inclusions of nanocrystalline hydrous aluminium silicate phase egg in superdeep diamonds from juina mato grosso state brazil abstract inclusions in alluvial diamond from juina mato grosso brazil have been investigated by tem methods electron diffraction hrtem aem haadf eels and raman spectroscopy the inclusion paragenesis of juina diamonds is dominated by ultrahigh pressure superdeep phases one of these diamonds sample 1 1 4 contains several micrometer sized approximately 200 m by 50 70 m inclusions which have been studied tem foils prepared applying focused ion beam fib technique revealed that these inclusions consist of a porous nanocrystalline groundmass which is composed of nanometre sized crystals of a hydrous aluminium silicate phase with al si approximately 1 1 and chemical composition of phase egg alsio 3 oh a minor volume fraction of nanocrystalline stishovite and pore space which was originally filled with a fluid or gas the nanocrystalline hydrous aluminium silicate phase is idiomorphic randomly oriented approximately 20 30 nm in size predominantly with tetragonal crystal structure a 0 0 743 nm c 0 0 706 nm the monoclinic structure of synthetic phase egg determined at ambient conditions m w schmidt l w finger r j ross r e dinnebier synthesis crystal structure and phase relations of alsio 3 oh a high pressure hydrous phase american mineralogist 83 1998 881 888 is only occasionally observed the fluid filling in the porosity has been released into the vacuum of the fib during tem specimen preparation quench products of the fluid containing minor concentrations of f p s cl k ca and ba were detected at the walls of the pores in addition phase egg is identified by raman spectroscopy within a second sample rs 43a from the same location the presence of phase egg in the inclusions in diamond may suggest that crustal material has been subducted to a depth of the lower transition zone although metastable growth of nanocrystalline high pressure phases or extension of their respective stability fields to lower pressure can not ruled out completely

late ordovician shallow marine environments due to glacio eustatic regression scrach formation mid wales an ashgill to llandovery offshore marine sequence in mid wales is punctuated by a shallow marine sequence the scrach formation attributed to late ordovician glacio eustatic regression the base of the regressive sequence is gradational and suggests progressive fall in sealevel during mid hirnantian late ashgill time an abrupt top to the scrach formation marks a rapid sealevel rise probably within the persculptus zone late hirnantian interbedded lenticular thin sands and muds dominate the scrach formation and represent shallow shelf environments with evidence of nw directed tidal ebb flow and storm currents sporadic medium bedded sand sheets represent the strom events locally lenticular thick bedded sand units embedded in the fine facies probably represent barrier facies in an area of high clastic input the scrach formation thins se into the present ne striking myddfai steep belt this is underlain by ne striking faults that actively controlled late ordovician to early silurian sedimentation the faults probably defined an emergent but low relief coast during the glacial regression offshore to the nw the scrach passed into parallel laminated mudstones then across a fault influenced slope the present tywi lineament into coarse clastic sediments deposited by gravity flows

structural geological and metamorphic significance of the kaapvaal craton limpopo belt contact abstract the hout river shear zone forms the terrane boundary between the granite greenstone terrane of the kaapvaal craton cratonic terrane and the high grade granulite terrane high grade gneiss province of the limpopo belt this structure is superimposed onto a thin skinned northward verging thrust system which exercises a significant control on the distribution of the ene wsw trending greenstone belts the hout river shear zone however dips steeply to the north and flattens out toward the south this structure was responsible for the emplacement of hot granulites against and over the cooler granite greenstone terrane and strongly influenced the metamorphic evolution of the area the shear zone acted as a conduit for infiltrating co2 rich h2o bearing fluids into still hot crust producing a retrograde isograd and associated zone of rehydration in the granulitic hanging wall of the structure retrogressed granulites in the hanging wall are characterized by fine grained kyanite as part of the retrograde assemblages while the greenstones in the footwall are characterized by coarse grained prograde kyanite the terrane boundary appears to be in thermal equlibrium with the lithologies in both the footwall and hanging wall so that tectonically mixed prograde greenstones and retrogressed granulites in and adjacent to the shear zone are apparently at a similar metamorphic grade the granulites appear to have had a hot iron effect on the cooler footwall rocks over which they have been displaced

