polydiapirism of the archean mount edgar batholith pilbara block western australia structural analysis of the mount edgar batholith and the enveloping greenstone belt indicates that the batholith and rocks around its margin were subjected to similar deformation events that involved progressive and stepwise diapiric uplift of the batholith these events may be correlated on the basis of fold style orientation and fabric development early recumbent to reclined structures d1 including thrusts were buckled d2 by rounded upright folds prior to shearing d4 along normal faults parallel with the batholith margin d4 deformation was responsible for uplifting the batholith to its present structural position a deep seated linear deformation zone d3 within the batholith did not affect the supracrustal belt syntectonic granite intrusion amphibolite facies metamorphism and migmatisation were associated with d1 in the batholith whereas greenschist facies metamorphism was associated with d1 in the lowermost unit of the greenstone belt the talga talga subgroup amphibolite facies conditions were maintained within the batholith throughout the structural history but this grade was reached in the supracrustal belt only near the batholith margin progressive deformation d1 d2 and metamorphism occurred in response to early massive granite generation and emplacement producing a first order plutonic dome that reached full maturity during d2 solid state diapirism d4 followed producing a gneiss dome of similar dimension restriction of early recumbent folds and thrusts in the supracrustal belt to areas within or adjacent to the batholith is consistent with the model elsewhere deformation is minor the late stage diapirism brought a gneiss complex within the batholith into juxtaposition with lowermost greenschist facies rocks of the pilbara supergroup this complex represents in part sialic material that existed below the pilbara supergroup before diapirism began the complex closely resembles other archean high grade gneiss terranes which suggests that some of these terranes are deeper crustal exposures of granite greenstone terranes

a deep subaqueous fan depositional model for the palaeoarchaean 3 46 ga marble bar cherts warrawoona group western australia the 3 46 ga marble bar chert member of the east pilbara craton western australia is one of the earliest and best preserved sedimentary successions on earth here we interpret the finely laminated thin bedded cherts mixed conglomeratic beds chert breccia beds and chert folded beds of the marble bar chert member as the product of low density turbidity currents high density turbidity currents mass transport complexes and slumps respectively integrated into a channel levee depositional model the marble bar chert member constitutes the oldest documented deep sea fan on earth with thin bedded cherts breccia beds and slumps composing the outer levee facies tracts and scours and conglomeratic beds representing the channel systems

granite greenstone terranes in the pilbara block australia as coeval volcano plutonic complexes evidence from u pb zircon dating of the mount edgar batholith the mount edgar batholith in the pilbara block western australia consists in part of a gneiss complex that resembles other archean high grade gneiss terranes the complex is extensively intruded by variably foliated granitoids which also intrude the surrounding greenstone succession the warrawoona group the contact between the greenstone belt and the gneiss complex is tectonic field evidence indicates that the banded gneisses are the oldest part of the batholith zircon u pb ages for three samples are 3429 13ma 3443 10ma and3448 8ma all very similar to the age of zircons in the felsic volcanic rocks near the base of the greenstone succession pidgeon 1978 1 two granitoids representing pre and post tectonic phases respectively give zircon ages of 3304 10ma and3314 13ma thereby constraining the major structural events to be contemporary with granite magmatism consistent with a diapiric model for the tectonic evolution of the batholith collins 1989 2 these ages are similar to the zircon age for terminal volcanism in the warrawoona group pidgeon 1984 3 but are 100ma older than the precisely defined rb sr whole rock ages obtained throughout the batholith collins and gray 1989 4 batholith wide resetting or late closure of rb sr isotopic systems is implied possibly associated with late fluid circulation the temporal coincidence between plutonism and volcanism in the mount edgar region strongly suggests that the greenstone succession contains the extrusive equivalents of both the 3300ma granites and the 3440ma gneisses in the batholith the isotopic evidence combined with the structural data indicates that the granites cannot be derived from the surrounding greenstone belt further there is no compelling evidence for a sialic basement to the greenstone succession

oxidation of methane at the ch4 h2o co2 transition zone in the external part of the central alps switzerland evidence from stable isotope investigations abstract with the aim of understanding the mechanisms that control the metamorphic transition from the ch 4 to the h 2 o co 2 dominated fluid zone in the helvetic domain of the central alps of switzerland fluid inclusions in quartz illite crystallinity index vitrinite reflectance and the stable isotope compositions of vein and whole rock minerals and fluids trapped in quartz were investigated along four cross sections increasing temperature during prograde metamorphism led to the formation of dry gas by hydrocarbon cracking in the ch 4 zone fluid immiscibility in the h 2 o ch 4 co 2 nacl system resulted in cogenetic ch 4 and h 2 o dominated fluid inclusions in the ch 4 zone fluids were trapped at temperatures 270 5 c the end of the ch 4 zone is marked by a sudden increase of co 2 content in the gas phase of fluid inclusions at temperatures 270 5 c in the h 2 o zone the total amount of volatiles within the fluid decreased below 1 mol with no immiscibility this resulted in total homogenization temperatures of h 2 o co 2 ch 4 nacl inclusions below 180 c hydrogen isotope compositions of methane in fluid inclusion have d values of less than 100 in the ch 4 zone typical for an origin through cracking of higher hydrocarbons but where the methane has not equilibrated with the pore water d values of fluid inclusion water are around 40 in isotopic equilibrium with phyllosilicates of the whole rocks within the ch 4 to h 2 o co 2 transition zone d h 2 o values in fluid inclusions decrease to 130 interpreted to reflect the contribution of deuterium depleted water from methane oxidation in the h 2 o zone d h 2 o values increase again towards an average of 30 which is again consistent with isotopic equilibrium with host rock phyllosilicates 13 c values of methane in fluid inclusions from the ch 4 zone are around 27 in isotopic equilibrium with calcite in veins and whole rocks the 13 c ch 4 values decrease to less than 35 at the transition to the h 2 o zone and are no longer in equilibrium with the carbonates in the whole rocks 13 c values of co 2 are variable but too low to be in equilibrium with the wall rock fluids compatible with a contribution of co 2 from closed system oxidation of methane differences in isotopic composition between host rock and alpine fissure carbonate are generally small suggesting that the amount of co 2 produced by oxidation of methane was small compared to the c budget in the rocks and local pore fluids were buffered by the wall rocks during precipitation of calcite within the fissures

geochronological evidence for continuous exhumation through the ductile brittle transition along a crustal scale low angle normal fault simplon fault zone central alps 1 major low angle normal faults juxtapose different structural levels of the crust that record both brittle and ductile deformation field relationships alone cannot establish whether these different responses to deformation represent 1 parts of a single process of exhumation along the detachment or 2 two separate events with the later more discrete brittle detachment exhuming a fossil ductile shear zone from depth these two general models are critically assessed for the low angle normal simplon fault zone sfz in the central alps the sfz shows a spatial transition from a broad ductile mylonitic shear zone to a discrete brittle detachment with identical kinematics the age of the ductile shear zone and ductile brittle transition is controversial we present a detailed geochronological study based on fission track 40ar 39ar and rb sr microsampling dating coupled with structural petrological and chemical analyses that provides tight constraints on sfz timing discontinuous mineral cooling ages over a broad range of temperatures across the fault zone argue for fault activity between 20 and 3 ma on the basis of synkinematic white mica in low temperature shear zones and necks of foliation boudinage the brittle ductile transition in the footwall could be dated at 14 5 10 ma overall the data presented here are consistent with a continuous transition from ductile shearing to a more localized zone of brittle deformation within the same geological framework over a period of 15 ma the sfz is therefore an example of a telescoped crustal section within a single major low angle fault involving a continuous period of exhumation rather than a two stage structure

