u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

behaviour of clay fouled ballast under drained triaxial testing contamination or fouling of rail ballast with external fines including slurried and pumped subgrade material e g clay and silt is one of the primary reasons for track deterioration fouling causes differential settlement of the track and also decreases the load bearing capacity owing to the reduction in the friction angle of the granular assembly in certain circumstances fouled ballast needs to be cleaned or replaced to maintain the desired track stiffness load bearing capacity and track alignment all of which influence safety this paper presents and discusses the results of a series of large scale triaxial tests conducted on latite basalt a rail ballast of volcanic origin commonly used in australia consolidated drained triaxial tests were conducted under three different levels of confining pressure and varying degrees of clay fouling stress strain degradation characteristics are discussed in detail this paper also describes the non linear strength envelope and a novel empirical relations

late archaean to early proterozoic granitoid magmatism and high grade metamorphism in the central limpopo belt south africa the central zone of the limpopo belt in southern africa has previously been interpreted as a segment of archaean crust which experienced its main deformation and metamorphism around 2 7 ga ago we report new single zircon u pb and pb pb ages for granitoid gneisses supracrustal rocks and anatectically derived granitic melt patches in the area around messina south africa the sand river gneiss is a composite suite of tonalitic to trondhjemitic rocks with protolith ages between 2 6 and 3 2 ga the singelele gneiss a heterogeneous granodioritic to quartz monzonitic rock has protolith ages between 2 55 and 2 58 ga since both the sand river and singelele gneisses experienced polyphase high strain ductile deformation this must have occurred later than 2 55 ga ago granulite facies pelitic gneisses of the beit bridge complex contain abundant spherical multifacetted zircons which reflect new zircon growth near or at the peak of metamorphism these zircons provide ages with a mean at 2026 5 6 3 ma which is interpreted as reflecting a high p t event 10 kbar 825 25 c granitic melt patches in the metapelites as well as in the sand river gneiss and anatectic granites are probably related to rapid near isothermal decompression to below 3 5 kbar and 600 750 c these rocks contain new magmatic zircons which yielded a mean age of 2005 6 4 4 ma and probably reflect a crustal melting event resulting from rehydration of the granulitic assemblage our zircon data support previous suggestions for only one single granulite facies event in the central zone and we suggest that this event occurred c 2027 ma ago since most of the deformation seen in the gneisses of the messina area must have occurred later than 2 55 ga ago it is likely that the limpopo orogeny at least in the central zone is not an archaean event but took place in the early proterozoic

carbonate silicate inclusions in garnet as evidence for a carbonate bearing source for fluids in leucocratic granitoids associated with granulites of the southern marginal zone limpopo complex south africa abstract we present a study of carbonate bearing polyphase inclusions in garnets from leucocratic granitoids intruding metapelitic granulites of the southern marginal zone smz of the neoarchean limpopo high grade complex south africa during the post peak stage 2710 2650 ma u pb ages for zircons and monazites ternary feldspar thermometry suggests that the granitoid magma cooled from temperatures 800 900 c at a pressure of ca 6 5 kbar abundant co2 fluid inclusions in quartz and t xco2 phase equilibria modeling via perple x imply action of an essentially carbonic fluid in the granitoids cores of almandine rich garnet grains from the granitoids contain polyphase carbonate bearing inclusions with a distinct negative crystal shape the major carbonate in the inclusions is a strongly zoned magnesite siderite variety whereas pyrophyllite is the predominant silicate phase raman spectra of unexposed inclusions revealed a presence of co2 as well as ch4 and h2o the carbonate bearing inclusions coexist with larger polyphase inclusions composed of biotite quartz k feldspar plagioclase sillimanite which are interpreted as relics of granitic melts modeling the mineral assemblage preserved within the carbonate bearing inclusions shows that their present mineral and chemical compositions are a product of interaction of the trapped aqueous carbonic fluid with host garnet during cooling below 400 c despite strong modifications the inclusions bear evidence for initial saturation of the fluid with mg carbonate this is taken as an evidence for an origin of the fluids by devolatilization of the mg rich carbonate bearing ultrabasic greenstone rocks of the kaapvaal craton that were buried under the smz being generated at temperatures between 650 and 700 c the fluid subsequently participated in anatexis and coexisted with the granite magma during exhumation and interaction of the smz granulites with cratonic rocks

replacement of primary monazite by apatite allanite epidote coronas in an amphibolite facies granite gneiss from the eastern alps accessory monazite crystals in granites are commonly unstable during amphibolite facies regional metamorphism and typically become mantled by newly formed apatite allanite epidote coronas this distinct textural feature of altered monazite and its growth mechanism were studied in detail using backscattered electron imaging in a sample of metagranite from the tauern window in the eastern alps it appears that the outer rims of the former monazites were replaced directly by an apatite ring with tiny thorite intergrowths in connection with ca supply through metamorphic fluid around the apatite zone a proximal allanite ring and a distal epidote ring developed this concentric corona structure with the monazite core regularly preserved in the center shows that the reaction kinetics were diffusion controlled and relatively slow quantitative electron microprobe analyses suggest that the elements released from monazite breakdown p ree y th u were diluted and redistributed in the newly formed apatite allanite and epidote overgrowth rings and were unable to leave the corona this supports the common hypothesis that these trace elements are highly immobile during metamorphism furthermore microprobe data suggest that the preserved monazite cores lost little possibly none of their radiogenic lead during metamorphism thus metastable monazite grains from orthogneisses appear to be very useful for constraining u th pb protolith ages on the basis of these findings and a review of literature data it seems that monazite stability in amphibolite facies metamorphic rocks depends strongly on lithologic composition while breaking down in granitoids monazite may grow during prograde metamorphism in other rocks such as metapelites

the structural framework of the southern margin of the limpopo belt south africa abstract the southern marginal zone is characterized by two prominent regional structural features i massive crustal wedges 20 km 50 km containing large oval shaped closed fold structures which are related to an early d 1 phase of deformation and which are bounded by ii major ductile shear zones that strike in a general e w direction the hout river shear zone forms the southern boundary of the southern marginal zone where granulites are juxtaposed against lower grade granite greenstones of the kaapvaal craton this fault system also acted as the sole thrust for the other high grade shear zones in the southern marginal zone and played a major role in the development of the zone of retrogression and the establishment of the orthoamphibole isograd in the southern marginal zone the hout river shear zone and related faults within the southern marginal zone clearly post date d 1 and a period of peak metamorphism m 1 they are composite structures showing dominantly reverse sense displacements and are responsible for the emplacement of the granulite terrane over and onto the kaapvaal craton in contrast to these older structures are the reactivated shear zones these structures clearly cross cut and displace the high grade shear zones in the southern marginal zone as well as the retrograde orthoamphibole isograd the fact that the hout river and related shear zones are in metamorphic equilibrium with the surrounding wall rocks suggests their age is that of the metamorphism i e 2670 ma barton and van reenen 1992