strike slip faulting along the church stretton lineament old radnor inlier wales the old radnor inlier comprises precambrian sediments unconformably overlain by wenlock age limestones and shales its structure is dominated by steep strike slip faults major ne sw faults probably bound the inlier and are linked by more northerly faults to form a strike slip duplex strike slip faults within the inlier match a riedel shear system with observed dextral offsets on a nw striking set and implied sinistral offsets on the major inlier parallel faults subordinate dip slip faults are anomalously oriented but may be riedel shears reactivated during progressive sinistral transtension across the inlier pre wenlock deformation is recorded by the tilting of the precambrian sediments and by some faults that do not cut the unconformity on regional grounds this deformation might be ashgill late ordovician or late precambrian early cambrian in age the strike slip component now predominating was imposed in post wenlock time probably during the regional sinistral transpression of the acadian late caledonian event in early devonian time later variscan and mesozoic components cannot be ruled out

further paleomagnetic results from the builth wells ordovician inlier wales a characteristic magnetization has been isolated from 18 sites in middle ordovician igneous and sedimentary rocks from the builth inlier wales two conglomerate tests demonstrate that the characteristic magnetization in two of the volcanic units was acquired early perhaps during initial emplacement six sites from the northern and central parts of the inlier give a mean direction of d 153 8 i 69 8 k 45 8 95 10 that passes a regional fold test however 12 sites from the southern end of the inlier in and near the llanelwedd quarry yield a significantly different direction after correction for local tilt d 176 4 i 44 5 k 42 4 95 6 7 the llanelwedd quarry data are anomalous with respect to both results from the northern parts of the inlier and those obtained from approximately coeval rocks from the shelve inlier we have searched for an explanation for the discrepancy between the magnetization directions of the two areas and have now ruled out some possibilities including superimposed magnetization components at one of the two areas the llanelwedd quarry lies within a wedge of dominantly andesitic volcanic rocks that appear only locally and which are overlain by two unconformities that are also not present elsewhere in the inlier one possible explanation for the anomalous nature of the llanelwedd quarry magnetization is that the strata were not horizontal at the time of magnetization either due to early volcano tectonic tilting or primary dips on the flanks of a volcano another possibility is that the llanelwedd quarry rocks were extruded rapidly enough that the geocentric coaxial dipole field hypothesis cannot be applied results from cenozoic volcanic rocks from the western united states suggest that even highly anomalous fields can be preserved virtually unchanged over a number of separate flow events we currently regard this to be the most viable explanation for the anomalous nature of the llanelwedd quarry magnetization we conclude that the data from the 6 northern sites represent the best estimate of the middle ordovician axial dipole field for the inlier

sequence stratigraphy of the palaeozoic welsh basin the timing and extent of unconformities in the welsh basin are investigated using rock preservation curves derived from outcrop stratigraphic logs four basin wide unconformities occur focussed in late precambrian late tremadoc pusgillian early ashgill and mid devonian times these bound three megasequences equating with newly defined lithostratigraphic units the dyfed gwynedd and powys supergroups less extensive unconformities bound 18 component sequences the majority of the sequence boundaries reflect a component of tectonic or volcanotectonic activity rather than a pure eustatic sea level change the megasequence boundaries are attributed to late precambrian to early cambrian onset of rifting to form a passive margin tremadoc onset of subduction with intra arc then back arc extension late caradoc end to subduction and late early devonian collisional deformation the megasequences and controlling events can be tentatively matched with other basins on the avalonian margin more generally this study shows that sequence analysis is feasible in onshore basins lacking well and seismic data and that the global eustatic interpretation of sequence stratigraphy is only partially applicable to active margin basins

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

menai strait fault system an early caledonian terrane boundary in north wales a steep fault system runs parallel to the menai strait which separates anglesey from the mainland of north wales the menai strait fault system is interpreted as a major terrane boundary along which terrane docking took place during the late precambrian or earliest cambrian the evidence is based on 1 marked geologic dissimilarities exhibited by the pre late early cambrian basement to these terranes and 2 the existence of early major ductile shear zones within the fault system rocks as varied as blueschists low grade melange and shallow level granitic complexes were juxtaposed along these steep shear zones prior to the eruption of ash flow tuffs and deposition of upper lower cambrian marine sediments in the welsh basin since the amalgamation of the anglesey area with the avalonian terrane then a part of gondwana frequent phanerozoic reactivation of the resulting composite terrane boundary has been restricted to mostly dip slip fault movements