tensile and compressive growth structures relationships between sedimentation deformation and granite intrusion in the archaean coppin gap greenstone belt eastern pilbara western australia abstract the coppin gap greenstone belt is composed of an east west striking asymmetric synclinorium between the muccan and mount edgar batholiths along the contact with the muccan batholith the vertical to overturned northern limb is intensely sheared folded and tectonically reduced the southern limb is relatively undisturbed and irregularly intruded by the mount edgar batholith progressive unconformities and sedimentary infilling of growth structures in the gorge creek group record the deformation history the unconformities constrain which of the now steeply dipping shear zones have to be rotated with their cover sequence before they can be interpreted kinematically four mid archaean clastic systems have been recognized the first and third are transgressive littoral quartzose and feldspathic arenites the second and fourth comprise conglomeratic and ferruginous alluvial fan related deposystems east verging thrust tectonics affected the belt after unconformable deposition of the first deposystem on the early archaean warrawoona group in the western extremity of the belt primary tectonic relief was filled with the second deposystem thrust folds were overprinted at right angles by folding sub parallel to the developing belt after truncation the belt was covered by the third and fourth systems in the eastern part of the belt relief created by belt parallel folds affecting the first deposystem was filled by conglomerate overlain by basalts a sequence also assigned to the whim creek group we relate systematic depocentre shifts as observed in the bamboo creek syncline to differential doming of the adjacent batholiths buried thrusts and shear zones may have caused km scale repetition of the underlying sequences the observed relationships place uncertainty on the validity of the existing stratigraphic correlations which now need geochronologic work to resolve part of the thrusting appears to have taken place by inversion on pre existing detachment shear zones in the underlying warrawoona basalts and ultramafic rocks the shear zones descend from a high stratigraphic level in the east towards deeper levels in the west the first motion has been west block down listric normal growth faulting in places accompanied by synsedimentary explosive intrusion of black chert veins mega brecciation and back tilt of fault blocks geometrically these early structures cannot be related to the geometry of batholith doming and are interpreted as the result of either regional tensile stress local caldera collapse or both mechanisms acting synchronously the mid archaean influx of siliciclastic sediments in the coppin gap belt coincides with this change from a tensile to a complex compressive deformation the latter caused by regional thrusting and interbatholith folding late deformation of the belt includes further steepening of the strata and eastward oblique dextral and 060 sinistral strike slip

exhumation of high p marbles of the samana terrane northern hispaniola insights from paleostress and microstructural imprints abstract paleostress variations and microstructural imprints of a subducted carbonate slab record changes in mechanical strength during its exhumation the slab studied here forms part of the high p samana terrane located on the north eastern margin of the hispaniola island cold cathodoluminescence images reveal relict cataclastic fabrics within the highest pressure marbles of the punta balandra and santa barbara schists structural units formed in the early stages of exhumation at p t conditions ca 2 0 gpa 500 c cataclastic flow was triggered after a moderate increase of water content 1 2

volcanic degassing hydrothermal circulation and the flourishing of early life on earth a review of the evidence from c 3490 3240 ma rocks of the pilbara supergroup pilbara craton western australia abstract new data gathered during mapping of c 3490 3240 ma rocks of the pilbara supergroup in the pilbara craton show that most bedded chert units originated as epiclastic and evaporative sedimentary rocks that were silicified by repeated pulses of hydrothermal fluids that circulated through the footwall basalts during hiatuses in volcanism for most cherts fossil hydrothermal fluid pathways are preserved as silica barite fe bearing veins that cut through the footwall and up to the level of individual bedded chert units but not above indicating the contemporaneity of hydrothermal silica veining and bedded chert deposition at the end of volcanic eruptive events silica barite fe bearing vein swarms are accompanied by extensive hydrothermal alteration of the footwall to the bedded chert units and occurred under alternating high sulphidation and low sulphidation conditions these veins provided pathways to the surface for elements leached from the footwall e g si ba fe and volcanogenic emissions from underlying felsic magma chambers e g co 2 h 2 s hs so 2 stratigraphic evidence of shallowing upward and subsequent deepening associated with the deposition of warrawoona group cherts is interpreted to relate to the emplacement of subvolcanic laccoliths and subsequent eruption and or degassing of these magmas heat from these intrusions drove episodes of hydrothermal circulation listric normal faulting during caldera collapse produced basins with restricted circulation of seawater eruption of volcanogenic emissions into these restricted basins formed brine pools with concentration of the volcanogenic components thereby providing habitats suitable for early life forms fossil stromatolites from two distinct stratigraphic units in the north pole dome grew in shallow water conditions but in two very different geological settings with different morphologies stratiform and domical stromatolites in the stratigraphically lower c 3490 ma dresser formation of the warrawoona group are intimately associated with barite and chert precipitates from hydrothermal vents suggesting that component microbes may have been chemoautotrophic hyperthermophiles evidence of shallow water to periodically exposed conditions active growth faulting and soft sediment deformation indicates that the volcanogenic emissions were erupted into a shallow water tectonically active caldera and concentrated therein to produce an extreme habitat for early life widespread conical and pseudocolumnar stromatolites in the c 3400 ma strelley pool chert at the base of the unconformably overlying kelly group occur in shallow marine platform carbonates silicification was the result of later hydrothermal circulation driven by heat from the overlying newly erupted euro basalt the markedly different morphology and geological setting of these only slightly younger stromatolites compared with the dresser formation suggests a diversity of microbial life on early earth the biogenicity of putative microfossils from this and younger hydrothermal silica veins in the warrawoona group remains controversial and requires further detailed study

constraints on the age of the warrawoona group eastern pilbara block western australia abstract zircons from mafic and felsic volcanic rocks in the type area of the warrawoona group the basal archaean greenstone succession of the eastern pilbara block have been dated precisely using the ion microprobe shrimp the results allow two alternative time frames for the duration of the warrawoona group dependent on how the dated zircons are considered to relate to the volcanic rocks our favoured interpretation requires a hiatus of 135 5 ma between the duffer formation at 3 46 ga and the overlying felsic volcanic rocks of the wyman formation and a hydrothermal or later magmatic origin for zircons of age 3 33 ga within one duffer formation sample and the underlying metabasalts the alternative time frame requires a short time for deposition of the entire group less than 15 ma at 3 33 ga and a xenocrystic origin for the 3 46 ga zircons of the duffer formation outside the type area of the warrawoona group the age of an intrusive granodiorite requires that greenstones be older than 3 43 ga and the group formed over an interval of 120 ma visibly different zircons within one of the duffer formation samples were found to be palaeozoic in age and presumably constitute hydrothermal growth of new zircon within the rock at low temperature similar zircons were found within samples from other rock units but with a spread of proterozoic ages

th pb ion probe dating of zoned hydrothermal monazite and its implications for repeated shear zone activity an example from the central alps switzerland th pb age dating of zoned hydrothermal monazite from alpine type fissures clefts is a powerful tool for constraining polyphase deformation at temperatures below 350 c and presents an alternative to k ar and 40ar 39ar dating techniques for dating brittle tectonics this study considers the relationship between cleft orientations in ductile shear zones and cleft mineral crystallization during subsequent brittle overprinting in the grimsel area located in the aar massif of the central alps horizontal clefts formed during a primary thrust dominated deformation while younger and vertically oriented clefts developed during secondary strike slip movements the change is due to a switch in orientation between the principal stress axes 2 and 3 the transition is associated with monazite crystallization and chloritization of biotite at around 11 5 ma quartz fluid inclusion data allow a link between deformation stages and temperatures to be established and indicate that primary monazite crystallization occurred in both cleft systems at 300 350 c while cleft monazite crystallization ceases at 11 ma in inactive shear zones monazite growth and or dissolution reprecipitation continues under brittle deformation conditions in vertical clefts during later deformation until 7 ma this younger shear zone activity occurs in association with dextral strike slip movement of the rhone simplon fault system with the exception of varying th u values correlated with the degree of oxidation there is only limited compositional variation in the studied cleft monazites

