mechanism of arrested charnockite formation at nemmara palghat region southern india abstract the formation of arrested charnockite is an excellent example of structurally controlled channellised fluid flow along specific sites accompanied by selective elemental mobility and mineralogical changes the present paper recognises and focuses study on three types of arrested charnockite formation from palghat region namely shear controlled foliation parallel and boudin neck types and address their spatio temporal relations to regional scale charnoenderbite the shear controlled and foliation parallel types post date deformation and migmatisation the boudin neck type on the contrary is coeval with partial melting and followed the path of cooling and decreasing water activity in the gneiss k feldspar veining around plagioclase and quartz symplectitic intergrowth of biotite quartz after orthopyroxene and k feldspar and fluid inclusion data suggests the presence of alkalic supercritical brine and low density co2 rich fluid during charnockite formation charnockite domains developed following the breakdown of hornblende biotite and quartz are characterised by a more or less pronounced depletion of fe ca mg and ti and trace elements y and zr compared to their counterpart gneiss ree spectra indicate a subtle depletion in the hree near the centre of the charnockite domain although close pair samples of gneiss charnockite are isochemical on a scale of a few millimetres bi directional element movement related to the formation of new mineral was noted it is postulated that arrested charnockite formation developed in situ on local scale within the granitic domains of the hornblende biotite gneiss in the presence of co2 rich fluids and alkalic supercritical saline brine this process post dated the time of regional granulite charnoenderbite and large regional scale retrogression and migmatisation

lu hf and pbsl geochronology of apatites from proterozoic terranes a first look at lu hf isotopic closure in metamorphic apatite abstract the mineral apatite is characterized by elevated and highly variable lu hf ratios that in some cases allow for single crystal dating by the lu hf isotopic system apatites from the adirondack lowlands and otter lake area in the grenville province and from the black hills south dakota yield lu hf ages that are consistently older than their respective pb step leaching ages isotopic closure for the lu hf system therefore occurs before u pb system closure in this mineral in the adirondack lowlands where h2o activity was low lu hf systematics of cm sized apatite crystals remained undisturbed during upper amphibolite facies metamorphism 700 to 675 c at 1170 1130 ma the relatively old lu hf ages of 1270 and 1230 ma observed for these apatites correlate with decreasing crystal size in contrast apatite from the fluid rich otter lake area and black hills yields unrealistically low apparent lu hf closure temperatures implying that in these apatites fluids facilitated late exchange the lu hf ages for the metamorphic apatites were thus controlled either by the prevailing temperature and grain size or by fluid activity

archaean granulites of the limpopo belt zimbabwe one slow exhumation or two rapid events the peak of high temperature low pressure anticlockwise granulite facies metamorphism in the northernmost part of the northern marginal zone of the limpopo belt has been dated at 2591 4 ma using conventional u pb zircon geochronology on a syntectonic charnockite intrusion the thermal peak of 800 c at 0 5 gpa reflects a strong perturbation of the geotherm and was reached during compressional deformation and coincides with vast granite intrusion into the overlying craton the coincidence of the temperature t maximum and compression is thought to be a response to lithospheric mantle thinning that resulted in granite emplacement and a consequent change in upper crustal rheology after initial exhumation and cooling these high temperature low pressure rocks resided at a comparatively shallow depth 10 km for 620 ma field microstructural and geochronological investigation of a low temperature overprint establishes that reworking and final exhumation occurred at 1 97 ga along discrete greenschist facies thrusts the 40ar 39ar laser ablation of hornblende and biotite and pb pb step leaching of sphene are shown to be useful geochronometers in dating the low grade assemblages our results indicate that early proterozoic deformation lasted 10 to 20 ma however a number of ages intermediate between the events at 2 6 and 2 0 ga have been reported for the northern marginal zone these include pb pb garnet ar ar step heating amphibole and u pb zircon ages such ages suggest that these minerals were partially rejuvenated below their commonly believed closure temperature reinterpreting these intermediate ages as partial overprint or inheritance was possible in the northern marginal zone due both to the large time gap between the two tectonometamorphic events and the use of a multimethod multichronometer approach these observations call for great caution when extrapolating mineral ages into cooling histories especially so when the resulting cooling is slower than thermally and or mechanically possible and suggests that the geochronometry in other regions that underwent multiple tectono metamorphic events may need to be reassessed