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

role of the tectonic facies concept in orogenic analysis and its application to tethys in the eastern mediterranean region abstract the purpose of this paper is to advocate the use of a tectonic facies concept which aids systematic and objective analysis of orogenic belts the tectonic facies concept as discussed here focuses on the recognition of discrete tectonic settings mainly those exposed at high crustal levels in orogenic belts application of the tectonic facies concept can help field geologists to relate plate scale processes to outcrop geology tectonic facies are recognised based on knowledge of the modern oceans and margins and well constrained land settings tectonic facies analogous to sedimentary facies can be identified from integrated sets of field data and related laboratory studies a number of tectonic facies models can be recognised in this way their identification aids interpretation and comparisons of orogenic belts palinspastic reconstructions require additional information particularly age kinematic and palaeomagnetic data in this paper the tectonic facies concept is tested by application to a well documented mainly high level orogenic segment tethys in the eastern mediterranean area and is found to be an effective way both of the describing and interpreting this region other methods of tectonic analysis e g recognition of tectono stratigraphic terranes may also be found useful in certain settings and can be used in conjunction with the tectonic facies concept

a thin skinned thrust model for variscan pembrokeshire wales abstract the east and west coasts of pembrokeshire sw wales provide two sections through the variscan fold and thrust belt the evolution of these structures is interpreted in terms of a thin skinned tectonic model balanced cross sections are constructed for the high level imbricate sequences and these allow reasonably accurate estimates of shortening to be made basement control on structures developed in the upper carboniferous cover rocks is minimal though some thrust ramp positions may be determined by the location of earlier normal faults the thrust belt may be divided into two parts according to the depth to the decollement horizon in the north imbricate fans developed from a shallow level detachment shortening is heterogeneous and both thrust periodicity and fold style are partly determined by rheology cumulative tectonic displacement increases to the west across pembrokeshire resulting in a net clockwise rotation of about 40

geology of the gouap iron deposit congo craton southern cameroon implications for iron ore exploration abstract the ntem complex represents the northwestern edge of the archean congo craton in southern cameroon where numerous banded iron formation bif hosted iron deposits have been discovered the gouap iron deposit which is located on the western edge of the ntem complex is one of the recent discoveries of iron deposits in this area in this study geological mapping detailed drillcore logging and petrographical studies reveal that the gouap iron deposit comprises of two lithostratigraphic units namely the bif and the country rocks the bif unit consists of amphibole bif and amphibole pyrite bif whereas the country rocks unit is made up of gneisses garnet amphibole magnetite and amphibole garnet pyrite gneiss schists amphibole magnetite and garnet magnetite micaschist quartzites and dolerite sills the bifs are well interlayered and observed to be in sharp or gradational contacts to their associated country rocks the mineral assemblages of the bifs and wallrocks except the dolerites indicate that both units have been metamorphosed under amphibole facies conditions two main iron mineralization types are identified in the gouap deposit the soft iron type which is mainly located in the western and northern parts and the hard iron type mostly confined to the southern sector of the deposit the structural controls of the gouap iron deposit are better observed in drillcore samples other than on the outcrop scale there are three principal deformation events d1 d2 and d3 that led to the development of foliation folding and faulting respectively the iron mineralization are mostly concentrated on the f2 fold hinge zones and fractures that acted as passage ways for the circulation of meteoric water at surface we suggest that the following three major processes that led to the formation of the gouap deposit i the chemical precipitation of chert and iron oxide followed by sedimentation and diagenesis that formed chert magnetite bifs ii secondly the amphibolite facies metamorphism and deformation that formed quartz magnetite bifs which constitute the main iron ridge located on thicker zones of fold noses and iii finally the gouap bifs have been upgraded to medium grade iron ore due to supergene enrichment processes which are restricted to the modern land surface and locally reached deeper levels through fault and fracture zones the western and northern parts of the gouap deposit have been identified as potential exploration target areas for iron mineralization even though the iron formation processes of the gouap deposit are not yet completely understood and many questions remain unanswered regarding the genesis of the soft and hard iron mineralization these new insights have important implications on the exploration drilling strategies of this deposit