geochemical characteristics of archean cherts and other sedimentary rocks in the pilbara block western australia evidence for archean seawater enriched in hydrothermally derived iron and silica abstract geochemical and mineralogical characteristics of archean sedimentary rocks such as cherts banded iron formations bifs shales and sandstones collected from the pilbara block western australia were studied lithologic features of various types of chert occurring within the pilbara supergroup are closely related to their chemical compositions red and brown bands of banded cherts generally show a higher fe 2 o 3 total iron as fe 2 o 3 concentration than do other cherts whereas grey greenish grey and pale yellowish grey cherts are characteristic of relatively higher concentrations of al and ti white cherts are generally composed almost exclusively of sio 2 geochemical characteristics of the cherts suggest that they were precipitated from hydrothermal solutions evidence for the hydrothermal origin includes 1 low mno fe 2 o 3 values 2 low concentrations of heavy metals 3 positive euanomalies 4 low co zn and ni zn values and 5 high concentrations of mn and fe in carbonates positive correlations between fe and heavy metals such as ni and zn imply the intermittent precipitation of fe oxides hydroxides along with these heavy metals in the ancient ocean cherts except for grey greenish grey and pale yellowish grey cherts are highly depleted in detrital materials this may have been caused by rapid precipitation of hydrothermally derived fe and si the archean pilbara ocean is believed to have been enriched in fe and si derived from hydrothermal activity

constraining deformation stages in brittle ductile shear zones from combined field mapping and 40ar 39ar dating the structural evolution of the grimsel pass area aar massif swiss alps abstract 40 ar 39 ar dating of syn kinematic white mica phengite and biotite is used to constrain the age of shear zones within the granodiorites of the grimsel pass area aar massif central alps three sets of steeply dipping shear zones have been mapped stage 1 shear zones are very broad 200 m wide zones of biotite bearing rocks with a progressive increase in intensity of ductile deformation from rim to core and have dip slip kinematics subsequent stage 2 shear zones are narrower contain phengite rich mylonites and show a gradual change from dextral strike slip to dip slip kinematics across strike strain localisation during the formation of stage 2 shear zones is interpreted to have been aided by reaction weakening during extensive fluid rock interaction brittle precursors of stage 2 shear zones are marked by biotite rich cataclasites preserved along the northern contact of the overprinting main stage 2 shear zone biotite from a stage 1 shear zone yields an age of 21 1 0 2 ma phengites in stage 2 mylonites in the grimsel pass area have a very narrow age range 13 8 12 2 ma which is interpreted to bracket the duration of ductile deformation during stage 2 deformation at mid crustal levels however the biotite rich cataclasites at the margin of a stage 2 shear zone may have localised fluid flow up to 2 ma prior to the onset of main stage 2 ductile deformation stage 3 shear zones are brittle strike slip faults containing cataclasites breccias and clay fault gouges earlier studies have established that these late faults formed at

insights into the tectonic history of the western alps through dating of fissure monazite in the mont blanc and aiguilles rouges massifs abstract hydrothermal monazite ce crystals from sub horizontal and younger sub vertical alpine fissures located in the mont blanc and aiguilles rouges massifs were dated by in situ sims th u pb methods to identify major growth phases during deformation sub horizontal clefts formed in a compressive regime that led to steep foliation and down dip lineation in the mont blanc shear zones sub vertical clefts formed during younger strike slip movements along boundaries on both sides of the mont blanc massif and along permocarboniferous deposits in the aiguilles rouges all analyzed monazite ce crystals show signs of recrystallized porous and complexly zoned grains indicative of dissolution reprecipitation processes consequently the crystals show three main growth phases at 11 5 10 5 ma 9 ma and 8 7 ma interpreted as representing re crystallization during the onset of dextral strike slip faulting and subsequent steps of activity along the dextral strike slip fault system whereas monazite ce from the older horizontal clefts may record all three crystallization steps monazite ce crystallization in vertical clefts from the aiguilles rouges massif starts only at around 9 ma comparison of monazite ce crystallization ages with thermochronometers indicates that the anastomizing network of vertical shear zones with down dip lineation in the mont blanc massif became overprinted at 11 5 ma and only traces of an older monazite ce generation survived heat advection seems to have been locally sufficient to reset zircon fission track ages the vertical clefts of ca 9 ma age in the aiguilles rouges massif indicate that strike slip deformation occurred not only on the south eastern but also the north western boundary of the mont blanc massif and may be linked to the increase in exhumation rate around that time the identified deformation phases show that the external massifs of the western alps aar mont blanc aiguilles rouges pelvoux not only share a comparable tectonic position within the alpine arc but also underwent a very similar tectonic evolution

dating brittle tectonic movements with cleft monazite fluid rock interaction and formation of ree minerals 1 two millimeter sized hydrothermal monazites from an open fissure cleft that developed late during a dextral transpressional deformation event in the aar massif switzerland have been investigated using electron microprobe and ion probe the monazites are characterized by high th u ratios typical of other hydrothermal monazites deformation events in the area have been subdivided into three phases d1 main thrusting including formation of a new schistosity d2 dextral transpression and d3 local crenulation including development of a new schistosity the two younger deformational structures are related to a subvertically oriented intermediate stress axis which is characteristic for strike slip deformation the inferred stress environment is consistent with observed kinematics and the opening of such clefts therefore the investigated monazite bearing cleft formed at the end of d2 and or d3 and during dextral movements along nnw dipping planes interaction of cleft filling hydrothermal fluid with wall rock results in rare earth element ree mineral formation and alteration of the wall rock the main newly formed ree minerals are y si y nb ti minerals and monazite despite these mineralogical changes the bulk chemistry of the system remains constant and thus these mineralogical changes require redistribution of elements via a fluid over short distances centimeter low grade alteration enables local redistribution of ree related to the stability of the accessory phases this allows high precision isotope dating of cleft monazite 232th 208pb ages are not affected by excess pb and yield growth domain ages between 8 03 0 22 and 6 25 0 60 ma monazite crystallization in brittle structures is coeval or younger than 8 ma zircon fission track data and hence occurred below 280 c

early tectonic dewatering and brecciation on the overturned sequence at marble bar pilbara craton western australia dome related or not cataclastic breccias and hydrothermal fault arrays of likely c 3400 ma timing are well developed and exceptionally well exposed in the marble bar chert member of the pilbara craton brecciation involved centimetre to metre scale clast transport distances in breccia zones up to 5 m wide cutting the c 60 m thick chert in a series of right lateral fault zones our observations of downward facing pillow basalts the geometry of the breccias and oxygen isotope data for rocks and the breccia matrix suggest the rocks were at least steeply overturned on this flank of the mt edgar dome prior to brecciation the breccias are inferred to represent steep conjugate fault zones developed by local transtension the history of overturning and brecciation predates the formation of dome related regional foliation and metamorphism and therefore occurred between 3460 and 3320 ma the established ages for deposition of the underlying duffer formation and intrusion of the mt edgar batholith respectively local overturning of the marble bar sequence prior to both brecciation and the main phase of dome formation suggests a protracted deformation history for this segment of the pilbara craton the transtensional movement along the breccias may be representative of strain accommodation accompanying an early doming phase or could be a deformation event that developed independently of doming fluids involved in brecciation were most likely formation waters expelled from the cherts and basalts in response to overpressuring induced by the overturning and progressive burial

extensional structures during deposition of the 3460 ma warrawoona group in the eastern pilbara craton western australia abstract structures in the granite greenstone terrain of the eastern pilbara were mapped in both the coongan belt greenstones and shaw batholith granitoids important structures that are coeval with an early magmatic event in both the granitoids and the greenstones were identified in the coongan greenstones a brittle extensional fault array is syn kinematic with felsic volcanics of the duffer formation dated at 3452 16 ma pidgeon 1978a in the underlying shaw batholith a major ductile shear zone was mapped at the contact with the coongan belt although the foliation in the shear zone forms part of a domal structure the stretching lineations constantly plunge to the ene the sense of shear as recorded in the shear zone is consistently east up with subsidiary sinistral strike slip movement on the e w trending part and dextral movement on the n s trending part a wedge shaped granitoid intruded into the shear zone during deformation this granitoid is part of the north shaw suite which has been dated at 3467 6 ma mcnaughton et al 1988 the structures in the greenstones and in the batholith have therefore formed during the same event at different crustal levels the structural kinematic magmatic metamorphic and geochronological data are consistent with a model in which these structures represent the downdip part of a dome with a core complex type of geometry in turn this implies that the domal geometry of the shaw batholith was initiated at mid crustal levels at a very early stage namely during deposition of the warrawoona group in the evolution of the pilbara craton as extension occurred during intrusion and extrusion of large volumes of magma the original distribution and thickness of granitoid bodies and greenstones were probably largely determined by the extensional geometry the initial domal shape of the shaw batholith has been enhanced during later deformation phases which caused steepening of n s trending structures

