelf stability and the contrasts in calving rates between tidewater and freshwater calving glaciers many unresolved issues remain however including the role played by the removal of backstress in the acceleration of retreating calving glaciers and the controls on melting at and below the waterline

archean crustal evolution of the aldan shield siberia geochemical and isotopic constraints abstract the relatively unknown aldan shield in eastern siberia is made up of archean granite greenstone and high grade gneiss terrains as commonly observed in many precambrian shield areas new results of geochemical and isotopic study on two terrains of contrasting metamorphic grades olekma granite greenstone and west aldan granulite gneiss terrains are present with the following principal conclusions being reached 1 the crustal evolution of the aldan shield started as early as 3 5 ga as suggested by the tdm model ages of a few high grade gneisses and a retrograded granulite enclave but the most important crust forming and tectonothermal event is firmly established at ca 3 0 ga 2 new shrimp and pb evaporation analyses on zircons have confirmed the presence of crustal rocks older than 3 ga however the zircon u pb systems appear to have been disturbed by later thermal effects and no components of 3 5 ga have been identified 3 komatiitic and basaltic amphibolites form important supracrustal sequences in the olekma terrain both group i and ii komatiites occur and garnet or majorite fractionation in mantle melting processes is likely to have taken place during the genesis of group ii komatiites overall depleted mantle sources were involved in the geneses of basic and ultrabasic magmas as evidenced from lree depletion and nd isotopic compositions nd t 2 crustal contamination was not significant as suggested by the low la nb ratios 1 in komatiites and metabasalts 4 as common to most archean tonalite trondhjemite granodiorite ttg rocks the aldan granitic gneisses also show highly fractionated rare earth element ree patterns with heavy ree depletions suggesting that separation of garnet and amphibole has occurred during melting of their mafic sources 5 although the olekma and west aldan terrains have undergone different metamorphic evolution there is no substantial difference in the time of major crust forming events at 3 0 ga and a strong proterozoic thermal disturbance at 2 ga furthermore from the geochemical comparison of the two terrains it is concluded that the nature of mantle sources for basic ultrabasic magmas and the process of ttg magma generation are similar in both terrains and that the granulite facies rocks cannot be considered as the restites of intracrustal melting

dynamic controls on glacier basal motion inferred from surface ice motion 1 current heuristic laws that relate the motion of glaciers due to sliding along the bed to the subglacial water pressure fail to reproduce variations in sliding speed on timescales of specific hydrologic events such as lake drainage rainfall or surging this may be due to the importance of subglacial cavity evolution and shifts in the glacier stress field both of which are not accounted for in typical sliding laws we use multiple time series of surface motion over a 66 day period at brei amerkurjokull iceland to infer changes in bed separation and longitudinal force budget we observe multiple distinct periods of increased surface motion and uplift corresponding to periods of rainfall and or increased temperatures we find consistent hysteresis and lags between motion and variations in both the bed separation and longitudinal stress gradient that we attribute to the redistribution of normal stresses at the bed during cavity growth increases in the longitudinal stress gradient suggest a downglacier stress transfer during increased basal motion that is consistent with increased drainage system efficiency toward the terminus our results suggest that the transient evolution of the subglacial drainage system and shifts in the glacier stress field are important controls on basal motion

the composition and petrogenesis of the lower crust a xenolith study granulite facies lower crustal xenoliths from a single basaltic vent hill 32 in the mcbride volcanic province north queensland australia illustrate the extreme lithologic diversity of the deep crust these xenoliths are dominantly mafic but intermediate and felsic granulites represent a significant proportion 20 of the xenolith population all xenoliths have high grade mineralogies and generally well equilibrated textures as well as decompression features which are indicative of derivation from deep crustal levels 0 7 1 0 gpa major and trace element chemistry for 12 xenoliths chosen to span the observed lithologic range are used to constrain the petrogenesis of these rocks in the smaller layered samples metamorphic differentiation may lead to nonrepresentative sampling however in most instances such secondary processes can be identified and the original chemical characteristics delineated the mafic xenoliths formed through a variety of processes including crystallization of mafic magma crystal accumulation from mafic and felsic magmas and partial melting of intermediate compositions to yield a mafic residuum the two analyzed intermediate xenoliths are metasediments based upon high alumina to alkali ratios and rare earth element patterns whereas the two felsic xenoliths have compositions similar to igneous rocks comparisons of noncumulate and or restite xenoliths with unmetamorphosed rock types show that k and rb are variably depleted whereas th and u are strongly depleted in all rock types the large ion lithophile element depletion patterns for xenoliths are similar to those of rocks from granulite facies terrains suggesting similar processes were operative suggestions of an anomalously ba and sr rich lower crust are not supported by the data by using the observed lithologic proportions of xenoliths at hill 32 and the chemical analyses presented here a weighted mean composition of the lower crust can be obtained and compared with recently proposed lower crustal compositions the weighted mean composition is mafic and formed through a combination of basaltic underplating crystal accumulation tectonic underplating and partial melting no simple model of lower crust formation through basaltic or andesitic underplating nor intracrustal melting is sufficient to explain lower crust formation in phanerozoic continental margin settings

two stage evolution in an archean tonalite suite the taschereau stock abitibi quebec canada the taschereau stock occurs north of timmins and val d or quebec in the abitibi greenstone belt of the superior province this late archean composite pluton is composed mainly of diorite tonalite trondhjemite cut by granitic rocks gold molybdenum occurrences are associated with a zone of albite rich rocks surrounding the granitic rocks diabase dykes and shear zones postdate all rock units field and geochemical evidence suggests that the taschereau stock was emplaced diachronously trace element geochemical modelling shows that trace element abundances rare earth elements ti zr of taschereau granitic rocks are consistent with partial melting of preexisting taschereau tonalitic rocks and implies that these two rock types are not end members of a single magma that evolved through fractional crystallization

microseismic activity within a serac zone in an alpine glacier glacier d argentiere mont blanc france a passive seismic study was carried out underneath glacier d argenti ere mont blanc france where an array of seismometers was installed in a subglacial access tunnel the data show a very high emissivity from the glacier fracturing can be discriminated from serac falls using the signal characteristics we apply seismic array methods to locate the sources of these signals using a two step grid search in the parameter space four clusters of activity are found close to the network showing that this fracturing does not take place uniformly over the glacier but rather in isolated small zones we compute a local magnitude using regional earthquakes for calibration the magnitudes follow a classical gutenberg richter law in the range ml 3 to 0 15 showing that no characteristic size events dominate the process we suggest that those spatial clusters of icequakes could reveal the heterogeneous nature of the friction at the base of the glacier with patches of high frictional stresses locally generating intense fracturing within the ice mass