the timing of sub solidus hydrothermal alteration in the central zone limpopo belt south africa constraints from titanite u pb geochronology and ree partitioning abstract in the central zone of the limpopo belt south africa palaeoproterozoic granulite facies metamorphism was superimposed on an earlier archaean orogenic history previously determined ages of 2030 2020 ma obtained from high temperature chronometers zircon garnet monazite are generally thought to provide the best estimate of the peak of palaeoproterozoic granulite facies metamorphism in the central zone whereas ages as young as 2006 ma from late melt patches suggest that temperatures remained above the wet solidus for an extended period we present a new mc icp ms 207 pb 206 pb age of 2030 9 1 5 ma for titanite found in amphibolite to greenschist facies alteration zones developed adjacent to quartz vein systems and related pegmatites that cut a strongly deformed central zone metabasite this age could potentially date cooling of rocks at this locality to temperatures below the wet solidus alternatively the titanite could be inherited from the metabasite host and the age determined from it date the peak of metamorphism integration of the geochronology with la icp ms trace element data for minerals from the metabasite the hydrothermal vein systems and comparable rocks elsewhere shows that the titanite formed during the amphibolite facies hydrothermal alteration not at the metamorphic peak or during the greenschist facies phase of veining this suggests that high grade rocks in the central zone have cooled differentially through the wet solidus and provides timing constraints on when palaeoproterozoic reworking in the central zone began this study illustrates the potential of combined geochronological and high resolution geochemical studies to accurately match mineral ages to distinct crustal processes

step leach pb pb dating of inclusion bearing garnet and staurolite with implications for early proterozoic tectonism in the black hills collisional orogen south dakota united states we report 207 pb 206 pb stepwise leaching pbsl ages of 1762 15 ma 1759 8 ma and 1760 7 ma for almandine garnet staurolite and garnet staurolite combined in an early proterozoic metapelite from the black hills collisional orogen south dakota pbsl derived 208 pb 206 pb th u trends reveal that staurolite contains inclusions of both cogenetic monazite ca 1760 7 ma and older detrital zircon 2040 ma whereas coexisting garnet contains only the zircon these results and petrologic data indicate prograde sequential growth of garnet monazite and staurolite during tectonic burial at temperatures between 400 c and 550 c i e well below pb closure temperatures we interpret the 1760 7 ma date as a maximum age for wyoming superior continental collision in the black hills which apparently postdated the 1800 1900 ma trans hudson orogeny in canada but was nearly synchronous with north directed accretion of the central plains orogen in southeastern wyoming

the limpopo mobile belt a result of continental collision the 600 km long limpopo mobile belt is discussed within the frame of a proterozoic supercontinent model piper 1976 evidence is presented that the rhodesian and kaapvaal cratons may have been separated by distances of more than 1000 km of oceanic crust from about 3350 ma ago the kaapvaal craton appears to have been driven intermittently n nw and then wnw against the rhodesian craton forming the ne sw trending collision zone the limpopo mobile belt and all the major fold and fracture patterns found this movement would be similar to the oblique movement of the pacific plate into the aleutian trench when collision ceased around 2500 ma ago it is likely that the great dyke and other complexes intruded along release fractures formed at right angles to the compression

evidence for metamorphic and igneous charnockites in the southern marginal zone of the limpopo belt abstract metamorphic and igneous charnoenderbite and enderbite can be distinguished on the basis of field relations and bulk rock geochemistry in the southern marginal zone of the limpopo belt termed here metamorphic charnockite and igneous charnockite respectively both varieties are petrographically similar they are massive homogeneous rocks which formed after the d1 fabric forming event associated with the limpopo orogeny and they at least experienced the entire retrograde portion of that orogeny the metamorphic charnockite probably developed first during peak metamorphism m1 and is often closely associated with a banded orthopyroxene bearing quartzo feldspathic gneiss the baviaanskloof gneiss the metamorphic charnockite baviaanskloof gneiss relations in the field are those of quiescent obliteration of foliation and the charnockite is often characterized by the presence of relict banding the chemical composition of the metamorphic charnockite is identical with that of the associated baviaanskloof gneiss the igneous charnockite locally retaining igneous textures is characterized by intrusive relationships with the baviaanskloof gneiss and in most cases by chemical compositions that are distinct from those of the older unit available geochronological data indicate that the metamorphic charnockite or its protolith formed at or after 2715 ma ago significantly before emplacement of the igneous charnockite of the matok complex at 2671 ma emplacement of igneous charnoenderbite and enderbite in the rest of the southern marginal zone and in the matok complex was not synchronous but occurred respectively during and after peak metamorphism m1 metamorphic charnockite with preserved relict banding probably formed by in situ biotite dehydration to orthopyroxene in the presence of a fluid phase with locally different activity of co2 h2o dilution through extraction of a granitic melt is considered to be unfeasable because of similar bulk chemistry the igneous charnockite crystallized from calc alkaline magmas derived by partial melting of lower crustal rocks similar to some phases of the baviaanskloof gneiss

geologic observations across a tectono metamorphic boundary in the babangu area giyani sutherland greenstone belt south africa abstract a narrow corridor across the tectono metamorphic transition between low grade and high grade granite greenstone terrains respectively from the giyani also known as the sutherland greenstone belt into the hydrated granulites of the southern marginal zone of the limpopo belt has been mapped in detail the two terrains are separated by a sharp tectonic mylonitic break the low grade greenstone terrain to the south of this break is part of a southward dipping flattened antiformal thrust stack and is thus probably allochtonous regional variation of the dominant tectonic fabric is from shallow to steep this is interpreted to be due to progressive deformation during northward directed tectonic transport the greenstone belt may lie on a flattish southward dipping sole thrust lubricated by synkinematic granitoids in the foot and hanging walls the northern margin of the greenstones is abruptly terminated by a late steeply oriented shear zone defined by mylonites north of the shear zone high grade granite greenstone terrain rocks hydrated granulites have been juxtaposed against and across the low grade greenstone terrain the tectonic break is either part of a transcurrent fault or a lateral ramp as part of a late southward directed overthrust system although the rocks of the low grade terrain have retained their early tectonic fabrics their metamorphic character more reflects that of the overriding hydrated granulites

petrogenesis of the magmatic complex at mount ascutney vermont usa the ascutney mountain complex of eastern vermont usa is a composite epizonal pluton of genetically related gabbro to granite intrusives nd isotopic data are reported for mafic rocks granites and nearby country rock the parental mafic magma producing the complex 122 m y ago had 87sr 86sr 0 7039 143nd 144nd 0 512678 nd 3 8 and 18o 6 1 indicating a mantle source with time integrated lithophile element depletion uniform initial radiogenic isotope ratios for granites which are undistinguishable from those for the most primitive gabbro suggest that the granite magma evolved from the mafic magma without crustal contamination and that the increase in 18o to about 7 8 is the result of fractional crystallization mafic rocks show a large range in initial 143nd 144nd ratio from about 0 51267 to 0 51236 nd 3 7 to 2 5 which is correlated with elevated 87sr 86sr ratios and 18o these data substantiate the production of mafic lithologies by fractional crystallization of the parental magma accompanied by assimilation of up to about 50 crust the local country rocks include gneiss and schist and assimilation involved representatives of both rock types the isotopic and chemical relationships preclude derivation from a single batch of magma undergoing contamination and indicate that a large magma body at depth evolved largely by fractionation with batches of melt issued from this chamber being variably contaminated at higher levels or at the level of emplacement

