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

structural geology and tectonic setting of the sutherland greenstone belt kaapvaal craton south africa abstract this paper presents the results of recent mapping in selected parts of the sutherland greenstone belt at the northern edge of the kaapvaal craton south africa structural studies in the northern and central parts of the belt have defined a tectonic regime dominated by north dipping ductile shear zones with an oblique to reverse sense of movement the fabric pattern related to this thrusting can be recognised in both the supracrustal rocks and the adjacent granitoid gneisses but is absent from the 2 65 ga granites bordering the southern margin of the belt this implies a minimum age for deformation in the sutherland belt coeval with that recognised in the immediately adjacent high grade limpopo belt the structures in the granitoid greenstone terrane are interpreted as a mechanical response on the kaapvaal craton to collision tectonics in the limpopo belt

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

the southern high grade margin of the dharwar craton the amphibolite facies to granulite facies transition at the southern margin of the dharwar craton has been studied in the krishnagiri satnur halaguru and kushalnagar areas of southern india in all three areas the transition appears to be a progressive metamorphic overprint on cratonal gneisses and their mafic and metasedimentary enclaves without major structural interruption in the kabbal satnur br hills section of karnataka a high grade charnockite massif with pronounced rb depletion is the culmination of an apparently continuous increase of metamorphic grade southward in this and the kushalnagar areas increase of paleopressure from near 6 to near 8 kbar with increasing grade indicates a depth zone relationship of the amphibolite and granulite facies incipient charnockite replacing peninsular gneiss first appears along n s shears parallel to the regional grain of the craton low p h 2 o high co 2 vapors were instrumental in the creation of orthopyroxene introduction from a deep so

tectono metamorphic evolution of the simatic basement of the pietersburg greenstone belt relative to the limpopo orogeny evidence from serpentinite abstract detailed studies of low grade ultramafic rocks are integrated with regional structural field observations in the pietersburg greenstone belt these results show that petrographic investigations of serpentinite can reveal more about the tectonometamorphic history of greenstone belts than has previously been appreciated the ultramafic rocks of the pietersburg greenstone belt were affected by an early deformation event d 1 which is reflected by the presence of a relic magnetite fabric s 1 in serpentinite the s 1 fabric is crosscut by the peak metamorphic assemblage of antigorite tremolite talc chlorite both s 1 and the peak metamorphic assemblage are deformed by a second deformation event d 2 on a macroscale d 2 is manifested as a series of northward verging thrust faults that affected the entire belt published u pb zircon data show that d 2 was active between 2 9 and 2 7 ga prior to uplift and southward thrusting of the limpopo belt lb major carbonatization of the simatic basement postdates peak metamorphism and occurred towards the end of d 2 coeval with the late evolution of the limpopo orogeny the alteration assemblages were subsequently deformed the sequence of events that affected the pietersburg greenstone belt is similar to that established for the high grade southern marginal zone of the limpopo belt but their absolute timing is different the results of this study demonstrate the important contribution that low grade ultramafic rocks can make to the understanding of the tectono metamorphic history of the archaean cratons and terrane accretion