evolution of high temperature hydrothermal fluids associated with greisenization and feldspathic alteration of a tin mineralized granite northeast queensland greisenization albitization and k feldspathization occur as pervasive background alteration and fracture controlled selvage style alteration in the go sam granite in the herberton mount garnet tin fields of northeast queensland optical sem and microthermometric investigations of fluid inclusions principally from vug filling minerals in selvage style alteration permit interpretation of the nature and evolution of hydrothermal fluids responsible for alteration of the granite saline hydrothermal fluids are dominated by na k ca fe chloride solutions with mg mn zn si al th ce and la relatively minor but important components salinities of up to 70 wt percent total dissolved salts have been calculated using the phase volume method indicated trapping temperatures of 600 degrees to 650 degrees c for many saline inclusions in vug filling minerals from feldspathic rocks record minimum temperatures for feldspathic alteration inferred temperatures for greisenization are similar and slightly lower 450 degrees 525 degrees c trapping temperatures and the composition of fluids in saline inclusions suggest late magmatic to early postmagmatic alteration of the granite by predominantly magmatic fluids inclusions containing relatively dilute fluids of possible meteoric origin homogenize between 100 degrees and 400 degrees c salinities as low as 2 equiv wt percent nacl are indicated and significant co super 2 3 hco super 3 or so super 2 4 may be present associated vapor rich inclusions indicate that the dilute fluids were probably boiling between 300 degrees and 400 degrees c a continuous range of salinities from dilute fluids to saline fluids has been found in the inclusions investigated dilution of very saline high temperature fluids could have occurred by separation of dissolved components as the fluid cooled and by mixing with dilute fluids below 400 degrees c data from fluid inclusions in magmatic quartz from the go sam granite are similar to those obtained from vug filling minerals in fracture controlled metasomatic rocks they suggest that pervasive alteration and fracture controlled alteration occurred contemporaneously and as a result of interaction with the same fluids consistently high temperatures inferred for the formation of all feldspathic rocks and associated greisens do not fit an established soviet model which emphasizes falling temperatures for the origin of comparable rocks a tentative model which emphasizes high temperature evolution of hydrothermal fluids due to changes in chemical parameters ph salinity na k f sub f 2 and confining pressures is discussed

multi element geochemical modelling of crust mantle interactions during late archaean crustal growth the closepet granite south india the closepet granite in the dharwar craton of south india is a large late archaean magmatic body its composition can be explained as a result of interactions between mantle derived melts and pre existing continental crust ttg gneisses using geochemical modelling based upon major and trace element compositions the following petrogenetic model is proposed for the formation of the batholith i an enriched mantle garnet and amphibole bearing lherzolite melts to produce a basaltic liquid ii the basaltic liquid undergoes limited fractional crystallization of biotite and amphibole f 0 9 iii the differentiated liquid rises into the old continental crust and induces water saturated anatexis of the ttg gneisses k feldspar qtz plagioclase melt iv both mantle derived and anatectic liquid mix and give rise to magma compositions ranging from quartz monzonite to granite the origin of the mantle enrichment is discussed in the context of the regional geological setting the most likely possibility seems to be metasomatism by slab melts the metasomatism of a mantle wedge by slab melts this suggests a two stage evolution for the dharwar craton during the late archaean i a subduction related event with the formation of ttgs sanukitoids and with associated mantle metasomatism and ii re melting of the metasomatized mantle this evolutionary history implies that at least some of the k rich late archaean granites are juvenile rather than products of intracrustal reworking as frequently assumed

late archaean 2550 2520 ma juvenile magmatism in the eastern dharwar craton southern india constraints from geochronology nd sr isotopes and whole rock geochemistry the results of field geochronologic geochemical and isotopic studies are presented for the granitoids that occur east of the closepet batholith up to the kolar schist belt ksb field data such as common foliation strong shear deformation occasionally leading to mylonitization together with petrographic data including reduction in grain size with corroded borders show characteristics of the syn kinematic emplacement of the granitoids single zircon evaporation ages define a minimum age of 3127 ma for the tonalitic trondhjemitic granodioritic ttg basement and 2552 2534 ma plateau ages for the emplacement of the granitoids which slightly predate 20 30 ma the emplacement of the 2518 ma closepet batholith major and trace element data together with isotopic data suggest at least four magmatic suites from closepet batholith to the east which have independent magmatic evolution histories the observed data are compatible with magma mixing for the closepet batholith melting of ttg and assimilation fractional crystallization processes for bangalore granites either melting of heterogeneous source or different degree of melting of the same source for the granitoids of hoskote kolar and fractional crystallization for the western margin of the ksb isotopic nd sr and geochemical data lree and lil elements suggest highly enriched mantle and ancient ttg crust for the closepet batholith enriched mantle and ttg crust for the bangalore granites c a chondritic mantle source for the granitoids of hoskote kolar and the quartz monzonites of the western margin of the ksb and slightly depleted mantle for granodiorites of the eastern margin of the ksb we interpret all these geochronologic geochemical and isotopic characteristics of granitoids from the closepet batholith to the east up to the ksb in terms of a plume model the centre of the plume would be an enriched hot spot in the mantle that lies below the present exposure level of the closepet batholith melting of such an enriched mantle hot spot produces high temperature magmas closepet that penetrate overlying ancient crust where they strongly interact and induce partial melting of the surrounding crust these magmas cool very slowly as the hot spot maintains high temperatures for a long time thus they appear younger 2518 ma on the contrary to the east the plume induces melting of c a chondritic or slightly depleted mantle that produces relatively colder and less enriched magmas which show less or no interactions with the surrounding crust and cool rapidly and appear slightly older 2552 2534 ma this plume model can also account for late archaean geodynamic evolution including juvenile magmatism heat source for reworking inverse diapirism and granulite metamorphism in the dharwar craton