crustal evolution in south india constraints from nd isotopes nd isotope analyses from a range of lithologies including charnockites metasediments and granitoids that comprise the high grade terrain of south india define contrasting age provinces north of the palghat cauvery shear zone the karnataka craton and the granulite blocks of nilgiri and madras are characterized by model nd ages from 3 4 to 2 4 ga with significant crustal growth during the late archean south of the palghat cauvery shear zone a pan african granulite facies event resulted in sm nd fractionation as observed in several incipient charnockite localities model nd age calculations that allow for sm nd fractionation at 550 ma yield ages in the range of 2 9 to 1 3 ga this range compares closely with published model nd ages from the high grade terrain of sri lanka mid proterozoic crustal growth 1 1 ga observed in the vijayan complex of sri lanka may have provided source material for metasediments characterized by young model nd ages in southern peninsular india the palghat cauvery shear z

high closure temperatures of the u pb system in large apatites from the tin mountain pegmatite black hills south dakota usa abstract the cores of several large 1 2 cm apatites from the early proterozoic tin mountain pegmatite black hills south dakota have concordant u pb ages the age uncertainties of these samples are less than 5 ma the lead isotopic compositions of muscovite and k feldspar coexisting with the apatites were used for initial lead isotopic corrections the resulting u pb concordia ages agree well with u pb and pb pb isochron ages based solely on multiple fragments from single apatites this agreement justifies the use of coexisting muscovite and k feldspar for initial lead corrections the precision of these u pb ages is comparable to those of the u pb systems of more commonly analyzed minerals such as zircon monazite and sphene the estimated u pb closure temperature t c of these apatites is about 620 c this t c is in close agreement with the t c calculated for 0 5 cm diffusion radii and published data for diffusivity and activation energy of lead diffusion in apatite apatite can therefore yield high precision u pb ages that are close to its crystallization age in igneous rocks

u pb single zircon pb evaporation and sm nd isotopic study of a granulite domain in se madagascar madagascar is a poorly known precambrian domain at the eastern border of the mozambique belt strongly affected by the pan african orogeny this large island represents a key part of gondwanaland and the accurate dating of the major metamorphic event is critical for the reconstruction of that supercontinent in the southeastern zone of the island metamorphism has reached high temperature and low pressure conditions 850 c and 5 kbar with moderate retrogression a u pb conventional multigrain single zircon stepwise pb evaporation and sm nd geochronological study on meta felsic rocks has shown that basement of at least lower proterozoic age was strongly metamorphosed around 570 580 ma this tectometamorphic event nearly obliterated any pre pan african zircon isotopic memory the granulite facies metamorphism is associated with the syn metamorphic emplacement of the anosyan charnock ites and granites a strong channelized fluid contribution at 545 ma has produced the re crystallization of biotite apatite

giant dune morphologies and dynamics in a deep continental shelf environment example of the banc du four western brittany france the aim of this paper is to improve the knowledge of spatio temporal variability of very large to giant dunes in deep tide dominated environment their growth mechanisms and dynamic evolution still remain in debate and constitute a significant scientific challenge in regard to difficulties to achieve accurate measures of migrations this motivated the realization of three recent swath bathymetry surveys across the wide dune field of the banc du four located offshore the western brittany the banc du four field is composed of more than 500 large dunes exhibiting a great diversity of morphologies ranging from 2d to 3d shapes and reaching the largest sizes of such sedimentary structure ever described wavelength over 1000 m and height over 30 m the analysis of the entire ensemble of dune parameters offers the opportunity to discuss the relative influence of forcings and the validity of the equations predicting the bedform geometry in shallow waters our results show a good height spacing power law correlation but do not strike the usual statistical regression observed in previous studies the steeper positive slope of the equation and the outstanding values of the height parameter reveal the complexity of local hydrodynamic regime interacting with bed load transport and sediment supply no clear relationship has been observed between asymmetry and size parameters water depth can not be considered as a major factor controlling the size of dunes otherwise the vertical sedimentary variation suggests the action of storm waves on dune crests and the horizontal migration rates of dunes in the eastern field of the banc du four were found to range from 3 to 20 m yr 1 such velocities were only mentioned before by van landeghem et al 2012 on deep continental shelves 70 m deep and attest of the still present morphodynamical equilibrium of the large dunes furthermore as was previously found for similar dunes in the irish sea no power law relationship between dune migration rates and dune size can be deduced similarly no strong correlation is established between migration rates and the degree of asymmetry nevertheless the dune asymmetry can be used to predict the migration direction as for dimension parameters the evolution asymmetry depends on the study area and migration rates more precisely when the crest lines of dunes form straight lines and their migration rates remain below 8 10 m yr 1 the degree of dune asymmetry increases with the migration rate this trend is reversed for the dunes with higher migration velocities and crests which are sinuous these results relativize the migratory predictions based on punctual geometrical observations