pbsl dating of garnet and staurolite constraints on the paleoproterozoic crustal evolution of the eastern block north china craton abstract it is well accepted that the north china craton formed by assembly of the eastern and western blocks along the trans north china orogen with cratonization in the paleoproterozoic 1 85 ma however there is controversy about the formation of the eastern and western blocks themselves especially the timing of cratonization of the eastern block the jiao liao ji orogenic belt is the most important paleoproterozoic orogenic belt in the eastern block and consists mainly of the north and south liaohe groups and the liaoji granitoids the timing of metamorphism of the liaohe group is the key to understanding the evolution of the eastern block of the north china craton because the isotope systematics of metamorphic minerals can be used to date parts of the prograde thermal histories of metamorphic belts dating of minerals such as garnet and staurolite has the distinct advantage that time information can be related directly to the pressure temperature deformation history recorded by the same mineral in this paper we present results of precise 207pb 206pb stepwise leaching pbsl dating of garnet and staurolite from the metasedimentary rocks of the south liaohe group the primitive pb and radiogenic pb are selectively extracted from garnet and straurolite by leach steps and determined by multiple collector mc icp ms the garnet and staurolite record two groups of 207pb 206pb ages at 1 93 1 91 ga and 1 86 ga respectively these constrain the time of peak metamorphism of the liaohe group during collision between the longgang block in the north and the liaonan nangrim block in the south and a subsequent post collisional event respectively our data for the south liaohe group and previously published data for the liaoji granite combined with key lithological structural and metamorphic information indicate that the cratonization of the eastern block of the north china craton occurred at 1 93 1 91 ga

geochronology of the hout river shear zone and the metamorphism in the southern marginal zone of the limpopo belt southern africa abstract in this paper monazite u pb and zircon evaporation dates stepleaching pb pb results on garnet staurolite and kyanite and hornblende ar ar data are presented which constrain the timing of granulite facies metamorphism in the southern marginal zone of the limpopo belt and its thrusting onto the kaapvaal craton the southern marginal zone of the limpopo belt is considered to be a lower crustal equivalent of the northern kaapvaal craton granulite exhumation is associated with southward thrusting along the hout river shear zone which is a set of thrust and strike slip shear zones zircon ages for the matok intrusive complex which was emplaced within the zone during this thrusting charno enderbites 2671 4 ma granodiorites between 2667 and 2664 ma have previously been interpreted as evidence for rapid exhumation of the southern marginal zone within only 7 ma we have obtained a u pb date of 2691 7 ma for monazite from the bandelierkop quarry in the southern marginal zone interpreted as the age of high grade metamorphism a single zircon evaporation pb pb date of 2643 1 ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation anatexis of metapelitic xenoliths within the matok intrusive complex was dated at 2663 4 ma by u pb on monazite indistinguishable from existing zircon ages for this complex pb pb step leaching dates obtained on synkinematically grown garnet 2691 20 ma staurolite 2712 37 ma and kyanite 2672 51 ma from the khavagari hills in the giyani greenstone belt in the immediate footwall of the hout river shear zone indicate that early thrusting was contemporaneous with peak metamorphism in the southern marginal zone ar ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations probably reflecting repeated or continuous tectonic activity of the hout river shear zone up to about 2600 ma

exhumation of limpopo central zone granulites and dextral continent scale transcurrent movement at 2 0 ga along the palala shear zone northern province south africa abstract the palala shear zone is part of a major tectonic lineament in southern africa which extends over 1000 km from central botswana to the soutpansberg in south africa the lineament is situated between the central zone of the limpopo belt and the kaapvaal craton at its main exposure in the koedoesrand window 50 km e of ellisras northern province the palala shear zone comprises a c 15 km wide mylonite zone which can be subdivided into a northern central and southern domain this study addresses the tectono metamorphic history of the palala shear zone in the koedoesrand window and its role in the tectonics of the limpopo belt in order to do this structural and petrographical work was combined with geochronology age data from relic mineral parageneses show that metapelites from the central domain underwent granulite facies metamorphism around 2 6 ga however because of an intense proterozoic overprint field relationships of the archean event are largely disrupted during a second high grade event at c 2 0 ga granulite facies metamorphism was reached in the northern domain and the adjacent limpopo central zone charnockites were emplaced in the northern domain after 2 1 ga and were affected by retrograde amphibolite and greenschist facies metamorphism around 1 97 ga post orogenic transcurrent faulting under sub greenschist facies conditions characterizes the central and southern domains field observations and microstructural investigations indicate a dextral shear sense for the retrograde mylonitisation these findings document that the palala shear zone played an important role during the exhumation of the proterozoic granulites in the limpopo central zone associated with a transpressive orogeny between 2 05 and 1 95 ga low grade post orogenic transcurrent faulting in the palala shear zone at c 1 9 ga may be associated with the transtensional opening of the soutpansberg and palapye grabens on the basis of the tectono metamorphic history in the koedoesrand window and regional geophysical geochemical and sequence stratigraphic data the palala shear zone is interpreted as a proterozoic suture zone along which the archean kaapvaal and zimbabwe provinces were juxtaposed

redistribution of rare earth elements thorium and uranium over accessory minerals in the course of amphibolite to granulite facies metamorphism the role of apatite and monazite in orthogneisses from southwestern norway the amphibolite to granulite facies transition has been studied in a high k calc alkaline hornblende biotite k feldspar megacrystic augen gneiss series from the rogaland vest agder rog va sector of the sveconorwegian province southwest norway hornblende begins to break down mainly to clinopyroxene at the cpx in isograd and biotite mainly to orthopyroxene at the opx in isograd abbreviations for minerals following kretz 1983 the magmatic accessory mineral association comprises titanite and allanite which begin to break down before the cpx in isograd titanite is preserved as relict inclusions in other minerals at higher grade monazite and thorite formed in the breakdown of allanite monazite abundance reaches a maximum between the cpx in and opx in isograds the middle and heavy rare earth element m hrees except eu content of apatite steeply increases with increasing metamorphic grade and is directly correlated to the decrease of the modal abundances of titanite hornblende and biotite the u contents of apatite are low and do not increase with metamorphic grade the light rare earth elements lrees and th content of apatite are not correlated to the breakdown of allanite around the cpx in isograd but increase around the opx in isograd a simplified equation is proposed for monazite crystallisation in the vicinity of the cpx in isograd that accounts for the allanite titanite and hornblende breakdowns and the m hree substitution in apatite 3 m hree 2o3in hornblende and litanite 3 lree 2in allaniteo3 2ca5 po4 3apatite f oh 6sio2quartz 6 lree pmonaziteo4 2ca2 m hree 3 sio4 3 f oh lessingite in apatite 6caoin plagioclase at the opx in isograd the increase of lrees and th content of apatite results either from the breakdown of some monazite 3 lree po4monazite 3sio2quartz 4caoin plagioclase f2 h2o fluid ca5 po4 3 f oh apatiteca2 lree 3 sio4 3 f oh less ingite in apatite or the breakdown of the remaining allanite 3 lree 2o3in allanite 6sio2quartz 4caoin plagioclase f2 h2o fluid 2ca2 lree 3 sio4 3 f oh lessingite in apatite the release of fluorine from the breakdown of biotite at the opx in isograd may increase apatite stability relatively to monazite in the granulite facies when compared to amphibolite facies the granulite facies augen gneisses do not show any u or th depletion this means that changes in the accessory mineral associations and the simultaneous breakdown of hydrous minerals at the amphibolite granulite facies transition do not inevitably result in depleted granulite facies rocks but rather to an isochemical element redistribution the coexistence of small amounts of metamorphic monazite and of relict inclusions of titanite in upper amphibolite facies augen gneisses suggests that high k calc alkaline orthogneisses are a suitable material to date with the u pb method prograde path of amphibolite facies regional metamorphism on monazite and the cooling path on titanite