ice shelf disintegration by plate bending and hydro fracture satellite observations and model results of the 2008 wilkins ice shelf break ups abstract satellite remote sensing observations of three break up events in 2008 for the wilkins ice shelf 28 february to 6 march 27 may to 31 may and 28 june to mid july provide unprecedented detail of ice shelf calving during rapid break up the observations reveal that the wilkins break ups occur through a distinctive type of shelf calving which we term disintegration as well as more typical rifting and calving here we focus on the disintegration process which is characterized by repeated rapid fracturing that creates narrow ice edge parallel blocks with subsequent block toppling and fragmentation forming an expanding iceberg and ice rubble mass we use these data to develop and test a model of floating ice plate disintegration in which ice plate bending stresses at the ice front arising from buoyancy forces can lead to runaway calving when free mobile water is available high resolution satellite images and laser altimetry of the first break up event provide details of fracture spacings ice thicknesses and plate bending profiles that agree well with our model predictions we suggest that surface or near surface meltwater is the main pre condition for disintegration and that hydro fracture is the main mechanism brine layers from near waterline brine infiltration can support a similar process but this is less effective unless regional ice stress patterns contribute to the net stress available at the crack tip for fracturing a combination of brine enhanced fracturing and changing internal net extensional stresses was the likely mechanism behind the latter two wilkins events

englacial drainage systems formed by hydrologically driven crevasse propagation recent work has shown that surface to bed drainage systems re form annually on parts of the greenland ice sheet and some high arctic glaciers leading to speed up events soon after the onset of summer melt surface observations and geophysical data indicate that such systems form by hydrologically driven fracture propagation herein referred to as hydrofracturing although little is known about their characteristics using speleological techniques we have explored and surveyed englacial drainage systems formed by hydrofracturing in glaciers in svalbard nepal and alaska in hansbreen svalbard vertical shafts were followed through 60 m of cold ice and 10 m of temperate basal ice to a subglacial conduit deep hydrofracturing occurred at this site due to a combination of extensional ice flow and abundant surface meltwater at a glacier confluence the englacial drainage systems in khumbu glacier nepal and matanuska glacier alaska usa formed in areas of longitudinal compression and transverse extension and consist of vertical slots that plunge down glacier at angles of 558 or less the occurrence of englacial drainages initiated by hydrofracturing in diverse glaciological regimes suggests that it is a very widespread process and that surface to bed drainage can occur wherever high meltwater supply coincides with ice subjected to sufficiently large tensile stresses

fluid percolations in a fault zone a study of fluid inclusion planes in the st sylvestre granite northwest massif central france abstract a method for relating fluid flow evolution direction of fluid percolation to the bulk brittle network microfracture system at each state in a sequence of deformational events is presented fluids percolating through cracks are generally trapped as fluid inclusion planes alignment of fluid inclusions the method involves the use of fluid inclusions not only as a tool for the establishment of the physico chemical conditions under which fluids were entrapped but also as a microstructural marker of the geometry of permeability the study shows that through the determination of the paleopressures for each stage of fluid migration the chronology of the cracks and p t conditions prevailing during each crack formation can be established this methodology has focussed on a granitic example from the french massif central but may be applied to other geological environments

metallogenesis of the french part of the variscan orogen part ii time space relationships between u au and snw ore deposition and geodynamic events mineralogical and upb data abstract mineralogical geochemical structural and isotopic u pb studies were carried out on representative uranium and gold deposits from the french part of the variscan orogen results are compared with complementary data from the literature in order to propose a reconstruction of the metallogenesis in the variscan terranes with a particular emphasis on u deposits the data show that the early metal concentrations or pre enrichments in the terranes are affected by successive metallogenic stages which caused significant metal transport and deposition thus the significant geodynamic events which affected the variscan terranes were responsible for multistage metal deposition or alteration of the early concentrations and for the superimposed features of the metal deposits u pb geochronological results complemented by a compilation of geochronological data on u and pb zn ba f deposits show that three major stages of u deposition took place in western europe in the periods 325 300 ma 290 260 ma and 190 170 ma in addition small uranium mineralization events have been dated at 425 ma 90 110 ma and as tertiary about 40 ma the permo stephanian stage is associated with late brittle deformation associated with major shear zone movements and constitutes the major stage of u concentration the jurassic phase has a prominent part in the hydrothermal remobilization of uranium and in f ba pb zn deposition throughout western europe especially in the eastern part of the massif central other stages correspond to subordinate uranium migration within variscan terranes during earlier or later geodynamic phases the silurian metamorphism in vendee and the mesozoic to tertiary tectonic movements related to the pyrenean or alpine orogenies in the south eastern part of the massif central and in the alps data available on au and sn w as sb deposits have been considered for comparison with the u metallogenesis stages related to the deposition of such metals display significant similarities with those related to uranium deposition they occurred mostly during visean to permian events related to the intrusion of the late visean sn w and late carboniferous sn w li f granitoids and to the late brittle deformation stages which affected the hercynian basement in the vicinity of major shear zones au as sb however the significant discrepancies between the physical chemical conditions of solubilization transport and deposition of the metal led to distinct deposit location independently of the metal source location within the terranes

two modes of accelerated glacier sliding related to water 1 we present the first glacier wide detailed measurement of basal effective pressure and related observations including bed separation to elucidate the role of water in sliding the hard bedded glacier instrumented in our study exhibited two phases of accelerated sliding motion apparently driven by separate mechanisms the first acceleration phase up to 5 fold increase in speed was closely tied to an increase in bed separation the faster second phase up to 9 fold increase in speed was related to an unusually high level of connectivity of subglacial waters we infer the first mode was related to cavity opening and the second mode was related to reduced ice contact with the bed glacier sliding over a hard bed is typically represented by sliding laws that include the effective basal pressure but neither sliding phase was accompanied by a simultaneous decrease in local or regional effective pressure

short term variations in glacier flow controlled by subglacial water pressure at lauteraargletscher bernese alps switzerland short term variations in horizontal and vertical surface motion were studied with high temporal resolution during the ablation season in lauteraargletscher bernese alps switzerland horizontal surface flow speed oscillated diurnally showing a correlation with the water level in a borehole flow speed increased as a function of the water level with an asymptote at the ice overburden level this observation implied that the flow variations were principally controlled by the local water pressure which enhanced basal motions detailed examination of the diurnal variations however showed that the speed was larger when the pressure was increasing than when it was decreasing greater speed with increasing pressure was interpreted by subglacial watercavity opening and or longitudinal stress coupling with the upper reaches of the glacier upward surface movements were observed when the glacier flow speed increased simultaneous measurement of internal vertical strain in a borehole showed that the uplift had two different sources vertical straining of ice and volume increase of subglacial water cavities the vertical surface movement was largely affected by the vertical strain and the uplift events could not be simply attributed to cavity opening