geochemistry of high grade supracrustal rocks from the kerala khondalite belt and adjacent massif charnockites south india abstract the kerala khondalite belt kkb comprises a suite of supracrustal rocks of probable proterozoic age that was metamorphosed to the granulite facies the kkb consists of an interlayered sequence of 1 garnet biotite orthopyroxene graphite gneisses 2 khondalites graphite garnet biotite sillimanite cordierite gneisses and 3 cordierite gneisses garnet biotite cordierite orthopyroxene as well as less abundant mafic granulites calc silicates and quartzites petrologic and geochemical data suggest that these rocks were originally arkosic and argillaceous sediments possibly accompanied by felsic volcanic rocks along with minor intercalations of basalts carbonates and quartz rich sandstones the ree patterns of the metasediments are typically lree enriched with large negative eu anomalies indicating a continental provenance the major and trace element compositions of some of the arkosic rocks of the kkb resemble those of the massif charnockites that bound the kkb to the north which suggests that the massif charnockites may have been the source of some of the kkb sediments modern day analogues of the pelite arkose lithologic association observed in the kkb occur in intracratonic rifts and rifted continental margins this association is clearly distinct from the graywacke mafic volcanic association found in the southern indian greenstone belts and many other lower grade archean sedimentary sequences the kkb supracrustals show both lithologic and chemical similarities to many proterozoic supracrustal successions

electron microprobe dating of monazite because monazite is extremely rich in u and th radiogenic pb pb accumulates very quickly and reaches in about 100 ma a level where it is possible to analyse it with the electron probe assuming that common pb is negligible and that partial loss of pb has not occurred the simultaneous measurement of u th and pb allows to obtain a geologically meaningful age from a single electron probe analysis here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method a specific statistical method to deal with the large number of data which can be obtained on a single sample is described and several guidelines illustrated by examples are suggested to optimize the method electron probe measurements carried out on samples of known age from 200 ma to 3 1 ga yield ages that always fall inside the confidence interval of the isotopically determined age demonstrating that this method is reliable the younger age limit is approximately 100 ma although it can be younger in some favourable cases in old monazites extremely high pb contents have been found up to 5 wt indicating that monazite can tolerate high radiation doses without experiencing lead loss the final precision on the age for a normal monazite is 30 50 ma for a total counting time of 600 s a complete dating procedure can be completed in less than 1 h first results indicate that old ages can be preserved in monazite either in small relict cores in crystals or by the coexistence of several generations of monazites in a sample this method has all the advantages of the electron probe it is non destructive has an excellent spatial resolution monazites as small as 5 i m can be dated and because it is possible to work on normal polished thin sections the petrographical position of the dated crystal is known this method offers a large number of geologists access to an in situ dating technique at moderate cost

holocene denudation rates from the superhumid southernmost chilean patagonian andes 53 s deduced from lake sediment budgets abstract holocene denudation rates and their regional variations in the superhumid and temperate climate of the southernmost patagonian andes are poorly surveyed therefore we have investigated denudation in five small lake catchments 0 11 1 62 km 2 across a precipitation gradient from 600 to 9000 mm yr 1 in southernmost chile at 53 s variations in denudation rates can be defined most precisely by using small catchments and short time spans holocene thus can be related to variable bedrock types vegetation cover elevation and climate minimum physical denudation rates were determined from sediment budgets based on sediment echosounder data and physical properties of sediment cores the holocene denudation rates vary from 0 08 mm kyr 1 in the highly elevated bare granitic rock catchments to 9 01 mm kyr 1 in the low and vegetated catchments with a basement of sedimentary rocks these values are comparable with those from similar areas in norway sweden and nw iceland the high annual precipitation in the study area has only a minor effect on denudation enhanced geomorphic activity during paraglacial conditions caused a faster denudation after the glacier retreat until either the clastic glacial detritus was removed or the catchment was covered with vegetation chemical comparisons between basement rocks and lake sediments indicate a pronounced chemical denudation in catchments with peaty soils and low soil water ph values of 3 to 5 the holocene surface lowering rates are on average 200 fold lower than cenozoic surface lowering rates calculated from fission track data for the southern andes this discrepancy can be explained by significantly higher erosion rates during glacial periods

dehydration melting phenomena in leptynitic gneisses and the generation of leucogranites a case study from the kerala khondalite belt southern india