deep crustal shear zones high grade tectonites and associated metasomatic alteration in the limpopo belt south africa implications for deep crustal processes large thrust sheets of complexly folded d 1 migmatitic granulites bounded by a regional system of anastomosing ductile shear zones d 2 characterize the southern marginal zone of the limpopo belt tectonites within these shear zones are subdivided into primary straight gneisses with preserved textures that can unambiguously be referred to the d 2 exhumation stage annealed straight gneisses that recrystallized during post d 2 cooling and classic mylonites that post date the limpopo orogeny primary straight gneisses within these shear zones contain textural and mineralogical features that suggest they developed by a shear related process of grain size reduction and fabric formation due to decreasing pressure and temperature under granulite facies conditions this process is evident from the synkinematic replacement of garnet by cordierite and hypersthene symplectites the resultant asymmetric sigmoidal corona textures enhance the shear fabric in contrast pelitic granulite

monazite u pb dating of staurolite grade metamorphism in pelitic schists a study of the occurrence of and relations between rare earth element ree minerals in pelitic schists indicates that monazite forms at or near the p and t of the staurolite isograd samples at staurolite grade from the silurian perry mountain formation in the rumford quadrangle of maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites the bulk chemistry of the metapelites as seen in the major element abundances and ree patterns does not vary significantly across the study area thus the appearance and disappearance of ree phases is assumed to reflect changes in metamorphic grade in a sample from the biotite zone scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 m scale the texture suggest that metastable detrital monazite breaks down distributing its ree components to allanite from samples below staurolite grade in which monazite is not present our observations suggest that rees are partitioned into allanite at or near the staurolite isograd monazite forms as a metamorphic mineral initiating its role as a geochronometer garnet biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite 525 c 25 c at 3 1 0 25 kbar a total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated each date is remarkably concordant even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes calculations indicate insignificant th disequilibrium in these monazites the conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade two distinct metamorphic events are resolved one at around 400 ma and one at about 370 ma the latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 ma this work suggests that in addition to dating plutonism and high grade metamorphism monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists

a crystal chemical basis for pb retention and fission track annealing systematics in u bearing minerals with implications for geochronology this study develops an empirical crystal chemical framework for systematizing the kinetics of pb loss and fission track annealing in u bearing minerals ionic porosity z the fraction of a mineral s unit cell volume not occupied by ions potentially accounts for kinetic behavior by monitoring mean metal oxygen bond length strength various tests of a general kinetics porosity relationship are presented based upon diverse mineral data including 1 pb diffusion parameters 2 measured closure temperatures tc for fission track annealing and 3 retentivities of both pb and fission tracks from apparent age data every kinetic parameter including tc and mineral age for both the u pb and fission track systems is inversely correlated with z within the sub assemblage zircon z 29 titanite 34 and apatite 38 assuming a diffusional closure model pb isotopic transport phenomena are described by a tc z scale calibrated with field based tc data for titanite 680 20 c and apatite 500 c extrapolation of this scale yields tc estimates for the following minerals staurolite tc 1060 c z 25 garnet 1010 c 26 5 zircon 900 c monazite xenotime and epidote 750 c 32 and ca clinopyroxene 670 30 c 34 1 depending on composition these empirical results imply that a u pb pb date for staurolite or garnet records the time of mineral growth not post growth isotopic closure as also concluded in recent field studies because z systematizes fission track annealing this recrystallization process like volume diffusion must also be rate limited by the strength of chemical bonds the extent to which other recrystallization processes are likewise rate limited is important to u pb geochronology because they potentially compete with diffusion as mechanisms for pb isotopic resetting in nature

u pb geochronology of zircon and monazite from mesoproterozoic granitic gneisses of the northern blue ridge virginia and maryland usa abstract mesoproterozoic granitic gneisses comprise most of the basement of the northern blue ridge geologic province in virginia and maryland lithology structure and u pb geochronology have been used to subdivide the gneisses into three groups the oldest rocks group 1 are layered granitic gneiss 1153 6 ma hornblende monzonite gneiss 1149 19 ma porphyroblastic granite gneiss 1144 2 ma coarse grained metagranite about 1140 ma and charnockite 1145 ma these gneisses contain three proterozoic deformational fabrics because of complex u pb systematics due to extensive overgrowths on magmatic cores zircons from hornblende monzonite gneiss were dated using the sensitive high resolution ion microprobe shrimp whereas all other ages are based on conventional u pb geochronology group 2 rocks are leucocratic and biotitic varieties of marshall metagranite dated at 1112 3 ma and 1111 2 ma respectively group 3 rocks are subdivided into two age groups 1 garnetiferous metagranite 1077 4 ma and quartz plagioclase gneiss 1077 4 ma 2 white leucocratic metagranite 1060 2 ma pink leucocratic metagranite 1059 2 biotite granite gneiss 1055 4 ma and megacrystic metagranite 1055 2 ma groups 2 and 3 gneisses contain only the two younger proterozoic deformational fabrics ages of monazite separated from seven samples indicate growth during both igneous and metamorphic thermal events however ages obtained from individual grains may be mixtures of different age components as suggested by backscatter electron bse imaging of complexly zoned grains analyses of unzoned monazite imaged by bse and thought to contain only one age component from porphyroblastic granite gneiss yield ages of 1070 1060 and 1050 ma the range of ages of monazite not reset to a uniform date indicates that the grenville granulite event at about 1035 ma did not exceed about 750 c lack of evidence for 1110 ma growth of monazite in porphyroblastic granite gneiss suggests that the short hill fault might be a grenvillian structure that was reactivated in the paleozoic the timing of proterozoic deformations is constrained by crystallization ages of the gneissic rocks d1 occurred between about 1145 and 1075 ma or possibly between about 1145 and 1128 ma d2 and d3 must be younger than about 1050 ma ages of mesoproterozoic granitic rocks of the northern blue ridge are similar to rocks in other grenville terranes of the eastern usa including the adirondacks and hudson highlands however comparisons with conventional u pb ages of granulite grade rocks from the central and southern appalachians may be specious because these ages may actually be mixtures of ages of cores and overgrowths

measurements of transient ground movements below a ballasted railway line this paper presents the results of a detailed investigation into the ground deformations that occur under a railway line during the passage of a train four horizontal boreholes were installed at different depths below a ballasted railway track ground deformations were measured using geophones at set distances from the centreline of the track within each borehole the results show vertical displacements reducing with depth from a maximum at the sleeper sleeper displacements are dominated by pairs of bogies at the ends of adjacent wagons which have a frequency of loading 1 hz although the effects of individual bogies 2 hz and axles 6 hz are also apparent higher loading frequencies attenuate with depth so that at a depth of 0 780 m below the sleeper soffit no axles are visible within the displacement data and by a depth of 1 98 m only the combined effect of pairs of adjacent bogies is apparent in contrast longitudinal horizontal motion is greatest at a depth of 0 78 m below the sleeper soffit

pb u and th diffusion in natural zircon zircon zrsio4 is one of the most widely used minerals for determining the age origin and thermal history of rocks by u th pb geochronology but the parameters describing the solid state volume diffusion rates of these elements in natural zircon which are themselves important for establishing the limits on the applicability of zircon for geochronological studies have remained poorly quantified1 this is because of the measurement difficulties associated with the low p p m concentrations and low diffusion rates of these elements in natural zircon and the chemical and physical heterogeneity present in most crystals here we present direct measurements of the uranium thorium and lead loss from a thermally treated gem quality natural zircon and show that lead diffuses much faster than uranium or thorium we find that the u th pb isotopic system in natural zircon will typically have a closure temperature greater than 900 c which explains why zircon is apparently such a robust geochrometer and is capable of remaining isotopically closed through extended periods of high grade metamorphism and partial melting of the host rock