geochemistry of shear zone hosted uranium mineralisation at the zadni chodov uranium deposit bohemian massif abstract samples from the zadni chodov uranium deposit of the bohemian massif were analysed using optical and chemical methods organic matter and brannerite were studied in detail to estimate conditions that prevailed in association with uranium enriched shear zones four major events are known in the high grade metasediments the pre ore stage is associated with the formation of shear zones during the uranium ore deposition stage temperatures ranged from 122 c to 258 c and coffinite and brannerite appeared two generations of post ore stage minerals are distinguishable a detailed study of a brannerite mineral grain revealed variations in its interior and surroundings but these are related to the composition of brannerite which is not a nominal mineral si and al are presented as chlorites but the main mineral phases are mixtures of anatase oxidized brannerite with non stoichiometric uranium oxide and carbonaceous matter carbonaceous matter in the samples studied appeared as a mixture of substances of different composition origin and age optical and structural characteristics showed variations in three coexisting types of metamorphosed carbonaceous matter including fine grained and larger lamella shaped particles that represent anthracite meta anthracite and semi graphite stages lamellas of crystalline graphite and pyrobitumen corresponding to impsonite the structure of organic particles was not affected by uranium minerals which is an interesting difference from changes that minerals undergo in for example coal biomarkers compounds used in assessing sources of organic matter were present in extracts but they were not a remnant of the original shear zone organic material they are likely products from a later impact probably of tertiary and younger sediments

carbonic fluid inclusions in granulites from tanzania a comparison of geobarometric methods based on fluid density and mineral chemistry abstract high density carbonic fluid inclusions with well developed negative crystal shape occur in host quartz plagioclase and garnet in intermediate to high pressure granulites from the furua granulite complex southern tanzania they are most abundant in intercalated metapelitic units detailed microthermometric data on inclusions in combination with electron microprobe analyses of associated solid phases are presented for a kyanite bearing garnet granulite and a garnet two pyroxene granulite both selected from the same locality the inclusions contain virtually pure co2 and are believed to be of primary syngenetic origin this is indicated by their distribution in the host minerals and by the homogenization temperatures which give well defined single maxima between 30 and 50 c the corresponding fluid pressures are only 1 3 kbar lower than the pressure range of 10 13 kbar that is inferred from the solid phase geobarometers based on element partitioning in garnet kyanite plagioclase and orthopyroxene clinopyroxene garnet plagioclase assemblages associated temperature estimates of 770 870 c are based on the compositions of garnet biotite garnet clinopyroxene and orthopyroxene clinopyroxene pairs although the co2 density data fit within the regional pressure regime of 7 12 kbar for the metamorphism in the furua area the detailed comparison with solid phase geobarometers at the scale of a single outcrop reveals a small underestimate of pressure which may be explained by minor adjustment of the inclusions after peak metamorphism or by partial h2o pressures in the order of 0 1ptot to 0 2ptot

albitization and quartz dissolution in paleoproterozoic metagranite central sweden implications for the disposal of spent nuclear fuel in a deep geological repository hydrothermal alteration resulting in albitization and quartz dissolution has been identified in paleoproterozoic metagranites down to 1000 m elevation at forsmark sweden the alteration features were discovered during investigations to locate a site for the disposal of spent nuclear fuel in a deep geological repository in general albitization occurs extensively but it is also observed locally adjacent to minor intrusive bodies of amphibolite the altered rocks show a marked decrease in k feldspar and an increase in quartz relative to the unaltered equivalents resulting in an epitonalitic composition plagioclase is metamorphic in character and generally richer in albite than in the unaltered rocks it is inferred that albitization was triggered by the input of basic or intermediate melts into the crust during igneous activity close to the peak of regional metamorphism at 1 87 1 86 ga the mineralogy of the epitonalites gives rise to an increased thermal conductivity and thereby a positive influence for the design and safety of a deep geological repository for spent nuclear fuel however the increased frequency of low conductive amphibolite in the albitized volumes consistent with the proposed mechanism for alteration gives a negative influence in sharp contrast to the albitization a majority of the occurrences of quartz dissolution which resulted in the formation of episyenite are located along fracture zones quartz dissolution took place between or after 1 8 1 7 ga when the bedrock was able to respond to deformation in a brittle manner most of the vugs left after the removal of quartz are to a variable extent refilled by hydrothermal assemblages including quartz albite k feldspar hematite chlorite and calcite the geometry and spatial distribution of episyenite argue against an extreme fluid rock ratio and it is inferred that the fluids had at least a moderate salinity with a temperature in excess of 300 degrees c the dissolution process was promoted by the generation of secondary permeability localized in columnar or pipe like volumes the close spatial connection to fracture zones provides a basis to avoid bedrock affected by this type of alteration and thereby reduce the negative mechanical and hydrogeological aspects for a deep geological repository c 2012 elsevier b v all rights reserved

propagating strain anomalies during mini surges of variegated glacier alaska u s a analyse des deplacements acceleres observes a l aide de jauges de deformation et de geophones 1979 1980 1981 evolution des mini surges deformations glissement basal

li na metasomatism related to i type granite magmatism a case study of the highly fractionated la pedriza pluton iberian variscan belt abstract this study describes rare albitites associated with highly fractionated i type granitic melts showing a mineral paragenesis that resembles those of high temperature metasomatized rocks fenites associated with alkaline and carbonatitic intrusions the studied albitites occur as lenticular bodies on the meter scale hosted in peraluminous granites and are scattered in a narrow sector next to the peraluminous and highly fractionated i type la pedriza leucogranite lpl pluton in the spanish central system albitites show sharp contacts and preserve textural features of the host granites i e isovolumetric metasomatism these rocks consist of a variably albitized magmatic feldspar framework 70 85 modal and interstitial domains where singular na li fe3 rich amphiboles and pyroxenes crystallized with albite or quartz titanite andradite grossular garnet mg li rich mica magnetite fluorite and apatite are metasomatic accessory minerals mass balance indicates enrichments of na2o feo mgo mno li f be and some hfse high field strength elements such as zn sc and sn and lile large ion lithophile elements depletion u pb data from metasomatic titanite yield 307 10 ma which is coeval with the lpl and within the range of the regional granite emplacement about 309 298 ma the lpl also shows strong compositional affinities f li na sn and zn enrichment with albitites similar sr nd d isotopic signatures between albitites and host granites suggest that magmatic fluids might be dominant during metasomatism however the heterogeneity of 18o which shifts toward lower values in the more distant outcrops suggests contribution of meteoric in origin low salinity waters during prograde metasomatism p t fo2 conditions are estimated at 0 2 0 3 gpa 645 700 c and relatively oxidized conditions we propose that na hfse flux rich residual melts of the lpl expelled two immiscible hot liquids an alkaline low density liquid and a high density hydrosilicate liquid hsl the low density fluid dissolved magmatic quartz and provided albitization of the magmatic feldspar framework by a diffusive mechanism quartz dissolution created microchannels for infiltration of the relatively low viscosity hsl forming tubular channels this fluid was enriched in network modifiers li na f be and cations ca al released after albitization and it precipitated in these microchannels to form the new metasomatic paragenesis quartz albite uncommon li rich mafic silicates and accessory minerals