geochemistry and geodynamic implications on the source of parana etendeka large igneous province evidenced by the 128 ma rosario 6 kimberlite southern brazil abstract the rosario 6 is a non diamondiferous hypabyssal kimberlite located above the rio de la plata craton and near the south eastern edge of the parana basin in southern brazil it is petrographically an inequigranular texture macrocrystal kimberlite fresh and the groundmass exhibits a microporphyritic texture and round megacrysts of olivine which are derived from disaggregated mantle xenoliths olivine is also present as macrocrysts microphenocrysts and in the groundmass together with phlogopite and apatite these microphenocrysts are immersed in a groundmass of olivine monticellite phlogopite catio3 perovskite apatite mg chromite and mg ulvospinel and melilite a mesostasis assemblage of phlogopite melilite soda melilite akermanite and calcium carbonate is segregated from the groundmass its geochemical signature is similar to those of transitional kimberlites of kaapvaal craton south africa and the u pb ages of 128 ma on perovskite reveal that rosario 6 kimberlite post dates the main pulse of volcanism in the parana etendeka large igneous province lip the high ti content of some minerals such as mg chromite mg ulvospinel phlogopite and melilite and the presence of perovskite suggest a ti rich source the petrographic geochemical and isotopic data indicate that the rosario 6 kimberlite source is a depleted mantle metasomatized by h2o rich fluids co2 rich and silicate melts derived from the recycling of an ancient subducted oceanic plate eclogite before the south atlantic opening although several authors indicate the influence of tristan da cunha plume for the generation of alkaline magmatism associated to the parana etendeka flood basalts our data demonstrates that tristan da cunha plume has no chemical contribution to the generation of rosario 6 kimberlite except by its thermal influence

paleomagnetic results from volcanic rocks of the shelve inlier wales evidence for a wide late ordovician iapetus ocean in britain abstract in order to shed light on the early paleozoic paleogeography of iapetus bordering continents samples for paleomagnetic study were collected from middle ordovician llanvirn volcanic rocks of the shelve inlier welsh borderland the folding within the inlier is constrained to late ordovician ashgill time thus offering a fold test with tight age brackets after stepwise thermal demagnetization a pre folding characteristic magnetization component was isolated from 95 specimens the mean of 11 site mean directions for the characteristic magnetization gives d i 116 4 67 9 k 89 4 95 4 9 and the paleomagnetic pole lies at 26 6 n 36 1 e p 6 9 m 8 2 the fold test is positive at the 99 confidence level indicating that the magnetization was acquired during the middle or late ordovician we infer from geological considerations that this magnetization is not a primary thermoremanence but was acquired during late ordovician time as a result of a thermochemical disturbance of that age our results indicate that southern britain was near 51 south latitude during the later ordovician since middle to late ordovician paleolatitudes for gondwana and laurentia are well determined our result can be used to test various scenarios for the closure of iapetus we conclude that 1 the iapetus ocean in britain still had a latitudinal width of some 30 at this time thus rendering a postulated ordovician closure unlikely 2 southern britain avalon and hercynian europe all occupied high latitudes during the later ordovician consistent with a paleoposition near the northern margin of gondwana and 3 the acadian orogeny in britain and north america could have been the result of the collision of gondwana and laurentia during early devonian time

palaeozoic terranes and their lithospheric boundaries within the trans european suture zone tesz a review the trans european suture zone tesz is a broad and complex zone of terrane accretion separating ancient lithosphere of the baltic shield and east european craton eec from the younger lithosphere of western and southern europe there is debate about the number of terranes involved and their origins the most significant terrane boundaries originally oceanic sutures are poorly exposed and are defined using faunal provinciality prominent geophysical lineaments may represent sub terrane boundaries within composite terranes the avalonia composite terrane was amalgamated to baltica in latest ician time the provenance of the malopolska and moravo silesian terranes whose neoproterozoic basement is suspect with respect to baltica but with which they nonetheless share faunal affinities is still debated the extent of the variscide rhenohercynian zone defines the southern limit of the early palaeozoic accreted terranes terranes within the bohemian massif exhibit hp metamorphism recording subduction driven collision prior to incorporation in the tesz collage itself associated with a hp event in romania the saxothuringian zone and rheic suture are directly juxtaposed with the eec and the crustal structure of the tesz has been much modified by post palaeozoic tectonism deep seismic reflection data where available show that the oceanic sutures are frequently associated with inclined reflectivity zones irz in the lithospheric mantle here inferred to mark relict subduction zones interpreting the age of subduction is complicated by offset of originally coplanar mantle and crustal segments of the irz resulting from early orogenic lithospheric delamination at the moho interface and or post orogenic reordering processes the mantle irz appear to indicate a n directed polarity for early palaeozoic subduction apparently incompatible with the geological evidence which indicates subduction of the tornquist sea beneath avalonia and a large amount of underthrusting of avalonia by baltica possible explanations for these apparently incompatible observations are evaluated