constraining deformation phases in the aar massif and the gotthard nappe switzerland using th pb crystallization ages of fissure monazite ce abstract fissure monazite ce hereafter called monazite commonly crystallizes during deformation at low metamorphic grade and offers the possibility to date protracted deformation over several millions of years and to identify distinct deformation phases we performed th pb geochronology at the microscale on 10 samples of fissure monazite from two alpine crystalline massifs aar massif and gotthard nappe ion microprobe ages show that the earliest stage of crystallization recorded by fissure monazite domain ages occurred around 15 9 ma in the gotthard nappe and about 1 my later in the aar massif with the latest crystallization event recorded at 6 ma protracted monazite crystallization in fissures indicates that deformation datable with fissure monazite lasted about 10 ma comparison of th pb crystallization ages of fissure monazite to existing thermochronological data shows that early monazite crystallization coincide with zircon fission track ages whereas the youngest monazite crystallization overlaps with apatite fission track ages monazite also grows contemporaneous with muscovite illite crystallization k ar ages in fault gouges as monazite can grow during dissolution precipitation cycles induced by tectonic activity their chronology allows to further constrain in the aar massif the final handegg phase at 12 11 5 ma and the coeval activity of the pfaffenchopf 11 5 9 ma and the oberaar and rhone simplon phases 11 5 and 6 ma in the gotthard nappe monazite crystallization constrains a major portion of the chiera backfolding phase at 14 13 ma and confirms that the south western termination of the nappe was affected by the rhone simplon phase

rare earth and trace element mobility in mid crustal shear zones insights from the mont blanc massif western alps the behaviour of rare earth elements ree during fluid rock interaction in mid crustal shear zones has received little attention despite their potential for mass balance calculation and isotopic tracing during deformation in this study several cases of large ree mobility during alpine fluid driven shear zone development in the pre alpine granitic basement of the mont blanc massif are considered on a regional scale the undeformed granite compositions range within 5 wt sio2 70 5 75 3 wt and magmatic chemical variations are of the order of 10 20 ascribed to minor effects of crystal fractionation major and trace element mobility observed in shear zones largely exceeds these initial variations shear zones developed a range of mineral assemblages as a result of shearing at mid crustal depths at not vert similar0 5 gpa 400 c five main shear zone assemblages involve muscovite chlorite epidote actinolite and calcite respectively as major phases in most cases selective enrichments of light or heavy ree and y ta hf are observed ree mobility is unrelated to deformation style cataclastic mylonitic the intensity of strain and to the shear zone s major metamorphic mineral assemblages instead the changes in ree concentrations are ascribed to the alteration of pre existing magmatic ree bearing minerals during deformation related fluid rock interaction and to the syntectonic precipitation of metamorphic ree bearing minerals mainly monazite bastnasite aeschynite and tombarthite minor proportions 2 of these accessory phases with grain sizes mostly 20 m account for enrichments of up to 5 1 compared to the initial granite whole rock ree budget the stability of the ree phases appears to be largely dependent on the altering fluid composition ree mobility is ascribed to changes in ph and to the availability of co32 po42 and so42 ligands in the fluid such processes are likely to influence the mobility of ree y hf and ta in shear zones

evidence for multiphase deformation in the archean basal warrawoona group in the marble bar area east pilbara western australia abstract the results of a structural analysis of the marble bar greenstone belt in the mcphee reward area of the eastern pilbara block in western australia indicate that the area has undergone a multiphase deformation on shears which approximately parallel the trace of stratigraphy cf primary igneous layering around the talga talga anticline early extensional movements were associated with the formation of the warrawoona group chiefly the duffer formation and indicate a wnw ese extensional direction similar to the syn warrawoona extension found in the area of the north shaw batholith extension was followed by thrusting with transport to the east the talga talga anticline is a culmination which formed during this event by thrust stacking a major shear in the area the duffada shear was reactivated during granodiorite emplacement with transport to the ne in their present geometry the shears have thrust movements on the south side of the talga talga anticline but normal movements on the north side after the reactivation came a nw se shortening which was accompanied by conjugate shear activity on north trending sinistral and west trending dextral shears and constrained to 2950 ma in the north shaw area the final movements were associated with a n s shortening which resulted in an e w trending thrust which cuts the northern flank of the talga talga anticline and nw and ne trending conjugate faults there is no evidence of radially oriented stretching lineations or slickensides which would be expected if the granitoid bodies which make up the mount edgar batholith were emplaced by a solid state diapiric process the net result of deformation was tectonic disruption of the stratigraphic stacking within the type ara of the talga talga subgroup in the warrawoona group this may offer an additional explanation for anomolously young dates of the talga talga subgroup with respect to the duffer formation and also indicates that the type section for the talga talga subgroup is a lithotectonic sequence

k ar dating of synkinematic clay gouges from neoalpine faults of the central western and eastern alps abstract with ongoing alpine convergence and deformation during exhumation there is a general progression from ductile to brittle behaviour on the periadriatic fault and kinematically related faults farther north e g the simplon rhone fault and a diffuse discontinuous zone of generally dextral strike slip movement on the southern boundary of the aar massif k ar dating of fine grained illite from clay fault gouges provides a reliable method for establishing the maximum age of this transition to brittle faulting and for short lived faults the approximate time of faulting itself the new results establish that brittle faulting on the northeastern segment of the canavese fault i e the part of the periadriatic fault southwest of lago maggiore occurred around 20 ma with south side up kinematics a k ar age of ca 17 ma for the crosscutting giudicarie fault in the eastern alps is effectively identical with an already published pseudotachylyte age and places a lower limit on major and continuous strike slip movements of the periadriatic fault the age of brittle faulting farther north on the southern border of the aar massif is from 13 6 0 3 to 8 3 1 1 ma consistent with the younger cooling and exhumation in this area the dominantly dextral brittle faulting becoming younger to the north reflects the continued indentation and anticlockwise rotation of adria as alpine units become exhumed and progressively welded to the southern block in contrast to the eastern alps there is no field evidence for westward lateral extrusion of alpine units relative to adria and the european foreland

stable isotope and ar ar evidence of prolonged multiscale fluid flow during exhumation of orogenic crust example from the mont blanc and aar massifs nw alps the spatial and temporal scales and the geometry of fluid pathways in a collisional orogen are investigated using stable isotope analysis o c and h and 40ar 39ar dating of vein minerals formed at circa 11 16 ma in the mont blanc and the aar external crystalline massifs in both massifs 40ar 39ar dating of veins adularia provides evidence for progressive crystallization from 16 to 9 ma and mainly at 11 12 ma following veins opening during shear zone activity the fluid flow duration thus ranges from 4 to 5 ma in the two massifs the 18o values of vein quartz and calcite are similar to those of undeformed crystalline and sedimentary host rocks suggesting rock buffering while carbon isotope ratios of vein calcites fall into three compositional groups a type veins have 13c values that are buffered by the helvetic metasediments which suggests that these veins formed in a closed system from a locally derived co2 rich fluid the fluid in equilibrium with c type veins has depleted 13c values similar to mantle co2 while the intermediate 13c values of b type veins suggest mixing between the a type and c type fluids these results are in agreement with crustal to lithosphere scale upward vertical fluid flow along vertical shear zones related to the strike slip system bounding the adriatic block since 16 20 ma connecting a deep seated fluid to some downward flow in the sedimentary cover of external crystalline massifs