melting of archaean sialic crust under high ah2o conditions genesis of 3300 ma na rich granitoids in the mount edgar batholith pilbara block western australia the voluminous 3300 ma old granitoids of the mount edgar batholith pilbara block western australia are typically trondhjemites and silicic tonalites with sio2 ranging from 65 75 wt they have high sr average 425 ppm at 70 6 sio2 and na2o 3 5 5 0 wt contents with k2o rapidly increasing 1 0 4 5 wt with increasing silica so that the most fractionated are granodioritic a lack of restite minerals calcic plagioclase cores and mafic microgranitoid enclaves suggests that the granitoids were produced from chemically evolved sources and that they crystallised from melts approximating the bulk composition of the batholith 70 6 wt sio2 based on experimental data mass balance and trace element modelling these silicic melts are considered to have been produced by high degrees of partial melting involving complete plagioclase dissolution of tonalitic dacitic precursors similar in composition to the dacites of the 3450 ma greenstone succession the data are consistent with field and isotopic evidence from the gneiss complex of the batholith where orthogneisses have formed virtually in situ from older 3450 ma banded tonalitic gneisses that resemble coeval metamorphosed felsic volcanic rocks of the duffer formation or deeper level equivalents a number of granitoids from other batholiths of the pilbara block are geochemically similar to the mount edgar batholith and probably were derived from similar sources the following three stage model of crustal growth is envisaged 1 production of a thick primitive mafic crust at 360 ma remnants of which exist as gabbroic anorthosite xenoliths in some granitoids mcnaughton et al 1988 2 partial melting of the mafic crust at 3500 3450 ma to produce tonalitic dacitic rocks at shallow mantle depths 60 km associated with formation of the major greenstone belts of the pilbara block 3 infracrustal melting of the tonalitic dacitic rocks at 3300 ma to produce the abundant silicic tonalitic trondhjemitic sodic granitoids uniformitarian and non uniformitarian tectonic models involving arc magmatism rifting mantle plumes and delamination are discussed for the three stage crustal growth process but the available geological evidence does not favour any particular model the data indicate that archaean granitoids are similar to those produced in modern subduction related environments but the process of archaean magmatism might have been quite different

the role of fluids in the development of a granulite facies transition zone in s india the bodinayakkanur area of tamil nadu lies in the transition zone p 5 kbar t 700 c from amphibolite to granulite facies metamorphism in the archaean high grade terrain of s india within the zone 3 closely associated rock units occur 1 precursor grey gneiss 2 pink migmatitic and metasomatic gneiss 3 green charnockitic granulites field observation shows that the complex inter relations between these 3 rock types can be related to influx of fabric and layer guided co 2 and h 2 o k 2 o rich fluid phases derived from a deep seated source

le magmatisme de la region de kwyjibo province du grenville canada interet pour les mineralisations de type fer oxydes associees the granitic plutons located north of the kwyjibo property in quebec s grenville province are of mesoproterozoic age and belong to the granitic canatiche complex the rocks in these plutons are calc alkalic k rich and meta to peraluminous they belong to the magnetite series and their trace element characteristics link them to intraplate granites they were emplaced in an anorogenic subvolcanic environment but they subsequently underwent significant ductile deformation the magnetite copper and fluorite showings on the kwyjibo property are polyphased and premetamorphic their formation began with the emplacement of hydraulic magnetite bearing breccias followed by impregnations and veins of chalcopyrite pyrite and fluorite and ended with a late phase of mineralization during which uraninite rare earths and hematite were emplaced along brittle structures the plutons belong to two families biotite amphibole granites and leucogranites the biotite amphibole granites are rich in iron and represent a potential heat and metal source for the first iron oxide phase of mineralization the leucogranites show a primary enrichment in ree rare earth elements f and u carried mainly in y u and ree bearing niobotitanates they are metamict and underwent a postmagmatic alteration that remobilized the uranium and the rare earths the leucogranites could also be a source of rare earths and uranium for the latest mineralizing events

episodic ice velocity fluctuations triggered by a subglacial flood in west antarctica height change anomalies in satellite altimeter data have been interpreted as the surface expressions of basal water moving into and out of subglacial lakes these signals have been mapped throughout antarctica on timescales of months to years but only broad connections have been made between active lakes and ice dynamics we present the first high frequency observations of ice velocity evolution due to a cascading subglacial lake drainage event collected over 5 years 2010 2015 using global positioning system data on whillans and mercer ice streams west antarctica we observed three episodic ice velocity changes over 2 years where flow speed increased by up to 4 as well as an 11 month disruption of the tidally modulated stick slip cycle that dominates regional ice motion our observations reveal that basal conditions of an antarctic ice stream can rapidly evolve and drive a dynamic ice response on subannual timescales which can bias observations used to infer long term ice sheet changes

geochronology and geochemistry of 2 5 to 2 4 ga granitic plutons from the southern margin of the north china craton implications for a tectonic transition from arc to post collisional setting abstract the xutai and lujiagou plutons exposed in the songshan jishan area in the southern margin of the north china craton ncc are studied to constrain the tectonic regime of the ncc between 2 5 and 2 4 ga the xutai pluton dated at 2509 33 ma mainly consists of tonalite and monzogranite with a calc alkaline affinity rittmann index

formation of granitic magmas during repeated orogenic events an example from the baltic shield abstract the southwestern 1 8 0 9 ga old part of the baltic shield sgc was formed later than its central svecofennian zone 2 0 1 75 ga after the formation of its continental crust the sgc was intruded repeatedly by anatectic granitoids this study tests the hypothesis that the younger anatectic granitoids were formed by partial melting of early granites intruded during the formation of the original continental crust the tests are based on rb sr isotopic major element and trace element data the sm nd data are generally consistent with the suggested hypothesis only one of the seven granite intrusions included in the test based on the rb sr data is rejected major and trace element data are available for four anatectic granite intrusions the tests suggest rejection of the hypothesis for two of these massifs for the remaining two no rejection of the hypothesis can be based on the outcome of the applied tests in conclusion it is found that macro and microscopically indistinguishable coeval granites which occur close to each other may have different origins