unraveling the record of successive high grade events in the central zone of the limpopo belt using pb single phase dating of metamorphic minerals abstract dating of relic metamorphic assemblages can provide important information about the timing and character metamorphic grade and or pt evolution of early high grade episodes in polymetamorphic provinces using pb stepwise leaching of metamorphic silicates we have dated multiple granulite facies metamorphic episodes in the central zone cz of the limpopo belt ages of 2 52 ga were obtained from sillimanite and cogenetic garnet and ages of about 2 01 ga from titanite garnet and clinopyroxene together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data these results lead us to a reinterpretation of the tectono metamorphic history of the cz three distinct high grade events at about 3 2 3 1 ga 2 65 2 52 ga and 2 0 0 05 ga are recognized each of these is suggested to correspond to a tectonic episode of distinct character a for the 3 2 ga event magmatic activity can mainly be identified best represented for example by the sand river gneisses or the messina layered intrusion the field relationships concerning the tectonometamorphic history of this early archean event are largely erased by at least two high grade metamorphic overprints b late archean 2 6 2 52 ga low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism the anticlockwise p t evolution of these granulites probably reflects deep crustal processes associated with magmatic underplating or in plating contemporaneous with vertical crustal growth of the zimbabwe craton around 2 6 ga c during the proterozoic event 2 05 1 95 ga tectonic thickening was caused by the collision of the kaapvaal and zimbabwe cratons the cz was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise p t evolution the structural metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny

the behaviour of nd and pb isotopes during 2 0 ga migmatization in paragneisses of the central zone of the limpopo belt south africa and botswana we have carried out major and trace element analyses and nd pb isotope measurements on different components of two migmatite outcrops in the limpopo belt lose quarry botswana and palala south africa in order to assess the effect of migmatization during a high grade metamorphic event on sm nd systematics and nd model ages the outcrops studied are located in the mahalapye tshipise straightening zone at the south western part of the limpopo central zone close to its contact with the kaapvaal craton leucosomes of peraluminous granitic composition were formed by incongruent biotite melting in an fe rich metagreywacke lose quarry and in a metapelite palala respectively monazite u pb and garnet sm nd ages yield close to 2 0 ga for the high grade event confirming other recent work in the central zone the chemical compositions of the main mineral phases feldspar biotite and garnet indicate minimum melt formation at t 750 c and low pressure isocon plots and pb isotope data indicate that the partial melts are locally derived however lil trace element distribution between the leucosomes and paleosomes indicate the incomplete equilibration of the nascent melts with the residual feldspars and biotite in the paleosomes sm nd results on leucosomes and paleosomes of the metagreywacke migmatite show considerable scatter when calculated back to 2 0 ga the paleosomes fall in a narrow range of nd 4 and give tdm model ages between 2 45 and 2 66 ga the nd values of leucosomes are on average four units lower causing them to have apparent tdm ages of 2 7 2 92 ga on average 250 ma older than those of the paleosomes the ree th p2o5 and zr budgets of paleosomes and leucosomes indicate dominance of the lree by monazite and the hree by apatite incorporation of proportionally more monazite than apatite into the melt is inferred as the cause for the nd isotope discrepancy and hence the uncertainty of the average protolith mantle derivation age in contrast pb pb results define an apparent age of 2025 29 ma reflecting nearly full pb isotope exchange in the case of the palala metapelitic migmatite the nd values at 2 0 ga of all but two migmatitic constituents cluster between 7 and 9 5 pointing out extensive nd isotope equilibration during migmatization whereas the pb isotopes yield an errorchron date of 2311 65 ma indicating incomplete pb isotope exchange here garnet dominates the hree distribution whereas the lree are controlled by feldspar and monazite considering that nd isotope equilibration was nearly reached during migmatization nd model ages on the whole rocks represent a good approximation of those of the original protolith 3 0 3 2 ga in the regional context the pb isotope data from the lose quarry locality combined with the nd model age uncertain but within the range of 2 5 2 9 are reminiscent of the northern marginal zone of the limpopo belt and the southern zimbabwe craton whereas the nd and pb isotope data from the palala locality are typical for the limpopo southern marginal zone and the northern kaapvaal craton

deep crystal response to continental collision the limpopo belt of southern africa the limpopo belt is an example of an archean 2700 ma continental collisional zone in which the thickened crust 70 km has virtually attained equilibrium with the adjacent non thickened plates by a process of upward movement and lateral spreading of the underthrust high grade metamorphosed plates

cretaceous reactivation and intensified erosion in the archean proterozoic limpopo belt demonstrated by apatite fission track thermochronology abstract cratons are generally assumed to be regions of long lasting tectonic stability in particular the study of the phanerozoic exhumation history of cratons has been largely hampered by the scarcity of suitable stratigraphic controls onshore this fact is even more pronounced in terranes lacking mesozoic or younger penetrative structural fabrics and metamorphic overprinting our study in the limpopo belt shows that modern apatite fission track thermochronology provides a hitherto unavailable perspective in the study of these rocks and has profound implications for the crustal evolution of the zimbabwe craton apatite fission track data from 35 samples taken along two transects in the southern edge of the zimbabwe craton and in the central zone of the limpopo belt suggest that extensive regions experienced kilometer scale exhumation in two discrete events as recently as the cretaceous the first occurred at around 130 ma and the second at around 90 ma basin subsidence and sedimentation loads on the mozambique margin support the timing of these events and provide strong indications of the source and pathways for the eroded material further the results indicate that young and old surfaces in a geomorphological sense may be structurally juxtaposed in regions of high elevation in zimbabwe this is contrary to early ideas of surface chronologies based on summit accordances or invoking pediplanation

evolution of the northern high grade margin of the kaapvaal craton south africa the northern kaapvaal craton is divided into three distinctive crustal zones 1 the high grade southern marginal zone smz of the limpopo belt in the north 2 a transitional zone of technically stacked metamorphic rocks which includes the area from the sutherland belt in the north to the pietersburg and murchison belts in the south and 3 a low grade granite greenstone terrane farther to the south the change across the smz boundary is expressed by an overall increase in the grade of metamorphism a strip of retrogression related to a zone of thrusting abrupt changes in intensity of deformation rock densities magnetic lineament patterns and isotopic ages in compositionally similar rocks regional metamorphism in the northern kaapvaal craton is demonstrably not a continuum in the sense that the highest grade rocks underwent all earlier stages of metamorphism rather that rapid local tectonic processes must have occurred a proposed evolutionary path for the smz was initiated with crustal thicke

thirty sixth canadian geotechnical colloquium advances in visualization of geotechnical processes through digital image correlation1 digital image correlation dic is an image processing technique that calculates fields of incremental displacement by comparing two digital images and locating numerous small regions in both image

an isotopic and geochemical study of the northern kaapvaal craton and the southern marginal zone of the limpopo belt are they juxtaposed terranes abstract to test existing lateral accretion models for the archean kaapvaal craton in south africa metapelites and leucocratic granitoids from two different provinces the granulite facies southern marginal zone of the limpopo belt and the northern part of the adjacent greenschist to amphibolite facies kaapvaal craton have been studied both provinces resemble each other in terms of isotope and trace element characteristics they show a general depletion of incompatible elements independent from the grade of metamorphism this observation conflicts with theories involving mobilisation during granulite facies metamorphism a further common fingerprint is the unusually high cr and ni concentrations in the metapelites in addition they define a very narrow range of nd model ages 2 9 3 05 ga constraining the average crustal age of their cratonic source region the granitoid gneisses show somewhat larger variations of nd model ages indicating some age heterogenity within the crust however the data from both provinces completely overlap the southern marginal zone and the kaapvaal craton also show uniform low 207 pb 204 pb relative to their 206 pb 204 pb ratios that differ clearly from other southern african provinces these results support the view that the southern marginal zone represents a part of the kaapvaal craton exposed at a deeper crustal level it rules out a hypothesis that both provinces are separate amalgamated terranes furthermore there is very little difference between granitoids and metapelites which indicates that the latter largely derived from the former the exposure of the granulite facies metapelites of the southern marginal zone at the same level as the lower grade metamorphic rocks of the kaapvaal craton indicates uplift along a sharp tectonic boundary