predictive mapping of prospectivity for orogenic gold giyani greenstone belt south africa we present a mineral systems approach to predictive mapping of orogenic gold prospectivity in the giyani greenstone belt ggb by using layers of spatial evidence representing district scale processes that are critical to orogenic gold mineralization namely a source of metals fluids b active pathways c drivers of fluid flow and d metal deposition to demonstrate that the quality of a predictive map of mineral prospectivity is a function of the quality of the maps used as sources of spatial evidence we created two sets of prospectivity maps one using an old lithologic map and another using an updated lithological map as two separate sources of spatial evidence for source of metals fluids drivers of fluid flow and metal deposition we also demonstrate the importance of using spatially coherent or geologically consistent deposit occurrences in data driven predictive mapping of mineral prospectivity the best predictive orogenic gold prospectivity map obtained in this study is the one that made use of spatial evidence from the updated lithological map and spatially coherent orogenic gold occurrences this map predicts 20 of the ggb to be prospective for orogenic gold with 89 goodness of fit between spatially coherent inactive orogenic gold mines and individual layers of spatial evidence and 89 prediction rate against spatially coherent orogenic gold prospects in comparison the predictive gold prospectivity map obtained by using spatial evidence from the old lithological map and all gold occurrences has 80 goodness of fit but only 63 prediction rate these results mean that the prospectivity map based on spatially coherent gold occurrences and spatial evidence from the updated lithological map predicts exploration targets better i e 28 smaller prospective areas with 9 stronger fit to training gold mines and 26 higher prediction rate with respect to validation gold prospects than the prospectivity map based on all known gold occurrences and spatial evidence from the old lithological map

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

hydration of cordierite and hypersthene and a description of the retrograde orthoamphibole isograd in the limpopo belt south africa

large scale fluid infiltration metasomatism and re equilibration of archaean basement granulites during palaeoproterozoic thrust belt construction ungava orogen canada abstract the metamorphic history of the archaean superior province crystalline basement in the palaeoproterozoic ungava orogen attests to the importance of structural and geohydrological controls on a retrograde amphibolite granulite transition two distinct metamorphic suites separated in age by nearly one billion years are recognized in extensively exposed tonalitic to dioritic metaplutonic gneisses the older suite comprises c 2 7 ga granulite facies assemblages orthopyroxene clinopyroxene hornblende plagioclase ilmenite biotite quartz that record moderate pressures 5 kbar and high temperatures 800 c a younger c 1 8 ga suite resulted from amphibolitization of the granulites and is characterized by regionally extensive amphibolite facies mineral zones that broadly parallel the basal decollement of the overlying proterozoic cape smith thrust belt deformation mineral growth relationships in the amphibolitized basement indicate that extensive hydration and re equilibration of the archaean granulites occurred during thrust belt deformation the transition from granulite facies to amphibolite facies assemblages is characterized by the growth of garnet hornblende quartz cummingtonite coronas between plagioclase and orthopyroxene clinopyroxene as well as titanite coronas on ilmenite multi equilibrium thermobarometry on the coronitic assemblages documents re equilibration of the granulitic gneiss to 7 7 kbar at 644 c in the south and 9 8 kbar at 700 c in the north the variably deformed amphibolite facies domain sandwiched between the coronitic garnet zone and the basal decollement is marked by significant metasomatic changes in major element concentrations within tonalite these changes are compatible with equilibrium flow of an aqueous chloride fluid down a temperature gradient the source of fluids for basement hydration metasomatism is interpreted to be dehydrating clastic rocks in the overlying thrust belt with fluid flow probably focused along the basal decollement

progressive metamorphism of mafic rocks from greenschist to granulite facies in the dharwar craton of south india in the archean dharwar craton of southern india n s trending belts of metabasic rocks are exposed which underwent regional metamorphism at about 2 5 ga ago the progressive changes in the assemblages and mineral chemistry of metabasites was studied in a n s traverse covering the chitradurga and nagaman gala belts the sargur area and the nilgiri hills towards the south with increasing metamorphic grade greenschists chl act ab ep pm carb qtz give way to amphibolites hbl plag pm qtz gar cumm and mafic granulites pyx plag gar pm hbl qtz the amphibole composition changes from actinolite in the greenschist zone to tschermakitic hornblende in the amphibolite zone and pargasitic hastingsitic hornblende in the granulite zone the ti content of the amphiboles systematically increases in this direction the plagioclase composition changes from albite in the greenschist zone to oligoclase andesine in the amphibolite zone and andesine labradorite in the granulite zone almandin