evidence for deep icequakes in an alpine glacier to obtain more reliable information about the focal depth distribution of icequakes in april 1997 we operated an array of seven portable digital seismographs on unteraargletscher central swiss alps over 5000 events were detected by at least two instruments during the 9 day recording period p wave velocities 3770 m f were determined from several calibration shots detonated at the glacier surface as well as in a 49 m deep borehole whereas s wave velocities 1860 ms 1 were derived from a simultaneous inversion for vp vs6 applied to 169 icequakes so far hypocentral locations have been calculated for over 300 icequakes besides confirming the occurrence of shallow events associated with the opening of crevasses our results show that a small but significant fraction of the hypocenters are located at or near the glacier bed one event was found at an intermediate depth of about 120 m three dimensional particle motion diagrams of both explosions and icequakes clearly demonstrate that all vertical component seismograms from shallow sources are dominated by the rayleigh wave on the other hand for events occurring at depths greater than about 40 m the rayleigh wave disappears almost entirely therefore a qualitative analysis of the signal character provides direct information on the focal depth of an event and was used as an independent check of the locations obtained from traditional arrival time inversions thus our results demonstrate that deep icequakes do occur and that simple rheological models according to which brittle deformation is restricted to the uppermost part of a glacier may need revision

observation of surface seismic activity changes of an alpine glacier during a glacier dammed lake outburst during the 2004 outburst flood of gornersee switzerland we observed surface microseismic activity so called icequakes near the glacier dammed lake on gornergletscher using surface waves we located near surface tensile fracturing events by inverting time delays using a quasi newton scheme we located 8520 near surface events with uncertainties of less than 10 m seismic activity increased during the lake drainage and the icequake epicenters migrated downglacier furthermore we identified events whose fault plane orientations are nearly perpendicular to the local pattern of surface crevassing these observations can be explained by the rotation of principal strains at the glacier surface deduced from local ice flow measurement which occurred during the flotation of the ice dam at the onset of the lake drainage in general we suggest that our technique is a useful tool to locate large data sets of microseismic events and can be used to monitor the temporal evolution of fracture processes and their dependence on glacier flow and glacier hydrology

a chlorite solid solution geothermometer the los azufres mexico geothermal system chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites in the active geothermal system of los azufres mexico electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition correlation coefficients among chemical constituents were calculated it is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content and there is almost no correlation with the octahedral aluminium and magnesium content a procedure is proposed to select end members and substitution vectors and to give a general formula for these chlorites

basal icequakes during changing subglacial water pressures beneath gornergletscher switzerland using dense networks of three component seismometers installed in direct contact with the ice the seismic activity of gornergletscher switzerland was investigated during the summers of 2004 and 2006 as subglacial water pressures varied drastically these pressure variations are due to the diurnal cycle of meltwater input as well as the subglacial drainage of gornersee a nearby marginal ice dammed lake up to several thousand seismic signals per day were recorded whereas most icequakes are due to surface crevasse openings about 200 events have been reliably located close to the glacier bed these basal events tend to occur in clusters and have signals with impulsive first arrivals at the same time basal water pressures and ice surface velocities were measured to capture the impact of the lake drainage on the subglacial hydrological system and the ice flow dynamics contrary to our expectations we did not observe an increase of basal icequake activity as the lake emptied thereby raising the subglacial water pressures close to the flotation level for several days in fact the basal icequakes were usually recorded during the morning hours when the basal water pressure was either low or decreasing during the high pressure period caused by the drainage of the lake no basal icequakes were observed furthermore gps measurements showed that the glacier surface was lowering during the basal seismic activity these observations lead us to conclude that such icequakes are connected to the diurnal variation in glacier sliding across the glacier bed

mineralogical and geochemical characteristics of hydrothermal alteration and episyenitization in the konigshain granites northern bohemian massif germany the late hercynian granites of konigshain underwent multistage hydrothermal processes extensive high temperature late magmatic alteration is for example indicated by low zr hf and an ree pattern displaying the tetrad effect intensive post magmatic alteration of the granite occurred along brittle structures at least two main stages of post magmatic hydrothermal alteration are involved the first high temperature stage which is characterized by albitization and or quartz leaching episyenitization resulted from fluid rock interaction with late magmatic fluids that very probably mixed with external low salinity fluids quartz dissolution was triggered by vapour condensation and or the cooling of these fluids below 450 c along brittle structures the high porosity resulting from quartz leaching during stage 1 assisted subsequent circulation of low temperature fluids at stage 2 the latter is characterized by the chloritization and illitization of episyenites almost all major and trace elements were enriched or depleted during one of the main alteration stages however zr hf th and ti were immobile during post magmatic alteration the significant depletion of lree and the enrichment of hree in albitized samples is controlled by the dissolution of monazite and the new formation of hree rich polycrase y or aeschynite y during post magmatic stage 1 negative ce anomalies of episyenites are associated with illitization and suggest oxidizing conditions during stage 2

origin of granites in an archean high grade terrane southern india archean deep level granites in southern india are similar geochemically to young granites from continentalmargin arc systems they exhibit light ree enriched patterns with variable but chiefly positive eu anomalies this is in striking contrast to the negative eu anomalies typical in high level archean granites in addition the deep level granites are relatively enriched in ba and sr and depleted in total ree and high field strength elements hfse one pluton the sankari granite has unusually low contents of ree and hfse most of the deep level granites appear to represent cumulates with variable amounts of trapped liquid and of minor phases resulting from fractional crystallization of a granitic parent such parental granitic magmas can be produced by batch melting of archean tonalite at middle to lower crustal depths the sankari granite requires a tonalitic source depleted in ree and hfse archean tonalites and tonalitic charnockites exhibit original igneous geochemical signatures and their average composition does not show a significant eu anomaly hence they cannot represent the positive eu anomaly complement to the negative eu anomaly high level granites our results suggest that archean deep level granites may represent this complement such granite may form in waterrich zones in the middle or lower crust and be produced in response to dehydration of the lower crust by a rising co2 rich fluid phase