archaean basin margin geology and crustal evolution an east pilbara traverse a palinspastic reconstruction of a 100 km long traverse through archaean rocks of the east pilbara western australia includes new observations of the deformation preceding the now visible greenstone belt pattern the restoration is time calibrated with all available u pb datings between incompletely preserved basin sequences two superposed palaeoarchaean volcano sedimentary basins the coongan and salgash basins are separated by an eastwards time transgressive interface tentatively interpreted as an onlap surface for over 140 ma the basin margin architecture was structurally controlled by superposed extensional growth fault arrays d 1 with associated dyke swarms in a curved pattern spatially not related to that of the actual distribution of granite domes and greenstone belts the basins are interpreted to have formed by collapse after arching above hotspots due to phase transitions by mini subduction of slabs of cooled water saturated basalt towards the base of an originally c 45 km mafic crust at c 3 31 ga the extension was replaced by plate driven regional nw se compression d 2 inferred from nw over se shear and ramp and flat thrusts partly reversing offsets of the d 1 extension the recognition of widespread d 2 pre doming compression is important because it triggered the c 3 18 ga start of formation of the dome and keel pattern d 3 visible today which culminated at c 2 9 ga supplementary material eight figures numbered supplementary material fig a h and a gps list of observation sites are available at https doi org 10 6084 m9 figshare c 3808243

fluid regimes during late stages of a continental collision physical chemical and stable isotope measurements of fluid inclusions in fissure quartz from a geotraverse through the central alps switzerland fluid evolution during neo alpine metamorphism during late stages of the continental collision between europe and africa was studied by analyzing fluid inclusions in alpine fissure quartz collected in forty nine localities along a geotraverse through the central alps switzerland the methods employed include microthermometry micro raman spectroscopy k na thermometry and stable isotope analysis early fluid inclusions provide evidence of close to peak metamorphic temperatures of the late tertiary or neo alpine metamorphic event fluid composition evolved along the geotraverse from north to south as follows higher hydrocarbons were dominant in the low and medium grade diagenetic zones methane was the main volatile in the high grade diagenetic and low grade anchizone water dominated in the highgrade anchizone and low grade epizone with co2 10 mol in the high grade epizone and in the mesozone higher hydrocarbons and ch4 were the products of kerogen maturation and cracking of preexisting petroleum large water supplies originated from the dehydration of cooler metasedimentary rocks that were overthrust by crystalline basements of the lepontines aar and gotthard massifs carbon isotope analyses suggest that the co2 component was derived from oxidation of graphitic matter especially in the vicinity of sulfate bearing metasediments and from decarbonation reactions in the aar and gotthard massifs as well as in the helvetic axen nappe and its underlying north helvetic flysch high fluid pressures prevailed and favored nappe transport by contrast in the southern lepontine area very low early fluid pressures were probably related to dry rocks and scarce metasediments and to high geothermal gradients that resulted from intense uplift and erosion between 26 and 18 ma retrograde fluid evolution was recorded by a succession of fluid inclusion populations in each alpine fissure it was controlled by uplift and cooling and characterized by decreasing contents of volatiles and an increase in 18o of host quartz tectonic activity led to episodic pressure drops of at least 0 5 to 2 kbar and promoted fluid unmixing channelized flow and rapid growth of skeletal quartz channelized rather than pervasive fluid migration at temperatures 450 c and under conditions of brittle deformation is documented by episodic increases in salinity and by fluid flushing through the massifs there is stable isotope evidence for involvement of meteoric water only in late crystallizing quartz formation of alpine fissures and fissure minerals was the result of a unique coincidence of late continental collision 450 c fluid expulsion from overthrust metasediments uplift and erosion

upb zircon geochronology of archaean felsic units in the marble bar region pilbara craton western australia abstract archaean supracrustal rocks that are well exposed in the marble bar region pilbara craton have been assigned to the warrawoona group and younger sequences the predominantly volcanic warrawoona group previously dated at 3300 to 3500 ma is largely basaltic with locally intercalated thick felsic volcanic units the supracrustal rocks have been folded and are distributed around the margins of large roughly circular to ovoid granitic gneissic batholiths a u pb zircon geochronology study was undertaken to obtain precise age constraints for some of the ore deposits in the area especially the big stubby north pole barite and miralga creek zn pb cu au deposits in support of efforts to improve archaean lead isotope models the results also help significantly in interpreting stratigraphic relationships and crustal evolution in the area the new conventional zircon u pb data indicate that most of the warrawoona group from the duffer formation upwards was deposited over a period of 20 ma from 3470 ma to 3450 ma the age of the duffer formation is closely constrained by results of 3471 5 ma and 3465 3 ma and the age of the upper salgash subgroup appears to be established at 3454 1 ma the panorama formation at the southern margin of the north pole dome is 3458 1 9 ma unless the duffer formation was deposited over a period of at least 10 ma this casts doubt on a previously suggested panorama duffer correlation the age of the lower part of the north pole succession is greater than 3458 ma consistent with the correlation of the basal greenstones at north pole with the talga talga subgroup the north pole chert barite unit which is well known for its preservation of early archean stromatolites microfossils and evaporite sediments is clearly older than 3458 ma the 3325 ma age now established for the wyman formation shows that this unit must be excluded from the warrawoona group the age of the talga talga subgroup remains uncertain a felsic schist in the mount ada basalt which has been dated at 3449 3 ma is interpreted to be a sill probably related to the 3450 ma granitoid bodies an age 3724 ma for a zircon xenocryst in the panorama formation is the oldest obtained so far for zircon from the pilbara granite greenstone terrane and indicates the existence of pre pilbara supergroup sialic basement ages of 3465 3 ma and 3449 2 ma applied to previous pb isotope data for the big stubby and miralga creek deposits respectively in combination with comparable data for other syngenetic archaean sulphide deposits yield a general single stage pb evolution model with the parameters t0 4560 ma a0 9 0818 b0 9 9002 and c0 29 343 this gives model ages in accord with known constraints for many archean deposits a model lead age of 3490 ma for the north pole deposit which is older than 3457 ma may be too great but attempts to directly date this deposit have not yet succeeded there are striking parallels between the warrawoona group ages reported here and those recently obtained for the barberton region kaapvaal craton south africa the talga talga subgroup and lower onverwacht tjakastad subgroup sequences have approximately equivalent constraints and an age for the upper salgash subgroup agrees closely with those for the hoogenoeg formation of the upper onverwacht group geluk subgroup

model for the development of kyanite during partial convective overturn of archean granite greenstone terranes the pilbara craton australia restricted occurrences of early syn and late kinematic kyanite adjacent to large domal batholiths in the archean granite greenstone terrane of the east pilbara craton australia are considered to result from partial convective overturn of the crust the analogue models of dixon summers 1983 and thermo mechanical models of mareschal west 1980 involving gravitionational overturn of dense greenstone crust that initially overlay sialic basement successfully explain the geometry dimension kinematics and strain patterns of the batholiths and greenstone rims application of these models suggests that andalusite and sillimanite are the stable aluminosilicate polymorphs in domal crests and rims where prograde clockwise p t t paths with small pressure changes should be recorded both aluminosilicates are predicted to overprint kyanite which is observed locally around the east pilbara domes kyanite is the predicted aluminosilicate polymorph in the deeper parts of domal rims and within sinking greenstone keels reflecting rapid near isothermal burial the narrow zones of kyanite bearing schists adjacent to some batholiths in the pilbara craton are metamorphosed highly strained equivalents of altered felsic volcanic rocks in the low grade greenstone succession dragged to mid crustal depths 6 kbar during greenstone sinking the schists rebounded as an arcuate tectonic wedge along the southern mount edgar batholith rim during the later stages of doming and were juxtaposed against regional greenschist facies low strain greenstones thus kyanite was preserved if the walls had remained at depth it would have been overprinted by the higher temperature aluminosilicate polymorphs during thermal recovery kyanite growth in the pilbara craton is unlikely to have resulted from ballooning of plutons mantled gneiss doming metamorphic core complex formation or early crustal overthickening the typical subvertical foliations and lineations of the tectonic wedge suggest that subvertical fabrics extended to mid crustal depths c 20 km before rebound providing a three dimensional glimpse of archean dome and keel structures the general occurrence of large granitoid domes in archean granite greenstone terranes restriction of rare kyanite to the adjacent high strain batholith margins and its absence from the batholiths suggest that partial convective overturn of the crust may have been a common process at this early stage of earth history