the oxygen isotopic composition of granulites and retrogressed granulites from the limpopo belt as a monitor of fluid rock interaction abstract from the southern marginal zone of the limpopo belt three rock systems cordierite bearing metapelites mafic rocks and ultramafic rocks are studied by whole rock oxygen isotope analysis in order to decide wether the observed variation in the o isotopic composition of the metapelites is due to granulite facies fluid rock interactions or due to earlier processes a discrimination index pelite index pi is proposed in combination with the whole rock oxygen isotopic composition this index enables one to put some constraints on the origin of the precursor rocks in the case of the limpopo metapelites it is concluded that the cordierite anthophyllite bearing metapelites are only in part true metasediments a metasomatic alteration of mafic precursors in an ocean crust environment could be responsible for both low pi indices and high with respect to unaltered mantle material 18 o values fluid rock interactions in this environment are also thought to be responsible for the observed chemical and o isotopic variation of mafic and ultramafic rocks the data do not support significant large scale co 2 infiltration during granulite facies metamorphism the retrograde regionally extended anthophyllite formation in the southern marginal zone of the limpopo belt is certainly triggered by some fluid infiltration model calculations show that the amount of fluids necessary for this readjustment is too small to have an imprint on the whole rock oxygen isotope composition

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

himalayan tibetan analogies for the evolution of the zimbabwe craton and limpopo belt abstract the limpopo orogeny which lasted from 2700 to 2600 ma was characterised by nnw sse directed shortening this resulted in both crustal thickening accommodated by folding and nnw directed thrusting and associated crustal extrusion whereby a number of crustal blocks including the central zone of the limpopo belt was displaced to the west southwest along wsw ene trending shear zones this was part of a regionally distributed phase of orogenic deformation which affected the adjoining kaapvaal and zimbabwe cratons from the witwatersrand basin in the south to the harare greenstone belt in the north this deformation which migrated northwards with time postdated an earlier phase of crustal accretion during which the archaean crustal unit now marked by the kaapvaal and zimbabwe cratons and the limpopo belt grew towards the north northwest shortening of this unit during the limpopo orogeny may be regarded as the last stage of the crustal accretion process this tectonic process of continental growth by accretion followed by crustal shortening with associated crustal extrusion has many features in common with tibetan tectonics from the mesozoic to the present

incipient charnockite characterization at the type localities abstract fifty years after the discovery of incipient charnockite orthopyroxene bearing granitic rocks partially replacing amphibole biotite gneiss in southern india this phenomenon has been found in transitional amphibolite facies to granulite facies terrains on every continent and may be a characteristic feature of progressive deep crustal metamorphism the alteration is essentially a metasomatic transformation mass balance calculations for localities in southern india and sri lanka for which analytic data are most comprehensive show that the main reaction producing orthopyroxene in orthogneiss is breakdown of hornblende by reaction with biotite with loss of feo mgo tio 2 and cao as well as h 2 o from the host rock there is concomitant gain of sio 2 no significant change of alkalis can be demonstrated as essential to the transformation though advanced charnockitic alteration at the type locality of kabbal karnataka may also show large increase of k 2 o with removal of mafic constituents very similar orthopyroxene bearing veins in garnet biotite gneiss at the ponmudi type locality in kerala show definite but more subtle metasomatism with loss of feo and mgo and increase of sio 2 and alkalis both the kabbal and ponmudi alterations occurred at mid crustal levels 5 6 kbar and 700 750 c it is postulated that both types of charnockitic alteration may have been produced as open system processes by much the same kind of metasomatizing fluids nearly coeval co 2 rich and ultrasaline fluids based on fluid inclusion evidence the paragneiss alteration is mainly biotite breakdown with some involvement of garnet and leads to a distinct metamorphic facies type the presence of graphite in some of the garnet biotite gneisses at ponmudi would have resulted in lower oxygen fugacity and probably lower h 2 o activity than in the similar kabbal type metamorphism the fluid rock ratios for both types were small about 10 mass percent of the rocks and the fluids whatever their origin were largely rock governed the source of the metasomatizing fluids has not been definitely identified carbon isotopes of fluid inclusions seem to show an igneous or primitive mantle source at both type localities but ultrasaline fluids may be more plausibly explained by metasedimentary sources and calcareous metasediments have been identified as a source of co 2 in some occurrences much remains to be learned about incipient charnockite and its significance in processes of crustal evolution