the triangle shearzone zimbabwe revisited new data document an important event at 2 0 ga in the limpopo belt abstract the limpopo belt in southern africa has been used to demonstrate that modern style continent continent collision operated during the late archaean 2 6 2 7 ga we have studied the age and pt conditions of strike slip tectonism along the important right lateral triangle shearzone our results substantiate existing proterozoic metamorphic mineral age data of prior uncertain significance using the pb pb and sm nd garnet chronometers and the ar ar step heating technique for amphibole we have dated pre and syn tectonic metamorphic minerals at 2 2 and 2 0 ga thus the triangle shearzone can now be regarded as an important proterozoic suture examination of corresponding high grade pt conditions reaching 800 c at 9 kbar indicates a clockwise metamorphic evolution with pronounced isothermal uplift although the evidence that thrusting of the marginal zones of the limpopo belt over the adjoining cratons occurred during the late archaean clearly remains it is now very uncertain to which event the various pt paths obtained in the limpopo belt may be assigned therefore the question of whether the 2 6 2 7 ga tectonism fits on its own a modern style continental collision model remains open and has to be reassessed

constraints on the timing of pan african granulite facies metamorphism in the kerala khondalite belt of southern india shrimp mineral ages and nd isotopic systematics abstract the supracrustal kerala khondalite belt kkb southern india constitutes an important but little understood terrain in the evolution of east gondwana here we report shrimp u pb zircon and titanite ages for a suite of metasedimentary rocks from the kkb metamorphic ages at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 530 2 pm 4 4 end document and documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd us

electron microprobe dating of monazites from high grade gneisses and pegmatites of the kerala khondalite belt southern india monazites of five samples one leptynitic garnet biotite gneiss one khondalite one augen gneiss and two pegmatites from the central and northern part ponmudi unit of the kerala khondalite belt kkb in southern india were analyzed with the electron microprobe dating technique monazites in the augen gneiss and the pegmatites yield grain sizes between 200 800 m th abundances are rather low 10 wt and the distribution of th u and pb within single grains is fairly homogeneous contrasting to this monazites in the leptynitic gneiss and the khondalite are small 150 m they often display very complex th u pb patterns and contain high th concentrations up to 20 wt the statistical treatment of individual ages from the investigated samples revealed three populations of lower proterozoic 1 9 ga upper proterozoic 580 ma and ordovician age 470 ma as well as mid proterozoic ages between 0 8 1 7 ga which are not regarded to be of geological significance lower proterozoic ages are preserved in the cores of monazites from leptynitic gneisses and khondalites they fairly agree with sm nd model ages for similar rocks of the kkb and give a minimum age for first monazite growth or complete homogenization the prominent pan african population with mean values between 540 and 580 ma is present in the leptynitic gneiss the khondalite and the augen gneiss and in line with published isotope ages for the kkb the ordovician population finally marks the emplacement of granitic pegmatites subsequent to the pan african high grade metamorphic event there is an obvious discrepancy between khondalites and leptynitic gneisses on the one hand and augen gneisses on the other concerning the presence of lower proterozoic ages while these are abundant in the former often rimmed by upper proterozoic ages they are completely absent in the latter it appears unlikely that lower proterozoic ages were completely reset during a pan african event exclusively in the augen gneisses while they were preserved in leptynitic gneisses and khondalites it is further concluded that the augen gneisses are of magmatic origin and were derived from porphyritic granites thus the upper proterozoic age of 605 37 ma calculated for the investigated sample approximates the time of magma emplacement which slightly precedes the peak stage of pan african high grade metamorphism in the kkb and of monazite crystallization from the granitic melt a characteristic feature of the investigated monazites is the resetting of lower proterozoic and pan african ages to significantly younger values due to partial pb loss monazites not shielded by other minerals e g garnet suffered selective mobilization of pb along fractures or at their rims while th and u concentrations remained almost unchanged the results presented in this study indicate that this was mainly due to fluid rock interaction it is concluded that thermal diffusion of pb even at the suggested temperatures of 900 c only had minor influence on the th u pb composition in monazite and that the closure temperature for this system must be significantly higher than previously assumed 750 c

pbpb dating of apatite by a stepwise dissolution technique abstract apatites from different geologic settings have been dated by a new sequential dissolution method the procedure is based on stepwise exposure of apatite to weak intermediate acids a pegmatite sample from the kerala khondalite belt in southern india yielded an age of 509 25 ma apatites from an enderbite sample from the northern marginal zone of the limpopo belt in southern africa yielded an age of 1970 26 ma which is in agreement with the high grade metamorphism at around 2 0 ga reported from the triangle shear zone and amphibolite facies metamorphism in the transition zone apatites from an amphibolite facies gneiss from the transition zone south of the nmz yielded an age of 1878 18 ma this age is somewhat younger than a sphene age on the same rock of 1971 17 ma and is believed to reflect the time of retrograde metamorphism in the presence of a fluid phase the sequential dissolution method is a new tool that allows dating of apatites which have relatively unradiogenic pb ratios because the spread can be significantly increased and no correction for common lead is needed this study further demonstrates that apatites from magmatic and high grade metamorphic rocks yield ages very close to those of monazite zircon or garnet unless the rock subsequently interacted with a fluid phase