detection of inherited monazite in the manaslu leucogranite by 208pb232th ion microprobe dating crystallization age and tectonic implications abstract although leucogranites are among the least petrologically variable of all igneous rocks ironically they are among the most difficult to reliably date the high himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt for a variety of reasons linked to the minimum melt composition of these leucogranites these dating studies have not been entirely successful we report results of a new ion microprobe dating method based on the decay of 232 th to 208 pb in monazite that has directly revealed the presence of inherited pb in monazite from the manalsu granite casting doubt on its previously accepted age monazite ages obtained from this leucogranite yield two distinct populations a large number of ages with a normal distribution and mean age of 22 4 0 5 ma 2 s e that we interpret to be the crystallization age and a smaller inherited fraction with an age of ca 600 ma because formation of the granite is thought to be related to slip on the main central thrust both this date and a second less precise result from a structurally similar pluton near mt everest indicate that the main central thrust was active in the interval 24 22 ma but do not constrain its initiation together with crosscutting relationships these data require that movement on the north himalayan fault occurred prior to 22 ma at both locations

protracted fluid rock interaction in the mesoarchaean and implication for gold mineralization example from the warrawoona syncline pilbara western australia abstract oxygen isotopic and geochemical analyses on whole rock and quartz veins are combined with structural observations in order to constrain the fluid circulation history within the mesoarchaean warrawoona syncline of the north pilbara craton western australia the plumbing system which is the focus of this study is localized in the so called fielding s find shear zone ffsz a km scale shear zone formed during the burial of greenstones and coeval exhumation of granitic complexes this shear zone runs parallel or close to the axial plane of the syncline it involves a prominent quartz vein network and is lined with strongly hydrothermally altered mafic felsic and sedimentary rocks towards the ffsz felsic and mafic volcanic rocks become intensely silicified with an increase in bulk rock 18 o values from 10 8 to 25 1 for altered felsic volcanics and from 7 1 to 18 3 for altered mafic volcanics geochemical modelling ascribes the silicification to a dissolution precipitation process ree and most other trace elements are strongly depleted in the silicified units with the exceptions of elements such as v cr ni and co which are enriched throughout the warrawoona syncline vein quartz 18 o data are within a small range of 13 2 2 significantly lower than their silicified host rocks these data are interpreted as the result of two main paleo fluid circulation stages intense silicification and 18 o enrichment represent alteration driven by low temperature hydrothermal convection probably involving archaean seawater in contrast the quartz veins network is related to the infiltration of metamorphic and or magmatic fluids during a later deformation episode these quartz veins represent the event responsible for the bulk of economic lode gold formation in the area the protracted fluid rock interaction history in the warrawoona syncline may have played a major role in setting the stage for the late mineralizing event the early hydrothermal circulation could have formed an efficient plumbing system characterized by high permeability low reactivity and possibly au enrichment upgrading the au endowment to the late hydrothermal fluids

paleomagnetism of the marble bar chert member western australia implications for apparent polar wander path for pilbara craton during archean time abstract the archean biosphere drilling project abdp drilled a continuous 270 m long oriented core from the towers formation which includes the marble bar chert member 3456 1 3476 0 ma in the pilbara craton northwestern australia a paleomagnetic study of 261 discrete specimens collected from a 158 5 to 182 0 m section of the marble bar chert member revealed two distinct magnetic components lt and mt the mt component yields seven different mean paleomagnetic directions clustered as mb1 to mb7 these together with the published paleomagnetic poles of early archean rocks from the pilbara craton draw a continuous paleomagnetic pole path which likely to be regarded as the early to late archean apparent polar wander path apwp for the pilbara craton the apwp implies that the pilbara craton underwent a latitudinal drift of about 21 during the interval when the magnetization of the marble bar chert member was acquired the estimated speed of the lateral drift is 12 112 cm yr 120 1120 km myr which is large compared with current plate motion velocities suggesting that continents might have moved during the archean faster than in the phanerozoic

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

stratigraphic continuity and early deformation of the central part of the coppin gap greenstone belt pilbara western australia abstract this article focuses on the discrepancy between the often advocated stratigraphic continuity of greenstone belts in the pilbara craton australia on the one hand and the observations of syndepositional deformation and other tectonic disturbance on the other hand the coppin gap greenstone belt of the east pilbara consists predominantly of mafic intermediate and felsic rocks with minor amounts of silicified sedimentary rocks of the early archaean warrawoona group several well defined zones subparallel to the bedding show intense brittle to ductile deformation testifying to differential movements of the tectono stratigraphic slices they delimit these deformation zones were at least partly related to west block down normal faults and accommodated upper block to the west transport however new u pb zircon dates geochemical analyses and detailed field observations show that the overall stratigraphic succession in the coppin gap greenstone belt has not been fundamentally disturbed the normal faults occur in several irregularly superposed arrays of km spaced faults part of which were active during deposition normal faults also occur throughout the entire belt combined the early deformation structures indicate east west extension over a minimum period of 20 ma from 3469 ma onwards likely the extension took place over an even longer period of time during much of the time of deposition of the warrawoona group from 3490 ma onwards syndepositional extension of the warrawoona group was geometrically unrelated to the doming of granitoid batholiths surficial i e uppermost crustal gravitational collapse during or shortly after deposition may have caused the apparent contradictory co existence of deformation zones and stratigraphic continuity although the deformation has not caused major disruptions of the stratigraphic succession the precision of the u pb shrimp ages is insufficient to rule out excisions or duplications of stratigraphy on the order of a few million years

tectonic control on the late stage exhumation of the aar massif switzerland constraints from apatite fission track and u th he data 1 we examine the hypothesis that pliocene exhumation of the external massifs in the central alps is controlled by climatic change new thermochronological data from the western gastern aar massif are used to investigate the timing extent and reasons for neogene exhumation our data reveal that exhumation was constant with 0 5 km ma over the last 10 ma in the north in the southern part exhumation was of the same order until 3 5 ma but then increased gradually toward the south to values of up to 1 2 km ma resulting in overall northward tilting of the western aar massif we explain this accelerated exhumation in the south after 3 5 ma mainly by tectonic denudation in the footwall of the rhone simplon fault and discuss changes in deep crustal configuration which may have triggered south directed normal faulting we propose that the rhone river was structurally trapped by the rhone simplon normal fault zone which additionally enhanced erosion in the southern section of the aar massif climatic forcing may have an impact in the very late stage of exhumation due to alpine glaciation in the late pliocene

stable isotope geochemistry and formation mechanisms of quartz veins extreme paleoaltitudes of the central alps in the neogene quartzveins ranging in size from less than 50 cm length and 5 cm width to greater than 10 m in length and 5 m in width are found throughout the central swiss alps in some cases the veins are completely filled with milky quartz while in others sometimes spectacular void filling quartz crystals are found the style of vein filling and size is controlled by host rock composition and deformation history temperatures of vein formation estimated using stable isotope thermometry and mineral equilibria cover a range of 450 c down to 150 c vein formation started at 18 to 20 ma and continued for over 10 my the oxygen isotope values of quartz veins range from 10 to 20 permil and in almost all cases are equal to those of the hosting lithology the strongly rock buffered veins imply a low fluid rock ratio and minimal fluid flow in order to explain massive nearly monomineralic quartz formation without exceptionally large fluid fluxes a mechanism of differential pressure and silica diffusion combined with pressure solution is proposed for early vein formation fluid inclusions and hydrous minerals in late formed veins have extremely low d values consistent with meteoric water infiltration the change from rock buffered static fluid to infiltration from above can be explained in terms of changes in the large scale deformation style occurring between 20 and 15 ma the rapid cooling of the central alps identified in previous studies may be explained in part by infiltration of cold meteoric waters along fracture systems down to depths of 10 km or more an average water flux of 0 15 cm3 cm 2yr 1 entering the rock and reemerging heated by 40 c is sufficient to cool rock at 10 km depth by 100 c in 5 million years the very negative d values of