emergence and evolution of santa maria island azores the conundrum of uplifted islands revisited the growth and decay of ocean island volcanoes are intrinsically linked to vertical movements while the causes for subsidence are better understood uplift mechanisms remain enigmatic santa maria island in the azores archipelago is an ocean island volcano resting on top of young lithosphere barely 480 km away from the mid atlantic ridge like most other azorean islands santa maria should be experiencing subsidence yet several features indicate an uplift trend instead in this paper we reconstruct the evolutionary history of santa maria with respect to the timing and magnitude of its vertical movements using detailed field work and 40ar 39ar geochronology our investigations revealed a complex evolutionary history spanning 6 m y with subsidence up to ca 3 5 ma followed by uplift extending to the present day the fact that an island located in young lithosphere experienced a pronounced uplift trend is remarkable and raises important questions concerning possible uplift mechanisms localized uplift in response to the tectonic regime affecting the southeastern tip of the azores plateau is unlikely since the area is under transtension our analysis shows that the only viable mechanism able to explain the uplift is crustal thickening by basal intrusions suggesting that intrusive processes play a significant role even on islands standing on young lithosphere such as in the azores

metamorphic p t profiles from the zimbabwe craton to the limpopo belt zimbabwe abstract metamorphic p t profiles in the eastern and western parts of the limpopo belt zimbabwe were examined on the basis of mineralogic and petrologic data from areas which include the southernmost zimbabwe craton the northern marginal zone and the central zone the zimbabwe craton consists of a granite greenstone terrane of 2900 ma to 3600 ma old the northern marginal zone represents a high grade equivalent of the granite greenstone lithologies the central zone has suffered granulite facies metamorphism during the late archean metamorphic temperature of the zimbabwe craton adjacent to the northern boundary of the limpopo belt is calculated to be 650 680 c for amphibolite and 710 720 c for quartzofeldspathic gneiss in the eastern part these temperatures are slightly lower than that calculated from the same assemblages in the northern marginal zone and central zone peak metamorphic conditions in the eastern part of the northern marginal zone are estimated at about 740 790 c and 7 5 7 9 kbar from the assemblage garnet core orthopyroxene plagioclase quartz in pelitic gneiss these conditions were followed by a retrograde event of about 630 660 c and 3 8 4 5 kbar which were calculated from the assemblage garnet rim cordierite sillimanite quartz in the same rock the p t path is very similar to the retrograde path from the southern marginal zone it implies that both the northern marginal zone and southern marginal zone may have shared a similar metamorphic evolution garnet bearing mafic gneiss in the central zone indicates somewhat higher p t conditions than those in the northern marginal zone central zone metamorphic conditions are estimated at 790 890 c and 11 0 13 3 kbar for garnet clinopyroxene plagioclase quartz assemblages and 750 870 c and 7 6 8 8 kbar for garnet hornblende plagioclase quartz assemblages the difference of p t conditions between the northern marginal zone and central zone may be related to movement along the pre triangle shear zone the northern marginal zone appears to have been displaced along the shear zone relative to the central zone during uplift carrying the deeper crust to the surface

the archean craton of southern india metamorphic evolution and p t conditions the archaean craton of southern india is characterized by a highly complicated and not yet fully understood geological history comprizing several cycles of sedimentation and volcanism deformation and metamorphism in the span between about 3400 and 2500 m y the large scale regional variation in metamorphic grade observed today is essentially related to a metamorphic event at about 2600 m y ago which affected an older migmatite gneiss greenstone terrain 2900 3400 m y