major and trace element geochemistry and crustal evolution of archaean granodioritic rocks from eastern finland in the eastern part of central finland vast areas of granodioritic gneisses surround the ca 2 65 ga greenstone belts these granodioritic gneisses can be divided into two chronological and lithological units 1 grey gneisses 2 86 ga old with a typical archaean ttg tonalite trondhjemite granodiorite composition 2 augen gneisses 2 51 ga old with a granodioritic to granitic composition the sr and nd isotopic data preclude derivation of the grey gneisses from very much older continental material the highly fractionated ree distribution patterns la yb n 37 103 with few or no eu anomalies militate against a direct mantle origin the data suggest that the grey gneisses have most likely been derived by partial melting of short lived basaltic crustal material garnet bearing amphibolite it is estimated that the basaltic protoliths were separated from a mantle source no more than 150 ma before the melting such an early basaltic crust might have been completely destroyed or might have left some remnants which are now represented by mafic inclusions in the grey gneisses the augen gneisses have a relatively high isr value 0 705 which suggests an origin by recycling of pre existing crustal material grey gneisses their ree patterns are less fractionated than those of the grey gneisses and sometimes have negative eu anomalies using the sr isotopic and trace element data a model calculation suggests that the augen gneisses could have been derived by partial melting 50 of mixed protoliths with ttg greenstone belt volcanics in a proportion of 70 30 by volume followed by fractional crystallisation

glacier velocity and water input variability in a maritime environment franz josef glacier new zealand short term glacier velocity variations typically occur when a water input is accommodated by an increase in the subglacial water pressure although these velocity variations have been well documented on many glaciers few studies have considered them on glaciers where heavy rain and glacier melt occur year round this study investigates the relationship between water inputs and glacier velocity on franz josef glacier new zealand we installed six gnss stations across the lower glacier during austral summer 2010 11 and one station during summer 2012 13 glacier velocity remained elevated at all stations for 7 days following large rain events during diurnal melt events we find velocity variations in the early afternoon 12 00 16 00 at 600 m a s l and in the late evening 20 00 01 00 at 400 m a s l we hypothesize that the late evening velocity variations occurred as an upstream region of high subglacial water pressures and accelerated ice motion propagated downstream this mechanism may also explain the increased longitudinal compression and transverse extension across the lower glacier during speed up events our results indicate that the subglacial drainage system likely decreases in efficiency upstream and that the water input variability can still cause short term velocity variations despite the large year round water inputs

the geochemistry of prograde and retrograde charnockite gneiss reactions in southern india abstract exposures in many quarries in southern india exhibit field evidence for incipient charnockitization of tonalitic and granitic gneiss prograde relationship or retrogression of charnockite to produce tonalitic gneiss retrograde relationship few systematic geochemical relationships exist between adjacent gneisscharnockite sample pairs during either prograde or retrograde reactions most elements and element ratios exhibit inconsistent variations however prograde chamockites appear enriched in ta pb volatiles chiefly co 2 and in transition metals relative to mg and depleted in ree and y compared to adjacent gneiss protoliths retrograde gneisses have higher rb pb th hf zn relative to co nb relative to ta hf relative to zr and volatiles chiefly h 2 o compared to parental charnockites of those elements u th rb cs pb significantly depleted in high pressure charnockites exposed south of the prograde transition zone only pb is significantly replenished during retrogression evidence suggests that prograde fluids are relatively rich in co 2 and retrograde fluids in h 2 o and that the typical non systematic geochemical variations during prograde and retrograde reactions reflect local effects at the wave front

the rožna uranium deposit bohemian massif czech republic shear zone hosted late variscan and post variscan hydrothermal mineralization three major mineralization events are recorded at the ro na uranium deposit total mine production of 23 000 t u average grade of 0 24 u 1 pre uranium quartz sulfide and carbonate sulfide mineralization 2 uranium and 3 post uranium quartz carbonate sulfide mineralization 1 k ar ages for white mica from wall rock alteration of the pre uranium mineralization style range from 304 5 5 8 to 307 6 6 0 ma coinciding with the post orogenic exhumation of the moldanubian orogenic root and retrograde metamorphic equilibration of the high grade metamorphic host rocks the fluid inclusion record consists of low salinity aqueous inclusions together with h2o co2 ch4 co2 ch4 and pure ch4 inclusions the fluid inclusion paragenetic and isotope data suggest that the pre uranium mineralization formed from a reduced low salinity aqueous fluid at temperatures close to 300 c 2 the uraniferous hydrothermal event is subdivided into the pre ore ore and post ore substages k ar ages of pre ore authigenic k feldspar range from 296 3 7 5 to 281 0 5 4 ma and coincide with the transcurrent reorganization of crustal blocks of the bohemian massif and with late stephanian to early permian rifting massive hematitization albitization and desilicification of the pre ore altered rocks indicate an influx of oxidized basinal fluids to the crystalline rocks of the moldanubian domain the wide range of salinities of fluid inclusions is interpreted as a result of the large scale mixing of basinal brines with meteoric water the cationic composition of these fluids indicates extensive interaction with crystalline rocks chlorite thermometry yielded temperatures of 260 c to 310 c during this substage uranium was probably leached from the moldanubian crystalline rocks the hydrothermal alteration of the ore substage followed or partly overlapped in time the pre ore substage alteration k ar ages of illite from ore substage alteration range from 277 2 5 5 to 264 0 4 3 ma and roughly correspond with the results of chemical u pb dating of authigenic monazite 268 50 ma the uranium ore deposition was accompanied by large scale decomposition of biotite and pre ore chlorite to fe rich illite and iron hydrooxides therefore it is proposed that the deposition of uranium ore was mostly in response to the reduction of the ore bearing fluid by interaction with ferrous iron bearing silicates biotite and pre ore chlorite the th data on primary mostly aqueous inclusions trapped in carbonates of the ore substage range between 152 c and 174 c and total salinity ranges over a relatively wide interval of 3 1 to 23 1 wt nacl eq gradual reduction of the fluid system during the post ore substage is manifested by the appearance of a new generation of authigenic chlorite and pyrite chlorite thermometry yielded temperatures of 150 c to 170 c solid bitumens that post date uranium mineralization indicate radiolytic polymerization of gaseous and liquid hydrocarbons and their derivatives the origin of the organic compounds can be related to the diagenetic and catagenetic transformation of organic matter in upper stephanian and permian sediments 3 k ar ages on illite from post uranium quartz carbonate sulfide mineralization range from 233 7 4 7 to 227 5 4 6 ma and are consistent with the early tethys central atlantic rifting and tectonic reactivation of the variscan structures of the bohemian massif a minor part of the late variscan uranium mineralization was remobilized during this hydrothermal event