petrochronology based on ree minerals monazite allanite xenotime apatite ree minerals monazite xenotime and allanite are ree1 minerals sensu stricto because lanthanides la lu and yttrium are critical constituents in them apatite does not require ree but because it contains substantial ree in many rocks it is included in this review all four minerals also host unusually high radionuclide concentrations notably th and u forming the basis of their utility as geochronometers this quartet of accessory minerals is playing an increasingly important role in petrochronology because they provide ways to link robust spot ages to petrogenetic p t conditions so can lend petrogenetic context to chronology based on other minerals part i of this review assembles the basic requisites prior to integrative petrochronologic analysis individual characteristics of the four ree minerals are addressed first i e their crystal chemistry and stability relations thermobarometers and trace element geochemistry used for tracing petrogenesis are discussed next and finally their chronology is summarized part ii presents case studies to highlight the specific strengths of ree minerals used to resolve the dynamics of a broad range of processes from diagenetic to magmatic conditions finally a brief section at the end outlines a few of the current challenges and promising perspectives for future work to introduce the four ree minerals in style let us recall the origins of their names the three phosphates have well respected greek grandparents and allanite has solid scottish roots yet of all four of them show idiosyncracies in etymology or type material apatite had long puzzled naturalists as it shows great chemical and physical variability and can resemble other minerals once properly identified abraham gottlieb werner named it apatite his reasoning referred to the greek root and giving the precise latin translation decipio taken literally both mean i deceive or i mislead which sounds like an apt confession from this mineral for having fooled

annealing kinetics of fission tracks in zircon an experimental study a series of laboratory annealing experiments on zircon fission tracks has been carried out under heating conditions of 350 750 c for 10 1 103 hr i e 4 5 min 40 days variation in the confined lengths of spontaneous fission tracks was determined using zircon grains from nisatai dacite the fading contours of normalized mean track length r on the arrhenius diagram showed as sets of straight lines we performed a series of model fittings called the parallel and fanning models in order to describe the decrease in r with increasing temperature or heating time the lowest temperature limit of the zircon partial annealing zone zpaz was defined as r 0 95 and the highest as r 0 4 which approximately corresponded to the total fading of surface tracks extrapolation of the results of the laboratory experiments to the geological timescale gives for a heating duration of 106 yr estimated values of the zpaz of 210 320 c 60 c 2 with the parallel model 190 350 c 50 c 2 with the fanning model critical temperature t0 and 170 390 c 50 c 2 with the fanning model t0 the temperatures of the zpaz decrease by 20 c for an annealing duration that is an order of magnitude longer because the estimated closure temperature of zircon fission track analysis approximately corresponds to the middle of the zpaz these results support the previously estimated closure temperature of 240 c by varying the etching time it was revealed that significant removal of radiation damage occurs at r 0 93

a field based estimate of the zircon fission track closure temperature abstract a field based estimate of the zircon fission track closure temperature is proposed from 13 zircon fission track cooling ages of the gold butte block se nevada using previously published and new thermochronological data the thermal history of the gold butte block is re evaluated resulting in a geothermal gradient estimate of 20 3 c km and a slow cooling rate of 0 55 0 02 c m y from the late jurassic to the early miocene before onset of rapid miocene cooling the rapidly cooled part of the gold butte block provides a slip rate of 10 5 km m y of the hanging wall that caused the miocene exhumation of this crustal section assuming these thermal conditions are correct the closure temperature of the zircon fission track system is determined at 205 18 c which is in good agreement with previous field based estimates because of the variation in zircon fission track annealing behavior with radiation damage field studies are a suitable way to calibrate the closure temperature of the zircon fission track system

in situ th pb dating of monazite in fibrous veins direct dating of veins and deformation in the shallow upper crust of the mexican orogen abstract few methods exist for directly dating orogenic events in the upper crust here we report in situ dates from 10 m diameter monazite grown in quartz calcite fibrous veins which precipitated at very low grade metamorphic conditions about 250 c in volcaniclastic and calcareous rocks of the early cretaceous trancas fm in the zimapan basin central mexico our integrated study includes regional and local structural and kinematic analysis of vein and folds detailed textural and mineralogical characterization of vein bearing minerals and texturally controlled th pb geochronologic analysis of monazite by la mc icpms we find that monazite in the veins preferentially crystallized at the interface between vein forming calcite and quartz fibers as well as at the vein host rock interface monazite ages range from 154 to 68 ma with the youngest sub population yielding a weighted mean age of 76 8 0 8 ma coincident with the age of basin wide folding as earlier constrained by ar ar illite dating the oldest monazite ages coincide with the timing of deposition of the volcaniclastic host rock given that veins are ubiquitous in shallow crustal rocks this approach to dating of veins with enclosed monazite has great potential to improve the constraints on temporal resolution of fluid rock interaction during of deformation

redistribution of ree y th and u at high pressure allanite forming reactions in impure meta quartzites sesia zone western italian alps accessory phases are important hosts of trace elements allanite may contain 90 of the ree in a bulk rock the mobility and redistribution of several trace elements notably hree th u and y is thus controlled by reactions involving allanite and other ree phases as well as several rock forming minerals as these elements are commonly concentrated in mature clastic sediments a suite of impure quartzite was studied two eclogite facies samples from the monometamorphic cover complex of the sesia zone western italian alps are presented in some detail as they reveal a remarkably rich spectrum of reaction relationships that involve ree phases two allanite forming reactions were inferred from textures and phase compositions monazite ca silicate fluid allanite apatite thorite 1 monazite thorite ca silicate fluid th rich allanite auerlite apatite 2 petrographic observations and thermodynamic models suggest that allanite entered the hp assemblage at 530 c and 17 18 kbar during prograde metamorphism in one sample allanite is rimmed by epidote rich in y and hree that grew at the expense of xenotime two net transfer reactions were derived xenotime allanite fluid y rich epidote apatite thorite 3 xenotime allanite fluid y rich epidote aeschynite thorite phosphate 4 textural relationships and trace element analyses of coexisting allanite monazite and xenotime y rich epidote reveal systematic partitioning of the ree partition coefficients for the hree are compatible with equilibrium fractionation whereas those for the lree show patterns that seem to be inherited from the precursor phases in this case zircon with variable lree composition

constraints on fluid evolution during metamorphism from u th pb systematics in alpine hydrothermal monazite abstract the timing and duration of hydrothermal activity during orogenesis are difficult to constrain because such systems are open and multistage using mm sized monazite crystals from two alpine clefts from the central alps switzerland we demonstrate that combined u th pb isotopic systematics of hydrothermal monazite can constrain both timing and fluid evolution during crystallization our data highlight four major characteristics of the cleft monazites i extreme th u ratios ii significant common pb in the u system up to 39 206 pb iii excess 206 pb up to 54 and iv precise and reliable th pb ages comparison of our results with literature data indicates that th u in monazite is a remarkable discriminant of geological conditions capable of distinguishing metasedimentary magmatic and hydrothermal origin hydrothermal monazite crystals are characterized by th u ratios up to 629 among the highest values ever reported extreme th u ratios in monazite enhance incorporation of 230 th a short lived intermediate product in the 238 u 206 pb decay chain generating 206 pb excess which if not accounted for results in 206 pb 238 u ages that are too old for example in the samples investigated here by as much as 300 for the two zoned monazite crystals studied in detail 208 pb 232 th ages are reliable for the different compositional domains visible in back scattered electron bse images 232 th 208 pb ages for the older domains 15 2 0 3 ma and 14 1 0 3 ma respectively are consistent with the structural relationships of the hydrothermal veins indicating early retrograde genesis the youngest rim age at 13 5 0 4 ma for the blauberg crystal marks termination of monazite precipitation the resolvable age difference is interpreted to reflect pulsed monazite growth over an extended period of hydrothermal activity in both crystals the outer rims have the highest 206 pb excess confirming a two stage crystallization two scenarios are envisaged to account for the 206 pb excess evolution in the first increasing 206 pb excess is caused by a modification of the th u fractionation between monazite and fluid due to an evolution of the fluid towards more oxidizing conditions that favor partitioning of hexavalent u into the fluid alternatively it is possible that 230 th increased with time in the fluid in this second case monazite growth would have occurred in a closed system from a fluid that was initially in disequilibrium with the 238 u 206 pb decay chain and progressively equilibrated t