paleoproterozoic amphibolite facies retrogression and exhumation of archean metapelitic granulites in the southern marginal zone of the limpopo belt south africa abstract granulite facies terrains that have undergone extensive retrogression provide insights into the recycling of volatiles in the crust during orogenesis the timing of retrogression is key to understanding the geodynamic circumstances of such events this study has investigated amphibolite facies retrogression of melt depleted granulite facies metapelites from the south marginal zone smz of the limpopo belt the granulites formed via incongruent partial melting reactions that consumed biotite and produced garnet and rutile as prominent peritectic products at 2 7 ga under conditions of 11 kbar and 875 c peak metamorphic garnet is replaced by cordierite orthopyroxene symplectites with cordierite subsequently partially replaced by fine grained intergrowths of amphibolite facies assemblages including rutile zr in rutile thermometry yielded a temperature range of 731 846 c for the granulite facies rutiles and 629 690 c for rutile in the retrograde assemblage all rutile generations yielded a youngest u pb age of 2 1 ga some granulite facies rutiles record older dates 2 55 2 25 ga which are non systematic with regard to crystal size and represent non reset relicts of metamorphic recrystallization during a potentially complex history of amphibolite facies metamorphism between 2 7 and 2 2 ga the youngest 2 1 ga rutile u pb age represents the time of final exhumation related cooling of the rocks following a discrete paleoproterozoic amphibolite facies retrograde event coexisting orthoamphiboles in the peak metamorphic assemblage indicate a maximum peak temperature of 625 c the lack of age size correlation and intragrain age zoning in rutile suggest fast cooling following the amphibolite facies retrograde event which is inferred to be not much older u pb dating of zircon from the same rocks confirms that the metapelites underwent granulite facies metamorphism at 2695 35 ma thus the metapelites of the smz record both an archean granulite facies event and a polymetamorphic paleoproterozoic amphibolite facies history that culminated in a mid amphibolite facies peak inducing partial retrogression of the granulites the paleoproterozoic amphibolite facies event is tentatively correlated with mantle melting documented between 2 3 and 2 1 ga to produce mafic and intermediate volcanic rocks in the pretoria group on the kaapvaal craton

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

upb geochronology of the sri lankan basement abstract u pb ages of zircons and monazites from ortho and paragneisses representing the major crystalline basement units of sri lanka establish a new geochronological framework which is consistent with previously published work on sm nd model ages rb sr whole rock ages and u pb mineral ages in the highland complex three newly dated orthogneisses one previously dated by baur et al 1991 and one dated by kroner and williams 1992 all intruded 1 85 to 1 9 ga ago this widespread intrusion event occurred soon after the end of sediment deposition which is estimated at 1 9 to 2 0 ga using upper intercept ages of zircons from metapelites in the highland complex within the boundaries of this complex established by sm nd model age mapping milisenda et al 1994 only one orthogneiss has been found which is substantially younger than 1 8 ga this is the metatonalite from polonnaruwa which intruded 670 ma ago baur et al 1991 three orthogneisses from the vijayan complex which lies to the east of the highland complex yield similarly consistent crystallization ages of 1023 8 1016 28 and 1033 121 ma in the wanni complex which lies to the west of the highland complex a sillimanite bearing gneiss which is apparently of metasedimentary origin but contains zircons of igneous appearance yields a primary crystallization age of 793 4 ma consistent with a previously dated orthogneiss from kurunegala of 770 ma baur et al 1991 in addition single zircon data by kroner et al 1994 indicate primary crystallization ages of 1 0 to 1 1 ga for orthogneisses from the vavuniya charnockite province which is considered to be part of the wanni complex these results confirm the age contrast between the highland complex on the one hand and the vijayan complex and wanni complex on the other as previously inferred from sm nd model ages milisenda et al 1988 1994 post tectonic granitoids and pegmatites intruded throughout all three complexes 550 ma ago the age of regional high grade metamorphism mostly granulite facies in the highland complex mostly amphibolite facies in the vijayan complex and both granulite and amphibolite facies in the wanni complex is constrained to be less than 670 ma the primary crystallization age of the polonnaruwa metatonalite it is more closely defined by lower intercept zircon ages which include one concordant age of 608 4 ma and one discordant but well defined age of 611 23 ma and by a nearly concordant monazite age of 592 2 ma this indicates that peak metamorphic conditions occurred 610 ma ago numerous other lower intercept ages between 590 and 550 ma and post tectonic intrusion ages of 550 ma set a lower age limit for high grade conditions and deformation the temperature history between 610 and 550 ma is not yet well resolved and it is not clear whether there was a separate thermal event 550 ma ago previously published whole rock rb sr and sm nd isochrons for gneisses from the highland complex yielding ages of 2 ga and greater and previously interpreted to date the time of granulite grade metamorphism must be reinterpreted in the light of the new data presented here as well as other recently published results e g burton and o nions 1990b baur et al 1991 and in view of the accumulated evidence for very limited element mobility during high grade metamorphism these whole rock ages are in fact identical or close to the ages of primary granitoid emplacement rather than the age of granulite facies metamorphism