compositional classification of kimberlitic and non kimberlitic ilmenite abstract ilmenite is one of the common kimberlitic indicator minerals recovered during diamond exploration and its distinction from non kimberlitic rock types is important this is particularly true for regions where these minerals are present in relatively low abundance and they are the dominant kimberlitic indicator mineral recovered difficulty in visually differentiating kimberlitic from non kimberlitic ilmenite in exploration concentrates is also an issue and distinguishing kimberlitic ilmenite from those derive from other similar rocks such as ultramafic lamprophyres is practically impossible ilmenite is also the indicator mineral whose compositional variety has the most potential to resolve provenance issues related to mineral dispersions with contributions from multiple kimberlite sources various published data sets from selected kimberlitic including kimberlites lamproites and various ultramafic lamprophyres and non kimberlitic rock types have been compiled and evaluated in terms of their major element compositions compositional fields and bounding reference lines for ilmenites derived from kimberlites sensu stricto ultramafic lamprophyres and other non kimberlitic rock types have been defined primarily on mgo tio2 graphs as well as mgo cr2o3 relationships

kimberlitic sources of super deep diamonds in the juina area mato grosso state brazil abstract the juina diamond field in the 1970 80s was producing up to 5 6 million carats per year from rich placer deposits but no economic primary deposits had been found in the area in 2006 2007 diagem inc discovered a group of diamondiferous kimberlitic pipes within the chapadao plateau chapadao or pandrea cluster at the head of a drainage system which has produced most of the alluvial diamonds mined in the juina area diamonds from placer deposits and newly discovered kimberlites are identical they have super deep origins from the upper mantle and transition zone field observations and petrographic studies have identified crater facies kimberlitic material at seven separate localities kimberlitic material is represented by tuffs tuffisites and various epiclastic sediments containing chrome spinel picroilmenite manganoan ilmenite zircon and diamond the diamond grade varies from 0 2 1 8 ct m3 chrome spinel has 30 61 wt cr2o3 picroilmenite contains 6 14 wt mgo and 0 2 4 wt cr2o3 manganoan ilmenite has less than 3 wt mgo and 0 38 1 41 wt mno the 176hf 177hf ratio in kimberlitic zircons is 0 028288 0 28295 with ehf 5 9 8 3 and lies on the average kimberlite trend between depleted mantle and chur the previously known barren and weakly diamondiferous kimberlites in the juina area have ages of 79 80 ma in contrast zircons from the newly discovered chapadao kimberlites have a mean 206pb 238u age of 93 6 0 4 ma corresponding to a time of magmatic activity related to the opening of the southern part of the atlantic ocean the most likely mechanism of the origin of kimberlitic magma is super deep subduction process that initiated partial melting of zones in lower mantle with subsequent ascent of proto kimberlitic magma

kinematics of strike slip faulting builth inlier mid wales abstract strike slip faulting in the builth ordovician inlier is demonstrated by large scale maps of the llanelwedd quarries near builth wells and by fault plane and slickenline data in the main quarry steep nnw striking strike slip faults dominate the structure together with significant strike slip displacement on the w dipping bedding surfaces and bedding parallel faults a zone of steep n striking extensional dip slip faults links two of the strike slip strands and there is a weaker e striking set of strike slip faults when the four fault sets are rotated so that the regional bedding is horizontal three become vertical and one horizontal probably their attitude during active life in late ordovician to early silurian time they formed a linked fault system capable of accommodating three dimensional bulk strain the fault flats have the same kinematic role in a strike slip system as lateral ramps or transfer faults in dip slip systems in the nearby gelli cadwgan quarry strike slip faults are again dominant but strike e or ese this heterogeneity of fault pattern in the southern builth inlier resolves into more homogeneous domains with areas of 0 1 to 0 5 km 2 separated by e to ne striking dextral strike slip faults domainal structure an important general feature of strike slip tectonics may be present on a variety of scales