fluid systems above basement shear zones during inversion of pre orogenic sedimentary basins external crystalline massifs western alps abstract in the inner part of the external alps inherited liassic basins were buried and inverted during the oligo miocene collisional phase of the alpine orogeny in northern oisans during crustal shortening the basement was locally sheared while the cover was disharmonically folded above the main basement shear zones that did not propagate into the cover in this contribution we analyze the witnesses of paleo fluid circulations associated with these crustal deformations focusing particularly on bourg d oisans and mizoen basins external western alps on the basis of structural and microstructural observations coupled to geochemical analyses cathodoluminescence o and c stable isotopes trace elements of vein versus host rock minerals we show that in the cover fluids mainly circulated over short distances closed system however trace element data also show that percolation of small amounts of basement derived fluids occurred over several tens of meters in cover rocks right above basement shear zones indeed the three successive vein sets recognized in the field display enrichments in basement derived ni co and cr which indicate that fluid transfer from the basement was efficient since the beginning of basin inversion therefore confirming the synchronous deformation of cover and basement fluid temperatures and pressures are estimated microthermometry coupled to 18o of vein minerals to about 250 400 c and 2 5 kbar for veins that most likely formed at or close to metamorphic peak conditions these results coupled to literature data are finally integrated into a model of fluid circulation evolution through progressive deformation of the whole external western alps

u pb dating of overpressure veins in late archean shales reveals six episodes of paleoproterozoic deformation and fluid flow in the pilbara craton fluid flow in the upper crust not only impacts the redistribution of heat and elements driving the formation of economic ore deposits but it also exerts control on metamorphism metasomatism and deformation however reconstructing the history of fluid flow in ancient basins is exceedingly difficult particularly in archean sedimentary rocks because of extensive overprinting and recrystallization here we report u pb ages for monazite and xenotime that grew in bedding parallel veins in 2 63 2 5 b y old shales along the southern pilbara craton australia the u pb ages define six discrete populations at 2 41 ga 2 30 ga 2 20 ga 2 10 ga 2 05 ga and 1 66 ga which formed 200 m y after deposition the abundance of bedding parallel crack seal and fibrous veins in banded iron formations bifs and underlying shales suggests a history of episodic buildup of fluid pressure followed by microfracturing fluid expulsion and mineral growth thermometry of vein minerals indicates temperatures between 230 c and 320 c implicating the migration of hydrothermal fluids the development of bedding parallel veins at 2 41 ga 2 20 ga and 1 66 ga was coeval with regional orogenic events known to have affected the craton whereas vein growth at 2 30 ga 2 10 ga and 2 05 ga reveals new episodes of deformation and fluid flow our results show that well preserved archean shales devoid of structural fabrics and 150 km inboard of the craton margin preserve a cryptic history of fluid overpressure crack seal vein development and hydrothermal fluid flow between 2 41 and 1 66 b y ago

hematite replacement of iron bearing precursor sediments in the 3 46 b y old marble bar chert pilbara craton australia the history of atmospheric oxygen prior to the great oxidation event 2 45 2 2 ga is not well understood hematite in the marble bar chert from a nasa funded drill hole abdp1 in the pilbara craton australia has been cited as evidence for an oxygenated ocean 3 46 b y ago however isotopic data from the same drill hole have been used to argue for an anoxic ocean it is generally agreed that the hematite is primary representing either a direct hydrothermal precipitate or a dehydration product of iron oxyhydroxides that formed during anoxygenic photosynthesis here we present new petrographic evidence from the marble bar chert in drill hole abdp1 that shows that hematite in jasper bands formed via mineral replacement reactions the hematite mostly occurs as sub micron sized inclusions within chert so called dusty hematite that are typically arranged into polygonal clusters surrounded by a rim of clear quartz resembling shrinkage structures the lateral transition from laminated chert enclosing minute inclusions of greenalite siderite and magnetite to chert dominated by dusty hematite provides evidence for in situ replacement of iron bearing minerals the presence of hematite rich bands containing octahedral crystals with residual cores of magnetite indicates that some of the hematite was derived from the replacement of magnetite this interpretation is supported by the widespread occurrence of magnetite in jasper displaying progressive stages of replacement from unaltered octahedral inclusions in quartz to hematite pseudomorphs along quartz grain boundaries the occurrence of dusty hematite in fractures sedimentary laminae and the outer margins of polygonal clusters containing greenalite is consistent with fluid mediated oxidation of iron rich precursor minerals the presence of syn sedimentary chert breccias comprising rotated fragments of laminated chert indicates that the precursor sediment was silicified shortly after deposition the abundance of dusty greenalite inclusions which are texturally the earliest components of the laminated chert suggests that the precursor sediment contained an iron rich clay mineral our results show that hematite has replaced ferrous rich minerals after deposition and provide a mechanism to explain the origin of hematite in the marble bar chert which is consistent with the origin of hematite in adjacent basalts a secondary origin for hematite invalidates arguments for an oxygen bearing ocean 3 46 b y ago and provides a viable explanation for the formation of archean jasper bands our findings show that misinterpretations about the origin of hematite in early precambrian cherts could lead to false conclusions about the chemistry of the ancient ocean and atmosphere

u th pb dating of collision in the external alpine domains urseren zone switzerland using low temperature allanite and monazite article i nfo dating metamorphism in external domains of orogenic belts is a challenging task due to the few chronometers available and common inheritance or disturbance of the isotopic systems at low temperature metamorphic con ditions in the external domains of the central alps urseren zone the occurrence of distinct populations of monazite and allanite in one single outcrop offers a unique chance to evaluate the potential of these two ree minerals to date successive metamorphic stages within a collisional cycle the studied outcrop locality of garvera exposes mesozoic metasediments that were metamorphosed under greenschist facies conditions t b 450 c under these conditions ree mineral grains or domains are typically small 5 50 m and in this case also occur as porphyroblast in pelites at the contact with carbonate layers or veins allanite grains n200 m based on the texture and mineral assemblages ree mineral growth conditions were attributed to be detrital mnz1 syn kinematic mnz2 and aln1 post kinematic at the thermal peak aln2 and retrograde during late stage deformation mnz3 to constrain temporally the successive crystallisation conditions ree minerals have been dated using shrimp ion microprobe u th pb analyses show that the alpine ree minerals have a high th u and while most of the 206 pb is non radiogenic f206 that can reach up to 99 for allanite th pb ages can be obtained using 206 pbc corrected th pb isochron the age of different generations of allanite and monazite constrains the timing of successive collisional stages for the prograde accretion at 22 5 1 5 ma mnz1 and aln1 thermal peak at 19 3 2 0 ma aln2 and late stage hydrothermal veining at13 6 1 4 ma mnz3 these novel geochronological data confirmtheregional diachroneity throughthe cen tral alps with younger ages burial thermal peak and exhumation toward the external domains the similarity betweenmetamorphicagesofsamplestakenintheurserenzoneandthenearbycrystallinemassifssuggeststhat themetasedimentarycoveroftheurserenzoneremainedcloselyjuxtaposedtotheexternalcrystallinebasement during the alpine collision cycle the monazite age of the late stage vein testifies for active tectonic deformation at 13 6 1 5 ma which likely played a crucial control on the exhumation of the external massifs in the middle miocene