foreshocks and delayed triggering of the 2016 m w 7 1 te araroa earthquake and dynamic reinvigoration of its aftershock sequence by the m w 7 8 kaikōura earthquake new zealand abstract we analyze the preparatory period of the september 2016 m w 7 1 te araroa foreshock mainshock sequence in the northern hikurangi margin new zealand and subsequent reinvigoration of te araroa aftershocks driven by a large distant earthquake the november 2016 m w 7 8 kaik ura earthquake by adopting a matched filter detection workflow using 582 well defined template events we generate an improved foreshock and aftershock catalog for the te araroa sequence 8 000 earthquakes over 66 d templates characteristic of the m w 7 1 sequence including the mainshock template detect several highly correlating events m l 2 5 3 5 starting 12 min after a m w 5 7 foreshock these pre cursory events occurred within 1 km of the mainshock and migrate bilaterally suggesting precursory slip was triggered by the foreshock on the m w 7 1 fault patch prior to mainshock failure we extend our matched filter routine to examine the interactions between high dynamic stresses resulting from passing surface waves of the november 2016 m w 7 8 kaik ura earthquake and the evolution of the te araroa aftershock sequence we observe a sudden spike in moment release of the aftershock sequence immediately following peak dynamic coulomb stresses of 50 150 kpa on the m w 7 1 fault plane the triggered increase in moment release culminated in a m w 5 1 event immediately followed by a 3 h temporal stress shadow our observations document the preparatory period of a major subduction margin earthquake following a significant foreshock and quantify dynamic reinvigoration of a distant on going major aftershock sequence amid a period of temporal clustering of seismic activity in new zealand

a two stage exhumation in western french massif central new geochronological evidences of syn collisional extension abstract systematic 40ar 39ar dating of granitoids and surrounding metamorphic rocks has been performed in the limousin area w of french massif central in order to provide new constrains concerning its late orogenic thermal and tectonic evolution the same 40ar 39ar age of ca 335 338 ma is found on the present day outcrop surface for most of the limousin metamorphic rocks hence indicating that the entire massif was uniformly cooled down at this period these ages are very close to the 40ar 39ar ages proposed in the sioule and brevenne areas in the literature but also to the coupled tectonic and magmatic activity in the south limousin these results help to document the existence of two exhumation stages each one associated with an extensional stage a first exhumation stage has been active at ca 335 340 ma available data indicate that this extension and associated exhumation and cooling occurred over a distance of more than 200 km in the internal zones of the belt whereas compression was still active in the northern and southern parts of the belt a second exhumation stage may be related to the late orogenic extension and collapse of the belt at ca 310 305 ma and is characterized by the uplift of the st sylvestre complex by unroofing of the overlying metamorphic and granitoidic formations

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

geochemistry and origin of granitic rocks scourian complex nw scotland concordant granite sheets from the granulite facies scourian complex n w scotland exhibit the following features 1 a common planar fabric with their host pyroxene granulites 2 the presence of an exsolved ternary feldspar phase 3 a low pressure water saturated minimum composition 4 k rb ratios 450 1 350 distinctly higher than most upper crustal granites but similar to the surrounding granulites 5 low absolute concentrations of the rare earth elements rees light ree enrichment and large positive eu anomalies

seismicity on the western greenland ice sheet surface fracture in the vicinity of active moulins we analyzed geophone and gps measurements collected within the ablation zone of the western greenland ice sheet during a 35 day period of the 2011 melt season to study changes in ice deformation before during and after a supraglacial lake drainage event during rapid lake drainage ice flow speeds increased to 400 of winter values and icequake activity peaked at times 7 days after drainage this seismicity developed variability over both diurnal and longer periods 10 days while coincident ice speeds fell to 150 of winter values and showed nightly peaks in spatial variability approximately 95 of all detected seismicity in the lake basin and its immediate vicinity was triggered by fracture propagation within near surface ice 330m deep that generated rayleigh waves icequakes occurring before and during drainage frequently were collocated with the down flow west end of the primary hydrofracture through which the lake drained but shifted farther west and outside the lake basin after the drainage we interpret these results to reveal vertical hydrofracture opening and local uplift during the drainage followed by enhanced seismicity and ice flow on the downstream side of the lake basin this region collocates with interferometric synthetic aperture radar measured speedup in previous years and could reflect the migration path of themeltwater supplied to the bed by the lake the diurnal seismic signal can be associated with nightly reductions in surface melt input that increase effective basal pressure and traction thereby promoting elevated strain in the surficial ice

from static alteration to mylonitization a nano to micrometric study of chloritization in granitoids with implications for equilibrium and percolation length scales strain accommodation in upper crustal rocks is often accompanied by fluid mediated crystallization of phyllosilicates which influence rock strength and shear zone formation the composition of these phyllosilicates is frequently used for pressure temperature time constraints of deformation events although it is often highly heterogeneous even in mylonites this study investigates the reactions producing a phyllosilicate chlorite in and below grenschist facies conditions and the variations in chlorite composition along a strain gradient in the variscan bielsa granitoid axial zone pyrenees compositional maps of chlorite including iron speciation are compared to nanostructures observed by transmission electron microscopy in increasingly strained samples and related to mechanisms of fluid percolation in the bielsa granitoid altered at the late variscan alpine age shear zones are found with high strain gradients the undeformed granitoid exhibits local equilibria pseudomorphic replacement and high compositional heterogeneities in chlorite this is attributed to variable reaction mechanisms at nanoscale and element supply little interconnected intra and inter grain nanoporosity and isolation of fluid evolving in local reservoirs in samples with discrete and mm sized fractures channelized fluid triggered the precipitation of homogeneous alpine chlorite in fractures preserving late variscan chlorite within the matrix in low grade mylonites where brittle ductile deformation is observed micro and nanocracks and defects allows the fluid percolating into the matrix at the scale of hundreds of m this results in a more pervasive but incomplete replacement of late variscan chlorite by alpine chlorite despite the high strain in studied granitoids deformed under greenschist facies conditions local equilibria and high compositional heterogeneities in phyllosilicates as chlorite are therefore preserved according reaction mechanisms and element mobility controlled by i matrix fracture porosity contrasts at nanoscale and ii the location and interconnection of nanoporosity between crystallites of phyllosilicates this preservation influences our ability to reconstruct the pre and syn kinematic metamorphic history of granitic rocks in low grade units of orogens

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes

geochemistry of the archean low to high grade transition zone southern india the transition zone between archean low and high grade rocks in southern india represents eroded crustal levels representative of 15 20 km it is comprised chiefly of tonalitic gneisses with some varieties showing incipient charnockitization and of minor amounts of granitic gneiss and charnockite both of which appear to have developed from the tonalitic gneisses