an overview of charnockite abstract charnockite a quartzfeldspathic rock containing orthopyroxene is an important component of high grade deep crustal terrains the origin of charnockite by both igneous and metamorphic processes not yet clearly defined has major bearing on the evolution of continents

the significance of rb sr ages of biotite and phlogopite for the thermal history of the central and southern marginal zones of the limpopo belt of southern africa and the adjacent portions of the kaapvaal craton abstract generally rb sr biotite and phlogopite ages range from 2050 ma to 1950 ma between the murchison thabazimbi and triangle tuli sabi shear zones of southern africa in the rocks comprising the central and southern marginal zones of the limpopo belt and adjacent portions of the kaapvaal craton in some shear zones however similar ages ocurr as young as 800 ma in contrast within the kaapvaal craton south of the murchison thabazimbi shear zone rb sr biotite ages range between 3150 ma and 2100 ma and show no regular distribution in all cases the ages are interpreted to indicate the time elapsed since the biotite last cooled below its closure temperature approximately 320 c the regular pattern of 2050 ma to 1950 ma rb sr biotite ages north of the murchison thabazimbi shear zone suggests that the present erosional surface approximates a paleo isothermal surface this surface was exposed by uplift and erosion which was completed by 1900 ma to 1800 ma ago when the sedimentary and volcanic rocks of the soutpansberg group were deposited the regional uplift was accommodated by motion on the murchison thabazimbi and triangle tuli sabi shear zones and probably was related to tectonism associated with the emplacement of the anorogenic bushveld and phalaborwa complexes other anorogenic granitic magmatism accompanied uplift the depth represented by the paleo isothermal surface and the magnitude of the uplift that exhumed it cannot be constrained precisely it is suggested however that the majority of the 10 to 15 km by which crust in this area is thinner than that of the adjacent cratons is the manifestation of this uplift

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

fluid inclusions in rocks from the amphibolite facies gneiss to charnockite progression in southern karnataka india direct evidence concerning the fluids of granulite metamorphism fluid inclusion studies of rocks from the late archaean amphibolite facies to granulite facies transition zone of southern india provide support for the hypothesis that co2 rich h2o poor fluids were a major factor in the origin of the high grade terrain charnockites closely associated leucogranites and quartzo feldspathic veins contain vast numbers of large co2 rich inclusions in planar arrays in quartz and feldspar whereas amphibole bearing gray gneisses of essentially the same compositions as adjacent charnockites in mixed facies quarries contain no large fluid inclusions inclusions in the northernmost incipient charnockites as at kabbal karnataka occasionally contain about 25 mol of immiscible h2o lining cavity walls whereas inclusions from the charnockite massif terrane farther south do not have visibile h2o microthermometry of co2 inclusions shows that miscible ch4 and n2 must be small probably less than 10mol combined densities of co2 increase steadily from north to south across the transitional terrane entrapment pressures calculated from the co2 equation of state range from 5 kbar in the north to 7 5 kbar in the south at the mineralogically inferred average metamorphic temperature of 750 c in quantitative agreement with mineralogic geobarometry this agreement leads to the inference that the fluid inclusions were trapped at or near peak metamorphic conditions calculations on the stability of the charnockite assemblage biotite orthopyroxene k feldspar quartz show that an associated fluid phase must have less than 0 35 h2o activity at the inferred p and t conditions which agrees with the petrographic observations high tio2 content of biotite stabilizes it to lower h2o activities and the steady increase of biotite tio2 southward in the area suggests progressive decrease of ah2o with increasing grade oxygen fugacities calculated from orthopyroxene magnetite quartz are considerably higher than the graphite co2 o2 buffer which explains the absence of graphite in the charnockites the present study quantifies the nature of the vapours in the southern india granulite metamorphism it remains to be determined whether co2 flushing of the crust can by itself create large terranes of largeion lithophile depleted granulites or whether removal of h2o bearing anatectic melts is essential