westward continuation of the craton limpopo belt tectonic break in zimbabwe and new age constraints on the timing of the thrusting the umlali thrust zone has been mapped between the granite greenstone rocks of the zimbabwe craton and the granulite facies rocks of the limpopo belt a tectonic break had not been previously recognized in the southwest because of the intrusion of syn to post kinematic porphyritic granites these intrusions locally obliterate evidence for the tectonic break although in some localities the associated mylonites are preserved as xenoliths within the intrusive porphyritic granite bodies the porphyritic granite bodies were emplaced during and after the main regional fabric forming event a u pb zircon age of 2627 7 ma obtained from a late kinematic microgranite is interpreted to be the minimum time of thrusting a whole rock rb sr age of 2583 52 ma obtained from plutonic rocks and retrogressed zones within the north marginal zone falls within error of the zircon age suggesting widespread magmatic and hydrothermal activity coeval with the thrusting

metamorphic p t conditions of the kerala south india khondalite belt a granulite facies supracrustal terrain the khondalite belt of southernmost india is an immense section of granulite facies metasediments of probable early proterozoic age dominant rock types are migmatized khondalites garnet sillimanite pm cordierite metapelites acid biotite garnet graphitic gneisses and charnockites quartzofeldspathic gneisses with orthopyroxene throughout much of southern kerala the biotite gneisses are in a state of arrested conversion to charnockite partially decrepitated co 2 inclusions in quartz at the ponmudi mixed biotite gneiss charnockite quarry have significant ch 4 and or n 2 and are texturally late the probable sequence of events in metasediments was 1 migmatization 2 charnockite formation 3 symplectitic cordierite formation and 4 late co 2 entrapment phase equilibrium diagrams of anatexis in aluminous systems and fe mg exchange between garnet and biotite indicate metamorphic temperatures of 650 850 c for the khondalites temperatures based on garnet orthopyroxene

proterozoic transpressive deformation in the northern marginal zone limpopo belt zimbabwe a major tectono metamorphic event affected the southern half of the northern marginal zone of the limpopo belt at 2 0 ga structural analysis along a 43 km long profile perpendicular to strike shows extension inclined toward the northeast extension parallel to axes of folds that uplift the southeast of the section i e the central zone of the limpopo belt and dextral shear these deformation patterns developed coevally the lineations form distinct tightly clustered groups that occur together in the profile there is no evidence for superimposed deformation or deformed lineations nor are lineations observed cross cutting each other this single episode of deformation can therefore be called transpression petrographic and thermobarometric evidence support this kinematic picture in that decreases of both pressure 2 5 kb and temperature 150 c are documented from southeast to northwest across strike 39 ar 40 ar step heating of metamorphic hornblende from amphibolites has yielded ages of

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

dating a variscan pressure temperature loop with staurolite staurolite with a complex internal metamorphic and structural relationship occurs in metapelite from the silvretta thrust sheet eastern alps stepwise leaching of a core and rim fraction of a single staurolite crystal resulted in a total pb pb isochron of 308 28 ma the variscan metamorphism has been confirmed by a further single bulk staurolite pb pb age of 298 42 ma which is in accordance with u pb ages of 306 8 ma and 304 6 ma respectively obtained from combined u pb leaching of the same crystals direct dating of the staurolite in reaction isograd was therefore successful our results date the prograde part of a variscan counterclockwise pressure temperature path in the silvretta thrust sheet proximity with white mica rb sr and k ar cooling ages suggests that the whole variscan episode was fast and took place within a few million years