single mineral pbpb dating a new stepwise leaching procedure applied to ca bearing silicates enables the selective recovery of radiogenic and common pb from a mineral and thus makes single phase pbpb dating possible this permits independent dating of rock forming minerals within a paragenetic sequence and the direct combination of age data with petrological results furthermore the stepwise leaching technique has the advantage over multi phase pbpb dating in that it does not require initial pb isotope equilibrium among the phases but may be used to test it the presence of microscopic solid inclusions which complicate dating of single phases if they are not in equilibrium with the host can be traced via the behaviour of the 208pb 204pb ratio through the various leaching steps this constitutes a major advantage over conventional bulk phase pbpb dating in addition the precision of an isochron age can be improved relative to that of the bulk mineral approach since the 207pb 204pb and 206pb 204pb data spread is significantly enhanced therefore the technique may both be used to date younger phases and or minerals with less favourable u pb ratios and to extend the applicability of self consistent p t t modelling

behaviour of saturated railway track foundation materials during undrained cyclic loading this paper presents results of a series of hollow cylinder tests carried out to investigate the undrained behaviour of saturated railway track foundation materials during cyclic loading involving p

field performance of a peat railway subgrade reinforced with helical screw piles rail tracks on peat subgrades can experience significant deflections which have led to derailments a potential ground stabilization strategy is to use screw piles to reduce rail displacements however limited research has been undertaken to investigate the effect of these piles and their performance under cyclic train loading a field site was instrumented and monitored before and after screw pile installation the piles were instrumented with strain gauges piezometers were installed in the peat and high speed cameras were used to measure track and subgrade displacements the load carried by each instrumented pile was approximately 50 60 kn three weeks after installation but reduced to approximately 30 kn after six months although one year s worth of piezometric data was collected before installation of the piles it is difficult to conclusively attribute the changes in pore pressures to the piles particularly because the piezometric data collected after the pile installation was only available for t

40ar 39ar age of plagioclase from acapulco meteorite and the problem of systematic errors in cosmochronology acapulcoites consistently yield some of the oldest 40ar 39ar ages reported for any meteorites all indistinguishable from the age of 4510 22 ma previously determined for whole rock samples of acapulco these ages are significantly younger at face value than the 4557 2 ma pb pb age reported for phosphates which dates cooling through 550 c metallographic evidence for rapid cooling between 650 and 350 c retention of 129xe in apatite and the absence of shock features all suggest that the 40ar 39ar ages might be expected to be more consistent with the pb pb age new 40ar 39ar data from plagioclase determined to be the sole k rich phase in acapulco yield a high precision isochron age of 4507 18 ma 2 based on currently accepted decay constants and standards but neglecting uncertainties in these quantities these data allow unambiguous association of the 40ar 39ar age with a closure temperature near 300 c and preclude the possibility of artifacts due to 39ar recoil low temperature alteration products or inherited pre solar grains the 40ar 39ar plagioclase age recalculated with decay constants believed to be more accurate than those conventionally used is 4554 ma weakening the basis of a previously inferred dramatic decrease in cooling rate shortly after closure of the pb pb system in phosphate systematic errors in 40ar 39ar dating associated chiefly with 40k decay constants and standards must be redressed before the method can be applied to high resolution inference of early solar system evolution similar limitations affect ages based on other long lived radionuclides such as 147sm

juvenile crust formation in the northeastern kaapvaal craton at 2 97 ga implications for archean terrane accretion and the source of the pietersburg gold abstract combined u pb and lu hf isotope data of detrital and magmatic zircon grains provide evidence that the murchison greenstone belt ne kaapvaal craton south africa hosts an important archean suture zone which primarily formed by a collision 2 97 ga ago between a back arc system to the north and an evolved terrane the proto kaapvaal craton to the south this model is supported by the observations that igneous rocks of the northern terrane comprising mafic rocks and tonalites of the rooiwater complex and felsic volcanics of the rubbervale formation were both emplaced at about 2 97 ga and show highly superchondritic hft of 4 4 to 5 1 in contrast u pb hf isotope data of detrital zircon grains from quartzitic schists of the murchison unit and the la france formation both part of the southern terrane provide evidence for a southern provenance which was affected by granitoid intrusions accompanied by crust re working at 3 53 3 42 ga hft 1 8 to 4 8 3 30 3 20 ga hft 1 8 to 6 3 and 3 13 3 05 ga hft 1 3 to 5 6 pointing to a connection with the barberton greenstone belt and swaziland a spatial separation of the two terranes prior to 2 97 ga is furthermore supported by the facts that the youngest detrital zircon grains of the southern terrane having ages between 2 99 and 2 97 ga show subchondritic hft between 6 5 and 1 5 in contrast to the highly superchondritic hft obtained from the contemporaneous magmatic rocks of the northern terrane the juvenile character of the rooiwater and rubbervale magmatic rocks along with the occurrence of vms deposits makes them a likely candidate for the source of the gold pyrite bearing conglomerates exposed in the adjacent pietersburg greenstone belt

shear zones of the limpopo belt and adjacent granitoid greenstone terranes implications for late archaean collision tectonics in southern africa abstract granulite metamorphism in the 2 65 ga limpopo belt of southern africa is related to crustal thickening due to late archaean collision tectonics the central zone of the belt is a major thrust sheet of distinctive lithological structural and isotopic character separated from adjacent terranes by crustal scale shear zones and the northern and southern marginal zones are up thrust deep level granulite facies terranes of the adjacent cratons geometrically the marginal zones are mirror images and the limpopo belt is essentially symmetrical in n s section conventionally regarded as a n s collisional orogeny the gross structural and lithological differences between the central zone and the marginal zones kinematic data from shear zones and geophysical data showing the three dimensional geometry of the central zone are here used to suggest that the direction of emplacement of the central zone was also the collision direction this was from northeast to southwest with the upthrust outwardly directed marginal zones developing in mechanical response to this collision

partial melting and the origin of metapelitic granulites in the southern marginal zone of the limpopo belt south africa abstract two potentially realistic models exist to explain the origin of the granulite facies rocks of the southern marginal zone i co2 fluxing inducing the necessary high temperature dehydration reactions and ii melting of hydrous minerals to yield an h2o undersaturated granitic melt in equilibrium with a dehydrated granulitic assemblage petrographic and field evidence from the bandelierkop quarry in the southern marginal zone indicate a sequence of melting reactions involving muscovite and biotite relict textural evidence indicates that the reaction responsible for the generation of most of the melt bi sil qz plag melt ga ksp was fluid absent the melting reactions can be shown to have occurred during the prograde metamorphic path with the highest temperature reaction bi qz plag hy cord melt occurring at approximately peak metamorphic conditions 850 c and less than 9 5 kbar this suggests a fluid absent mode of granulite formation a subsequent retrograde reaction cord ksp h2o ky qz related to a rise in h2o during in situ crystallisation of the melt did not destroy the granulite assemblages and hypersthene remains stable in areas of the southern marginal zone that record pressures of greater than 6 5 kbar during retrogression