metamorphism denudation and sea level in the archean and cooling of the earth abstract over long enough time scales an antagonistic balance acting between tectonics uplift and rifting and the sedimentary cycle erosion and deposition probably keeps continental surfaces close to sea level we argue that this balance constitutes the primary mechanism for keeping long term continental freeboard constant by regulating the continental thickness an important implication is that the long term uplift or down warp history of ancient continental segments provides direct information on secular changes in the isostasy of the adjacent ocean basins through time the amount of uplift can be estimated from the regional metamorphic grade the burial pressures at present exposure of ten undisturbed archean granite greenstone segments are estimated to be 1 5 0 5 kb from this we infer a maximum long term uplift and erosion of 5 2 km for stable continent since 3 0 ga implying a mean continental thickness of 46 km at the time of crustal stabilization for a constant mean ocean depth isostasy requires a palaeo oceanic crust 14 2 km thick in the middle archean alternatively if little continent existed then the oceans would have been shallower and the palaeo oceanic crust 21 2 km thick assuming plate spreading did occur in the archean potential temperatures in the shallow mantle at 3 ga were 90 30 c or 170 20 c for no continent hotter than those prevailing today these temperatures translate into time averaged cooling rates for the shallow mantle of 57 c ga 1 and are comparable to estimates of the present day cooling rate for the whole earth the low burial pressures are a strong argument against the existence of catastrophic mantle flush events in the geological past which have been predicted by some mantle convection models taken together these constraints suggest a plume related geological setting i e oceanic plateau like flood basalt for the initial stage of greenstone belt evolution

high pressure granulites of east siberia in terms of archaean and proterozoic evolution abstract the southern borderland of the aldan shield comprises high pressure granulites with dominant mafic components combined into the kurulta series these granulites pertain to the archaean and palaeoproterozoic history of the lower crust of the marginal part of the siberia craton while the upper crustal layers are composed of largely amphibolite facies rocks of the stanovik series which together with the kurulta series constitute the stanovik fold area the evolution of the area is characterized by recurrent tectonic activity of the aldan early stanovik stanovik and jeltulac cycles the earliest aldan events at 3 5 3 1 ga gave rise to primary granulite assemblages formed under geothermal gradients of 32 28 km homogeneous over the entire shield at 2 7 3 0 ga the northern more stable terrains were separated from the mobile stanovik area by the sutam belt which both controlled the special magmatism and high pressure metamorphism these events were governed by a low geothermal gradient of 22 20 km high pressure granulites resulted from sub crustal accretion including the aldan assemblages and early stanovik basic and calc alkaline magmatic rocks recrystallized at 750 950 c and 9 12 kbar the following stanovik activity at 2 4 ga resulted in uplift and interstacking of altered granulites together with the stanovik series metamorphosed under almandine amphibolite facies conditions after all this the jeltulac orogenic movements at 2 2 2 1 ga were accompanied by further retrogression and tectonic ascent of granulite fragments along local tectonic zones these events anticipated the emplacement of mafic dykes in the tukuringra zone at 1 8 1 9 ga when high grade metamorphism was resumed at deep crustal levels

structures in granitoid gneisses and associated migmatites close to the granulite boundary of the limpopo belt south africa abstract river platform exposures of quartzofeldspathic gneiss and associated migmatites occur close to the boundary of the southern marginal zone of the limpopo belt with the adjacent low grade granite greenstone terrane of the kaapvaal craton the complex gneisses and migmatites are described and are considered to represent the tectono thermally reworked equivalents of the pietersburg sutherland greenstone belts and early tonalitic and trondhjemitic intrusive granitoids structural features associated with the hout river shear zone and the limpopo orogeny 2700 2650 ma old and which include subhorizontal overthrust fault slices produced as a result of lateral extrusion of rock material following transpressional displacement along a strike slip fault or thrust zone support structural metamorphic and geophysical evidence suggesting southwestwards overthrusting of the high grade southern marginal zone rocks onto lower grade rocks of the kaapvaal craton