late yeadonian upper sandstone group incised valley supply and depositional systems in the south wales peripheral foreland basin implications for the evolution of the culm basin and for the silesian hydrocarbon plays of onshore and offshore uk abstract the upper sandstone group of south wales has previously been interpreted as a simple nnw sse trending incised valley that was filled during the late yeadonian namurian g1b data presented in this study indicate a far more complex scenario the group is well developed in the pembrokeshire sub basin but it is absent from the main part of the south wales basin located to the east of the penlan axis the principal source area for sediment flux was the forebulge of the basin located to the north in lower palaeozoic foreland strata variscan flexural uplift of this source area and an accompanying rapid high magnitude fall in sea level produced a very high sediment budget thus during the late yeadonian lowstand of sea level massive amounts of sediment were being generated and transported via an incised valley system across the western part of the south wales peripheral foreland basin the location and orientation of the incised supply system and its related interfluves were controlled by intrabasinal synsedimentary variscan structures many of which were reactivated caledonian extensional faults during the succeeding sea level rise the incised valley was filled initially by fluvio deltaic sands and later by estuarine argillaceous sediments the implications of this combined tectonic sedimentological high resolution sequence stratigraphic model for the evolution of the late namurian crackington formation turbidites of the culm basin are discussed examples of silesian incised valley fill sandstone reservoirs developed in the east midlands oilfield and the southern gas basin are briefly summarised and their spatial and temporal distributions are appraised data generated from these case studies are assessed in terms of their use for the modelling of incised valley fill sequences in foreland basins and other tectonic settings

reaction history of garnet sapphirine granulites and conditions of archaean high pressure granulite facies metamorphism in the central limpopo mobile belt zimbabwe sequential reaction textures in archaean garnet corundum sapphirine granulites from the central zone of the limpopo belt document a progression from early coarse grained high pressure p 9 5 kbar granulite facies assemblages m1 to late low pressure p 6 kbar granulite facies sub assemblages m2 the stable m1 assemblage was garnet 57 pyrope mg mg fe 62 sapphirine corundum gedrite phlogopite rutile late m1 boron free kornerupine grew at the expense of garnet and corundum and coexisted with garnet sapphirine and gedrite partial or complete breakdown of coarse garnet and kornerupine during m2 resulted in the development of pseudomorphs and coronas consisting of fine grained symplectic intergrowths of cordierite gedrite and sapphirine later spinel the majority of reaction textures can be explained in terms of a stable reaction sequence and a model time sequence of mineral facies can be constructed when compared with a qualitative petrogenetic grid of fe mg discontinuous reactions in the fmash multisystem sapphirine garnet corundum spinel cordierite gedrite kornerupine the facies sequence indicates decompression at essentially constant t assuming constant a h2o exhumation of m1 corundum inclusions during m2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite a second disequilibrium reaction of the spinel with cordierite produced sapphirine the operation of such reaction while pressure was decreasing the opposite dp from that implied by the texture if assumed to be the product of an equilibrium reaction has serious implications for the use of reaction textures in the construction of p t vectors garnet biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures of ca 850 c for the m1 stage a minimum late m1 pressure of ca 7 kbar is indicated by the former association of kornerupine and corundum relict m1 kyanites reported by other workers indicate a minumum early m1 pressure of 9 5 kbar implying metamorphism at depths of at least 33 km probably 38km the high pressure granulite facies metamorphism was followed by an almost isothermal pressure decrease of 5 kbar indicative of rapid uplift the p t path is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite facies terranes