pan african charnockite formation in kerala south india sm nd mineral ages of gneisses and associated granulites from the ponmudi incipient charnockite locality south india indicate that granulite metamorphism occurred at or shortly after 558 ma proterozoic ages recorded by garnet separates reflect a detrital age or an earlier metamorphic event preserved by inclusions within garnet the age of post metamorphic uplift 440 460 ma is constrained by sr isotope equilibration between biotite and plagioclase since charnockite formation and subsequent uplift north of the palghat cauvery shear zone had terminated by earliest proterozoic time these results confirm two distinct periods of granulite formation in south india and suggest that the palghat cauvery shear zone represents the boundary between two blocks of strongly contrasting geological histories both incipient charnockite formation and subsequent uplift at ponmudi may be correlated with the tectonothermal evolution of the highlands group in sri lanka the similarity between nd and sr model ages for charnockites and gneisses from ponmudi indicates that no significant rb sr fractionation has occurred during the crustal history of these incipient charnockites pb isotopic ratios suggest that th u ratios were fractionated during charnockite formation at about 500 ma in contrast to charnockites found north of the palghat cauvery shear zone fractionation of u pb during the archaean did not occur in the ponmudi granulites

intercrystalline stable isotope diffusion a fast grain boundary model we formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals and the intragranular distribution of stable isotopes in each mineral as functions of mineral grain sizes and shapes diffusivities modes equilibrium isotopic fractionations and the cooling rate of a rock one of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes this fast grain boundary fgb model describes interdiffusion between any number of mineral grains assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled the model can be used for a rock containing any number of minerals and number of grain sizes of each mineral several grain shapes and any thermal history or domain size desired previous models describing stable isotope interdiffusion upon cooling have been based on dodson s equation or an equivalent numerical analogue the closure temperature of dodson is the average bulk temperature recorded between a mineral and an infinite reservoir by using dodson s equation these models have treated the closure temperature as an innate characteristic of a given mineral independent of the amounts and diffusion rates of other minerals such models do not accurately describe the mass balance of many stable isotope interdiffusion problems existing models for cation interdiffusion could be applied to stable isotopes with some modifications but only describe exchange between two minerals under specific conditions the results of fgb calculations differ considerably from the predictions of dodson s equation in many rock types of interest actual calculations using the fgb model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient and may differ from that predicted by dodson s equation by hundreds of degrees c either or both of two minerals may preserve detectable zonation which may in some instances be larger in the faster diffusing mineral rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone the results of fgb diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations concordant or discordant recorded within any rock in which diffusion could have acted over observable scales the predictions of this model are applicable to thermometry evaluation of open or closed system retrogression and determination of cooling rates or diffusion coefficients

nd isotopic evidence for the position of southernmost indian terranes within east gondwana abstract sm nd model ages of orthopyroxene bearing massif charnockites from the cardamom hills massif and adjoining supracrustal rocks from the kerala khondalite belt in southernmost india are used to infer some of the relationships within these rocks and between them and neighboring areas most of these rocks have model ages of 2 1 2 8 ga with most charnockites in the range 2 2 2 6 ga thus 3 0 3 4 ga archean rocks to their north did not contribute material to either suite and the two suites may have been juxtaposed after formation of the supracrustal rocks the similarity of sm nd isotope systems in the two units studied here supports an argument that the massif charnockites were the primary sole source of the detritus incorporated into the supracrustal rocks a cordierite gneiss representative of a relatively minor lithology in the supracrustal belt has a model age of 1 3 ga the protolith of this gneiss not only formed from much younger material than the rest of the belt but also formed significantly after the other metasedimentary rocks the source material of the gneiss protolith may have been located in the wanni and vijayan complexes of sri lanka the overlap of the model ages of rocks in this area and those in the highland complex of sri lanka supports the notion that these two sets of rocks were joined to each other in gondwana they belong to a belt that ran from antarctica through sri lanka and india into madagascar this belt was involved in pan african tectono metamorphism as reflected in the 550 ma age of the last granulite forming event throughout the belt

reply to rigby m j eriksson p g and mavimbela p k comments on structural and compositional constraints on the emplacement of the bushveld complex by b clarke r uken and j reinhardt