metamorphic fluids in the deep crust most of the inferences about fluid action in the deep crust are drawn from field petrographic and geochemical studies in the granulite facies terrains deductions on the nature of fluids are made from major and minor element distributions and isotopic patterns in metamorphic rocks calculations of fluid mineral equilibria based on observed assemblages fluid inclusions and field evidence of fluid pathways discussion of these features has focused attention on several major problems concerning the timing flow mechanisms and origin of the fluids a key issue concerns episodicity versus secular activity of fluids there are strong arguments that most fluid action in the deep crust is linked to thermal deformational events including magmatism and regional meta morphism discrete radiometric ages of high grade terrains often define relatively short periods of crystallization or recrystallization the late archean granulite facies metamorphism of southern india is an example where u pb systems of zircons buhl et al 1983 whole rock u pb data peucat et al 1 987 and sm nd systems of whole rocks and minerals bernard griffiths et al 1 987 all show a major high temperature recrys tallization event inferred by several workers to have involved pervasive fluid action centered closely around 2 5 ga another geochemical indi cation of episodic fluids comes from stable isotope studies of progressive metamorphic sequences as in the damara orogen namibia where whole rock b 180 increases slightly at isograds representing arrested dehydration events but is monotonous between isograds hoernes hoffer 1 985

boudinage in nature and experiment abstract deformation of rocks produces structures at all scales that are in many cases periodic folding or boudinage with variable amplitude and wavelength here we focus on boudinage a process of primordial importance for tectonics in the present study we carried out measurements of natural boudins and experimentally tested the effects of two variables on boudinage layer thickness and compression rate the models were made of a competent layer mostly brittle as in nature of either elastic soft paper or viscoelastoplastic clay material embedded in a ductile matrix of linear viscous silicone putty the competent layer lied with its greatest surface normal to the principal shortening axis and greatest length parallel to the principal stretching axis the model was then subjected to pure shear at constant piston velocity and variable competent layer thickness model 1 or at different piston velocity and constant layer thickness model 2 the results of model 1 show an exponential dependence of boudin width on competent layer thickness in disagreement with data from the studied natural occurrence this indicates that variables other than competent layer thickness are hidden in the linear relationship obtained for the natural boudinage the results of model 2 show that the higher the velocity the smaller the boudin width following a power law with exponent very similar to that of analytical predictions the studied natural boudinage occasionally occurs in two orthogonal directions this chocolate tablet boudinage can be the result of two successive stages of deformation buckling followed by stretching of competent sandstone layers or buckling followed by rotation of reverse limbs into the extensional field of simple shear

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

emplacement at granulite facies conditions of the sesia lanzo metagabbros an early record of permian rifting the evolution of the pre alpine corio and monastero metagabbros points to strong chemical and mineralogical similarities with that of other permian gabbro bodies of the alps which are concentrated in the southalpine and austroalpine domains the structural and metamorphic pre alpine evolution of these gabbros records a re equilibration following the emplacement in the deep crust p 0 6 0 9 gpa and t 850 70 c exhumation through amphibolite facies conditions p 0 5 0 35 gpa and t 570 670 c followed by a greenschist facies imprint 0 25 p 0 35 gpa and t 550 c this retrograde p t evolution suggests that the exhumation occurred in a high thermal gradient regime such as that induced by upwelling of an asthenospheric plume during continental rifting this would be consistent with the crustal thinning known to have occurred in both the southalpine and austroalpine domains during permian times the gabbros and their country acid granulites are spatially associated with the serpentinised subcontinental mantle of the lanzo massif this lithologic association and the metamorphic evolution is similar to that of the fedoz gabbro austroalpine domain of the central alps and completely different from that observed in passive margins where no remnants of the lower crust occur and the upper granitic crust directly overlies the serpentinized lherzolites the location of permian gabbro bodies in the austroalpine and southalpine domains and their absence in the helvetic domain is evidence for asymmetric rifting