the significance of 3000 ma granulite facies mafic dikes in the central zone of the limpopo belt southern africa abstract structural petrographic and chemical relationships indicate that a suite of mafic dikes possibly 3000 ma old were emplaced at high crustal levels deformed and metamorphosed epicontinental supracrustal rocks of the central zone of the limpopo belt these dikes were subsequently metamorphosed at granulite grade conditions probably during the limpopo orogeny about 2700 ma ago the earlier metamorphism was under conditions of at least amphibolite grade and isotopic data suggest that it may have occurred between about 3200 ma and 3100 ma ago these relationships indicate that the rocks comprising the central zone did not follow a single p t time loop but instead were involved in two distinct orogenies the orogenies were separated by a period of crustal stability during which the supracrustal rocks presently and originally exposed at the surface were again moved near to the surface this tectonic history also implies that the supracrustal rocks of the central zone and the sand river gneisses are of great antiquity if the sand river gneisses with their unique suite of older mafic dikes form a previously deformed basement to the supracrustal rocks then the sand river gneisses have been involved in three orogenies pb isotopic data indicate that the mafic dikes were derived from a mantle source with a high average ratio at some stage in its evolution prior to 3200 ma all archean lithologic units within the central zone of the limpopo belt are characterized by having been derived from rocks that had a large mu ratio at some time in their history and this relationship suggests that a link existed between the crustal rocks of the central zone and this mantle source from at least 3200 ma ago until it was destroyed by or after the limpopo orogeny the tectonic history of the central zone is different from those of the lithologies forming the adjacent terranes and suggests that it evolved separately from these terranes until they were juxtaposed during the limpopo orogeny

interpretation of the regional gravity and magnetic surveys of wales using shaded relief and euler deconvolution techniques regional gravity and aeromagnetic data of wales have been processed using a variety of techniques image processing has greatly assisted qualitative interpretation whilst automated procedures have provided additional quantitative information the shaded relief images emphasize gradients in the potential fields and are useful for displaying strong linear features the euler deconvolution method produces plotted solution maps which define the position of the source of the gravity and magnetic anomalies euler solution maps of wales and the adjacent continental shelf are presented for the first time these maps are interpreted in relation to the known geology with special emphasis on the lower palaeozoic welsh basin it is proposed that the euler solutions define a network of fault bounded blocks within the precambrian basement

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

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

super deep diamonds from kimberlites in the juina area mato grosso state brazil abstract one thousand three hundred sixty five diamonds from seven newly discovered kimberlitic pipes in the juina area were comprehensively studied these diamonds like the ones from previously studied juina placer deposits are very homogeneous in their morphology and optical properties two diamond populations exist in the pandrea pipes the major population with a highly aggregated nitrogen impurity n b 75 100 and a secondary population with a moderately aggregated nitrogen impurity n b 20 65 while only one major population is present in the diamonds from placers the diamonds from the pipes have a permanent relatively high hydrogen impurity concentration among the mineral inclusions in diamonds from the juina pipes ferropericlase is predominant chrome spinel picroilmenite mn ilmenite mgcasi perovskite phase rutile sulphide native iron and iron oxides were also identified most of the inclusions belong to the lower mantle paragenesis some rutile and sulphide are of eclogitic paragenesis mineral inclusions in diamonds from kimberlitic pipes are different in composition from the same minerals in placer diamonds both kimberlitic and placer diamonds belong to the same carbon isotopic population but have differences in the 13 c distribution and were probably formed from different local carbon sources these data indicate that diamonds from both groups kimberlites and placer deposits in the juina area belong to the same genetic population with most of the stones originating within the super deep conditions however there are differences between these two groups which indicate that besides the known pandrea pipes which may have partly supplied diamonds to the placer deposits there may be other still unknown primary sources of diamonds in the juina area

sedimentation of deep water slope base and inner fan deposits the drosgol formation ashgill west central wales abstract the sedimentology of the drosgol formation is summarised in terms of nine sedimentary facies with particular attention given to the pebbly mudstones and major arenites sedimentary facies geometry and palaeocurrents suggest the interpretation of the major arenites as inner fan channel deposits locally preserving their levees the lower less organised part of the formation suggests a slope base environment with small scale channelling slumping and development of mass flow deposits a new definition of the upper boundary of the formation is proposed and map references are given to key localities