zircon and monazite petrochronologic record of prolonged amphibolite to granulite facies metamorphism in the ivrea verbano and strona ceneri zones nw italy abstract in order to improve the understanding of thermal tectonic evolution of high grade terranes we conducted a systematic study of textures ree content and u pb ages of zircon and monazite grains extracted from migmatitic metapelites across the amphibolite to granulite facies metamorphic gradient exposed in the ivrea verbano and strona ceneri zones italy this study documents the behaviour of these accessory minerals in the presence of melt the absence of relict monazite grains in the metasediments and the gradual decrease in the size of inherited zircon grains from amphibolite to granulite facies cores indicate partial to total dissolution of accessory minerals during the prograde path and partial melting the retrograde path is marked by i growth of new zircon rims r1 and r2 around inherited cores in the mesosome ii crystallisation of stubby zircon grains in the leucosome especially at granulite facies and iii crystallisation of new monazite in the mesosome stubby zircon grains have a distinctive fir tree zoning and a constant th u ratio of 0 20 together these features reflect growth in the melt conversely the new zircon grains with r1 rims have dark prismatic habits and th u ratios in the ivrea verbano zone the interquartile range iqr of u pb ages on zircon and monazite are interpreted to represent the length of zircon and monazite crystallisation in the presence of melt accordingly they provide an indication on the minimum duration for high temperature metamorphism and partial melting of the lower crust 20 ma and 30 ma in amphibolite and granulite facies respectively in amphibolite facies zircon crystallisation between 310 and 294 ma iqr is interpreted to reflect metamorphic peak condition and earlier retrograde history conversely monazite crystallisation between 297 and 271 ma iqr reflects cooling under 750 c to a temperature close to the solidus in granulite facies zircon crystallisation between 295 and 265 ma iqr is interpreted to reflect high temperature conditions which were attained after peak of metamorphism during isothermal decompression and subsequent cooling under 850 950 c the observed decrease of u pb ages in metamorphic zircon and monazite from amphibolite to granulite facies i e from the middle to the lower crust is interpreted to record slow cooling and crystallisation of the variscan orogenic root at the transition from orogenic collapse to opening of the tethys ocean

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

p t t development of archaean granulites in varpaisjarvi central finland ii dating of high grade metamorphism with the u pb and sm nd methods exposed blocks of lower crustal archaean granulites separated by palaeoproterozoic faults occur near the western boundary of the karelian craton in central finland the southwesternmost granulite block in the study area the jonsa block differs from other granulites by having younger zircon u pb and sm nd tdm model ages the u pb ages on zircons and monazites from leucosomes and mesosomes of mafic and pelitic granulites in the jonsa block are ca 2 63 ga which is interpreted as the age of granulite metamorphism igneous enderbites which make up a considerable part of the bedrock in granulite blocks are ca 50 ma older outside the jonsa block both conventional and ion probe zircon ages of mesosomes of migmatitic mafic granulites are 3 2 3 1 ga which are the protolith ages sm nd tdm model ages of the mafic granulites are 2 7 2 9 ga in the jonsa block but ca 3 2 ga in the other granulites the age differences are interpreted to represent terrane accretion that juxtaposed 3 2 ga rocks next to younger rocks all granulites were metamorphosed in similar pt conditions at 2 63 ga granulite metamorphism affected the whole area but was not able to reset zircons in older rocks in all blocks the sm nd garnet whole rock ages are younger ranging from 2 48 2 59 ga this reflects relatively low closure temperature of the sm nd system in garnet in these rocks the u pb age of a palaeoproterozoic dolerite which cuts the granulites is 2 3 ga close to the k ar age of amphibole in a retrogressed fracture

geochronological problems related to polymetamorphism in the limpopo complex south africa abstract the integration of new and published geochronologic data with structural magmatic anatectic and pressure temperature p t process information allow the recognition of high grade polymetamorphic granulites and associated high grade shear zones in the central zone cz of the limpopo high grade terrain in south africa together these two important features reflect a major high grade d 3 m 3 event at 2 02 ga that overprinted the 2 63 ga high grade neoarchaean d 2 m 2 event characterized by sw plunging sheath folds these major d 2 m 2 folds developed before 2 63 ga based on u pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high grade gneissic fabric the d 3 m 3 shear event is accurately dated by u pb monazite 2017 1 2 8 ma and pbsl garnet 2023 11 ma age data obtained from syntectonic anatectic material and from sheared metapelitic gneisses that were completely reworked during the high grade shear event the shear event was preceded by isobaric heating p 6 kbar and t 670 780 c which resulted in the widespread formation of polymetamorphic granulites many efforts to date high grade gneisses from the cz using pbsl garnet dating resulted in a large spread of ages 2 0 2 6 ga that reflect the polymetamorphic nature of these complexly deformed high grade rocks

tectonic model for the evolution of the limpopo belt abstract constraints on which any model for the tectonic evolution of the limpopo belt must be based include the following data crustal thickening to at least 65 km between 2700 and 2650 ma ago was responsible for the formation of the granulite terrane exposed in the limpopo belt today this crustal thickening probably resulted from the thrusting of the kaapvaal craton over the zimbabwe craton along the south dipping triangle tuli sabi shear zone other northwardverging thrusts related to this event occur in the kaapvaal craton south of the limpopo belt in the central and southern marginal zones and the kaapvaal craton this shortening is directed to the southwest while in the northern marginal zone it was directed to the north northwest peak metamorphism was superimposed on the thickened crust and then nearly isothermal decompression of several kbars occurred throughout the belt during this decompression rocks moved upward and spread outward onto the adjacent cratons from the zone of thickened crust along several inward directed shear zones creating a regional pop up high grade rocks were thrust over low grade rocks on the cratons producing along the terrane boundary retrograde metamorphism in the limpopo belt and prograde metamorphism on the cratons

isotopic and ree characteristics of the intrusive charnoenderbite and enderbite geographically associated with the matok pluton limpopo belt southern africa abstract intrusive charnoenderbite and enderbite in the southern marginal zone of the limpopo belt are genetically linked with peak metamorphism m1 during the 2700 ma to 2650 ma limpopo orogeny they occur geographically associated with and are intruded by the granodiorite and granite of the matok pluton which were emplaced between the response phase m2 and the rehydration phase m3 of that orogeny rb sr pb ree and sm nd elemental and isotopic data for whole rock samples and u pb data for populations of zircon suggest that the charnoenderbite and enderbite were intruded 2671 ma ago and were derived from a crustal precursor the granodiorite and granite of the matok pluton were intruded at approximately the same time between 2667 ma and 2664 ma ago they also were derived from a crustal precursor nd model ages and ree patterns for the charnoenderbite and enderbite and the granodiorite are distinct this relationship may imply that they are not cogenetic alternatively these rock types could be cogenetic if the magma from which they were derived was assimilating older crustal material as it evolved nd isotopic data indicate that the magma giving rise to the charnoenderbite and enderbite formed by partial melting of rocks compositionally similar to some portions of the baviaanskloof gneiss data for the granodiorite are consistent with the possibility that this unit formed in a similar manner from rocks compositionally similar to other portions of the baviaanskloof gneiss the close temporal relationship between the emplacement of the charnoenderbite and enderbite and the granitic rocks of the matok pluton indicates that the retrograde portion of the limpopo orogeny beginning 2670 ma ago occurred rapidly perhaps over 7 ma