the significance of titanite and apatite u pb ages constraints for the post magmatic thermal hydrothermal evolution of a batholithic complex berens river area northwestern superior province canada abstract titanite apatite and in one case rutile from a suite of twenty seven samples representing the major phases of an extensive and well dated archean plutonic domain of the northwestern superior province have been dated by u pb to determine the temporal progress of thermal and hydrothermal events accompanying and following major plutonism and to assess the isotopic behaviour of these geochronometers most samples contain 1 dark brown titanite with 350 20 ppm u showing shapes and textural relationships indicative of a primary magmatic origin and yielding ages corresponding either to the time of magmatic crystallization or to that of secondary metamorphic or magmatic events known in the area 2 pale titanite with 30 2 ppm u showing textural evidence for a secondary origin by breakdown of biotite hornblende oxide or older titanite and yielding ages that in general post date the major plutonic and metamorphic events in the area 3 apatite containing 30 5 ppm u occasionally up to 150 ppm u showing igneous shapes and textural relationships but yielding post magmatic ages similar to those of coexisting pale titanite 4 rutile in one sample postdating the ages of coexisting titanite and apatite the common coexistence of two noncoeval titanite generations demonstrates that the often invoked mechanism of pb diffusion during slow cooling in this case has no general validity the distribution of primary dark titanite ages however could be explained by late closure to diffusion or alternatively as the result of resetting by the thermal or hydrothermal effects caused by younger plutonic activity the ages of the secondary pale titanites probably reflect crystallization during discrete retrogressive events promoted by moderate heating or enhanced fluid activity the correspondence between pale titanite and apatite ages suggests that the same processes also caused the isotopic resetting of apatite on a regional scale the data pattern permits the identification of domains characterized by differences in the length and intensity of the thermal and hydrothermal processes the most protracted activity is recorded in northern domains of the berens river area whereas the gold mineralized red lake area in the south underwent a very rapid decline of the thermal hydrothermal activity these data also highlight important differences between the red lake and other gold camps of the superior province such as hemlo and val d or where hydrothermal activity locally associated with mineralization occurred as much as 30 80 m y after the major orogenic developments

initial isotopic heterogeneity and secondary disturbance of the smnd system in fluorites and fluid inclusions a study on mesothermal veins from the central and western swiss alps abstract this paper presents sm nd data on hydrothermal fluorites fluid inclusions and leach experiments on samples from the mont blanc and gotthard units swiss alps both areas have experienced alpine greenschist facies overprint 350 and 450 c respectively mont blanc samples represent late variscan fluorites from three individual veins whereas gotthard samples were taken from one hand specimen of probable late variscan age nd concentrations of fluorites range from 3 6 to 23 ppm 147 sm 144 nd ratios from 0 12 to 0 35 none of the samples sets yielded an isochron relationship imperfect correlations indicate apparent ages between late variscan and alpine reflecting neither the formation nor the age of metamorphic overprint reasons are wall rock influence during formation and post formation disturbance secondary fluorite gotthard shows the highest nd concentration and a low sm nd ratio close to that of fluid inclusion fractions a leach experiment showed the same trend even more pronounced 7 of nd and 2 of sm were removed from fluorite by 0 18 n hcl at 100 c hence secondary fluids are likely to leach and fractionate sm and nd and may easily cause dissolution and reprecipitation of fluorite consequently the system is highly suspicious if secondary fluid flow cannot totally be ruled out this is in sharp contrast to previously published conclusions however evidence for rejuvenation of the sm nd system in fluorites is also found in published data sets

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

arrested charnockite formation at kottavattam southern india at kottavattam southern kerala india late proterozoic homogeneous leptynitic garnet biotite gneisses of granitic composition have been transformed on a decimetric scale into coarse grained massive charnockite sensu stricto along a set of conjugate fractures transecting the gneissic foliation charnockitization post dates the polyphase deformation regional high grade metamorphism and anatexis and evidently occurred at a late stage of the pan african tectonothermal history geothermobarometric and fluid inclusion data document textural and chemical equilibration of the gneiss and charnockite assemblages at similar p lith t conditions 650 700 degrees c 5 6kbar in the presence of carbonic fluids internally buffered by reaction with graphite and opaque mineral phases x co2 0 7 0 6 x h2o 0 2 0 3 x n2 0 1 logf o2 l7 5 mineralogical zonation indicates that charnockitization of the leptynitic gneiss involved first the breakdown of biotite and oxidation of graphite in narrow outward migrating transition zones adjacent to the gneiss followed by the breakdown of garnet and the neoblastesis of hypersthene in the central charnockite zone compared to the host gneiss the charnockite shows higher concentrations of k na sr ba and zn and lower concentrations of mg fe ti v y zr and the hree with a complementary pattern in the narrow transition zones of biotite breakdown the p lith t x h2o data and chemical zonation patterns indicate charnockitization through subsolidus dehydration reaction in an open system subsequent residence of the carbonic fluids in the charnockite resulted in low grade alteration causing modification of the syn charnockitic elemental distribution patterns and the properties of entrapped fluids we favour an internally controlled process of arrested charnockitization in which during near isothermal uplift the release of carbonic fluids from decrepitating inclusions in the host gneiss into simultaneously developing fracture zones led to a change in the fluid regime from fluid absent in the gneiss to fluid present in the fracture zones and to the development of an initial fluid pressure gradient triggering the dehydration reaction