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

seismic studies in the limpopo belt abstract the structure of the archaean limpopo belt of southern africa has been investigated by means of a deep seismic reflection profile and the recording of the traveltimes of mining induced tremors at regional distances interpretation of the traveltimes suggests that a seismic low velocity zone exists in the upper mantle of the northern kaapvaal craton and limpopo belt beginning at a depth of about 42 km the seismic reflection profile provides evidence for a northward dip of the contact between the kaapvaal craton and the southern marginal zone of the limpopo belt no mid crustal decollement zone is detected the tectonic model of van reenen et al 1990 is in accord with these observations the lower crust is essentially seismically transparent and no reflection moho is observed

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

constraints on the composition of fluids involved in retrograde anthophyllite formation in the limpopo belt south africa abstract orthoamphibole occurs widely in the metapelitic gneisses and other lithologies within the southern marginal zone of the limpopo belt in southern africa petrographic relations indicate that the orthoamphibole formed as a retrogressive replacement of the granulite facies minerals cordierite and orthopyroxene the existence of a discrete retrograde orthoamphibole isograd that can be mapped for over 150 km has led previous workers to suggest that the orthoamphibole formed as a result of equilibrium high temperature reactions which occurred in response to regional influx of fluids at the close of the limpopo orogeny 2670 ma ago however in order to substantiate this interpretation it is necessary to show from phase equilibrium analysis that the metapelitic minerals and their retrograde products comprise assemblages stable at elevated pressure and temperatures rather than disequilibrium assemblages formed at unspecified lower temperatures by monotropic reactions some of the properties of the infiltrating fluids may in turn be specified if the alteration products represent equilibrium assemblages it is shown that the presence of anthophyllite and the absence of talc in two analysed metapelites from the retrograde isograd require an associated vapour phase to have been relatively low in water ah2o 0 7 and temperatures to have been below 620 c when the anthophyllite developed from the orthopyroxene the fast kinetics of formation of talc and the relatively sluggish kinetics of formation of anthophyllite in experimental systems make it highly unlikely that the orthoamphibole could have formed metastably at low temperatures and high water activities a more general phase equilibrium analysis based on the coexistence of cordierite garnet biotite enstatite plagioclase and quartz with retrograde anthophyllite and kyanite indicates that this assemblage was in equilibrium at temperatures between 450 to 650 c for pressures of 5 to 7 kbars and water activities between 0 6 and 0 1 the temperatures pressure and water activities are highly correlated and lower pressures and water activities are required for equilibrium at lower temperatures the reduced water activities calculated require that a substantial quantity of another volatile be present in the vapour phase in the absence of stable scapolite or carbonates this volatile cannot be confirmed to be co2 the required infiltration of late orogenic fluids with reduced water activities is apparently a major feature of the limpopo event and may have parallels in other large precambrian high grade terrains in as much as orthoamphibole replacing orthopyroxene is a commonly observed mineral assemblage

direct current resistivity and lotem model for the deep structure of the northern edge of the kaapvaal craton south africa abstract the transition from the high grade metamorphic southern marginal zone smz of the limpopo belt to the low grade kaapvaal craton was studied by means of geoelectrical techniques the d c data showed conclusively that the high grade rocks were thrust over the low grade rocks this contradicts earlier geological ideas but agrees with recent seismic reflection data the area where high grade rocks occur above low grade strata was mapped out and the southern boundary of the smz was established the conductor that occurs at lower crustal depths in the kaapvaal craton was traced into the smz using the lotem method the conductor shallows from 30 to 22 km over 12 km on the surface but does not reflect the difference in metamorphic depth of 30 km seen at the surface this indicates that the conductor is a transient feature of the crust the conductor is not associated with a seismically reflective crust the cause of the conductor remains enigmatic