shear zones bounding the central zone of the limpopo mobile belt southern africa abstract contrary to previously suggested north directed thrust emplacement of the central zone of the limpopo mobile belt we present evidence indicating west directed emplacement the central zone differs from the marginal zones in rock types structural style and isotopic signature and is an allochthonous thrust sheet it is bounded in the north by the dextral tuli sabi shear zone and in the south by the sinistral palala shear zone which are crustal scale lateral ramps published gravity data suggest that the lateral ramps are linked at depth and they probably link at the surface in a convex westward frontal ramp in the vicinity of longitude 26 30 e in eastern botswana two phases of movement the first between 2 7 and 2 6 ga and the second between 2 0 and 1 8 ga occurred on both the tuli sabi and the palala shear zones

discrete metamorphic events in the limpopo belt southern africa implications for the application of p t paths in complex metamorphic terrains essentially all models for the tectonic evolution of the high grade limpopo belt of southern africa invoke a single orogenic event at 2 7 ga involving the collision of the zimbabwe and kaapvaal cratons with a distinct central zone by implication rocks within the belt must have followed parallel pressure temperature time p t t paths the main reasons for this situation are the belief that syn to postdeformational granitic magmatism in the belt was coeval and the fact that p t paths are similar new geochronological data indicate however that this magmatism occurred at distinct times in different parts of the belt over a span of at least 70 m y in addition the data indicate that high grade metamorphism and associated tectonism in the central and southern marginal zones occurred at about 2 0 and 3 15 ga respectively therefore p t t paths of these zones were independent until their juxtaposition more recently than 2 0 ga

constraints on the form of the p t loop in the southern marginal zone of the limpopo belt south africa abstract a generalised clockwise p t path consisting of a prograde portion a high grade isothermal decompression portion and an essentially isobaric cooling portion has been suggested for the metamorphic evolution of the southern marginal zone previous studies have concluded that the high grade decompression path started in the kyanite stability field peak metamorphic mineral compositions have been reset during the equilibrium decompression event and hydration during the cooling dominated isobaric portion occurred in the kyanite stability field the present petrographic study from the bandelierkop quarry a locality in the granulite zone of the southern marginal zone identified a series of sequential anatectic reactions which record a period of prograde heating from approximately 700 c to 850 c during this period pressure is constrained to within the sillimanite stability field by the production and preservation of prograde generations of sillimanite the maximum temperature melting event is closely followed by two divariant reactions ga qz sil cord and ga qz cord hy which occur in response to a period of general isothermal decompression a subsequent cooling dominated rehydration period is indicated by a crystallisation hydration reaction cord ksp melt bi ky qz which occurred at a minimum temperature of approximately 640 c and pressure greater than 6 5 kbar the present data confirms the clockwise form of the loop and also supports the previously suggested flat slope of the retrograde cooling portion additional mineral chemistry data relating to the second decompression reaction indicate that the zonation profiles of cordierite and garnet as well as the growth of small quantities of garnet after the reaction are in strong support of the suggested p t path the present mineral compositions in the southern marginal zone reflect a near total re equilibration during the high grade cooling dominated portion of the p t path and as a result reflect p t conditions considerably lower than those of the peak metamorphism

when was the limpopo orogeny abstract the term limpopo orogeny refers to the common high grade metamorphic and tectonic event that affected the archaean rocks of the central and southern marginal zones of the limpopo belt and its influence on the adjacent portions of the kaapvaal craton available geochronological data from muscovite kyanite prograde metamorphic assemblages zircon in igneous charnoenderbite and enderbite muscovite in syntectonic pegmatite bodies formed in shear zones partial melts and syn and post tectonic plutons of granodiorite and granite indicate that this orogeny occurred during the time interval from 2700 ma ago to 2650 ma ago the retrograde phase of this orogeny from the onset of the response phase m 2 until the rocks were subjected to rehydration m 3 was very rapid probably lasting 7 ma from 2671 ma to 2664 ma ago similar but limited data from the northern marginal zone and adjacent portions of the zimbabwe craton suggest that rocks there were subjected to a more recent interval of high grade metamorphism and tectonism extending from 2600 ma until 2550 ma

u pb systematics of monazite and xenotime case studies from the paleoproterozoic of the grand canyon arizona monazite is accepted widely as an important u pb geochronometer in metamorphic terranes because it potentially preserves prograde crystallization ages however recent studies have shown that the u pb isotopic system in monazite can be influenced by a variety of processes that partially obscure the early growth history in this paper we attempt to interpret complex monazite and xenotime u pb data from three paleoproterozoic granite dikes exposed in the grand canyon single crystal monazite analyses from an unfoliated granite dike spread out along concordia from the crystallization age of the dike defined by u pb zircon data to be 1685 1 ma to 1659 2 ma a span of 26 million years back scattered electron bse imaging reveals that magmatic domains within most crystals from this sample are truncated by secondary domains associated with prominent embayments at the grain margin fragments of a single crystal yield contrasting concordant dates and fragments from the edges and tips of crystals yield the youngest dates based on these observations we suggest that the secondary domains formed at least 26 million years after the crystal formed monazite and xenotime dates from the second sample a sheared dike that cross cuts the previous dike spread out along concordia over 16 million years and range up to 2 4 normally discordant again bse imaging reveals secondary domains that truncate both magmatic zoning and xenocrystic cores fragments sliced from specific domains of a previously imaged monazite crystal demonstrate that the secondary domain is 13 million years younger than the core domain textures revealed in bse images suggest that the secondary domains formed by fluid mineral interaction normal discordance appears to result from both radiation damage accumulated at temperatures below 300 c and water mineral interaction monazite data from the third sample exhibit dispersion in both the 207pb 206pb dates 1677 1690 ma and discordance 1 6 to 3 1 reverse discordance in these monazites cannot be explained by incomplete dissolution or excess thorogenic 206pb sliced fragments from several crystals reveal dramatic intragrain u pb disequilibrium that does not correlate with either th or u concentration or position within the crystal we suggest that reverse discordance resulted from mechanisms that involve exchange or fractionation of elemental u or elemental pb and that neither the u pb dates nor the 207pb 206pb dates are reliable indicators of the rock s crystallization age given the large number of processes proposed in the recent literature to explain monazite u pb systematics from rocks of all ages our results can be viewed as another cautionary note for single crystal and multi crystal monazite geochronometry however we suggest that because individual crystals can preserve a temporal record of primary and secondary monazite growth micro sampling of individual monazite crystals may provide precise absolute ages on a variety of processes that operate during the prograde peak and or retrograde history of metamorphic terranes

re os and pb pb geochronology of the archean salobo iron oxide copper gold deposit carajas mineral province northern brazil rhenium osmium ages were determined for two molybdenite samples and a pb pb age was derived from bornite chalcopyrite magnetite at the salobo iron oxide copper gold deposit to determine the timing of mineralization and its relation to the nearby old salobo granite rhenium osmium dating of molybdenite spatially associated with copper sulfide minerals yields ages with weighted means of 2576 8 and 2562 8 ma removing the error multiplier introduced by the decay constant uncertainty appropriate for comparing ages from the same isotopic system these data convincingly argue for two temporally separated pulses of molybdenite deposition at 2576 1 1 4 ma n 2 and 2561 7 3 1 ma n 3 the 2576 8 ma age coincides with a previously published u pb age of 2573 2 ma for the old salobo granite suggesting that main stage ore formation may have been contemporaneous with granite magmatism the slightly younger 2562 ma age most likely represents new molybdenite precipitation associated with the development or reactivation of local shear zones lead lead stepwise leaching of copper sulfide minerals yields a less precise isochron age of 2579 71 ma and supports an archean age for the salobo ores this is the first documentation of an archean iron oxide copper gold deposit and the re os and pb pb geochronology herein support 2580 2550 ma estimates for basement reactivation and regional granite magmatism associated with the development of brittle ductile shear zones