a revised sedimentary and biostratigraphical architecture for the type llandovery area central wales the global standard for the llandovery series early silurian in central wales is re assessed in the light of detailed geological surveying biostratigraphical sampling and a rigorous examination of published datasets a new sedimentary and biostratigraphical architecture is presented key graptolite brachiopod acritarch and for the first time chitinozoan assemblages are critically assessed upper hirnantian to aeronian strata record events that followed the late ordovician glacial maximum and comprise a series of progradational sequences bounded by flooding surfaces but inferred still to be glacioeustatic in origin significant faunal renewals associated with many of the flooding levels underpin their potential for international recognition compound non sequences are a feature of proximal parts of the system where erosion associated with fault footwall uplift was an important process extensive slump sheets contribute to further stratal loss and displacement in distal facies a re assessment of the aeronian stage gssp reveals shortcomings with the biostratigraphical criteria used in its selection telychian portions of the succession display the disrupting effects of intra wenlock synsedimentary sliding hence the relevance of key published fossil assemblages and the criteria used to erect the stage gssp are undermined however the llandovery area remains one of the best studied early silurian successions in the world this together with regional considerations supports the retention of the series standard in mid wales where the contiguous deep water basinal succession affords internationally cited exposure of richly graptolitic facies for the whole series and significantly for the post sedgwickii biozone interval

alluvial fan and wetland interactions evidence of seasonal slope wetlands from the silurian of south central wales uk distal environs of the ludlow age trichrug formation from south central wales uk detail deposition on the outer fringe of an ephemeral debris flow dominated alluvial fan debrites and subordinate sheetflood deposits are interbedded with sporadic thin sandstone dominated heterolithic units deposited in shallow ephemeral ponds in the axial valley the latter slope wetland system fringed the permeable alluvial fan deposits being maintained by streamflow precipitation and ground water recharge a prolonged high water table is indicated by low chroma intervals interpreted as the result of gleying a variety of redoximorphic indicators including colour mottling and ferricrete concretions are evidence of iron mobilization and concentration associated with a seasonally fluctuating water table the slope wetlands were colonized periodically by skolithos generating organisms most likely to have been arthropods the widespread occurrence of redoximorphic indicators and ferricrete contrasts markedly with calcrete vertisols in penecontemporaneous continental deposits lower old red sandstone of the region it is likely that the alluvial fan toe area was the site of a wetland depression which maintained a high though fluctuating water table in contrast the alluvial channel reaches of the surrounding drainage network were significantly better drained with floodplains prone to wetting and drying and vertisol palaeopedogenesis

rb sr whole rock isochron evidence for the age of the malvern hills complex the malvernian igneous complex of the malvern hills consists mainlyof plutonic rocks and their locally deformed equivalents the igneous rocks are calc alkaline and range from ultra mafic diorite to granite in composition although diorite and tonalite are the most abundant rock types a rb sr whole rock isochron plot for representative samples yields an age of 681 53 ma with an initial 87 sr 86 sr ratio of 0 7049 0 0005 2 errors this is interpreted as the age of crystallization of the igneous complex and the relatively low initial 87 sr 86 sr ratio is considered to preclude a long crustal history for the source region the malvernian complex is thus regarded as a late precambrian addition to the crust it has been compared on geological grounds with the johnston complex and the stanner hanter complex and isotopic ages between 640 and 700 ma for all these complexes indicate that they may represent the earliest phase of igneous activity in the evolution of continental crust below england and wales

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

crustal growth and late precambrian early palaeozoic plate tectonic evolution of england and wales comparison of the basement rocks of britain reveals major contrasts to the n and s of the iapetus suture solway shannon line north of the suture polymetamorphic high grade basement schists and gneisses and igneous rocks date from about 2900 ma to c 400 ma the latter age corresponding to closure of the iapetus ocean by contrast s of the suture in england and wales the basement is dominantly low grade metamorphic rock together with intrusive and extrusive igneous and sedimentary rocks there is little geological or geochemical evidence for the presence of basement older than c 900 ma below this area and the predominant expression of crustal growth in england and wales is indicated by radiometric ages between c 700 ma and 450 ma reflecting late precambrian palaeozoic magmatism and metamorphism palaeomagnetic data from scotland demonstrate that the crust in northern britain developed as an integral part of the laurentian shield during mid proterozoic times 1600 1000 ma the palaeomagnetic record from the pre ordovician rocks of england and wales shows that this area was unrelated to the laurentian shield prior to closure of the iapetus ocean the apparent polar wander a p w path is that of a distinct microplate involved in rapid growth of iapetus during early cambrian times and emplaced in its present configuration adjacent to northern britain as a result of subduction of the ocean basin between cambrian and silurian times we conclude that whereas the basement to the north of the iapetus suture formed an essentially continental unit from 2900 to 400 ma the basement of england and wales is a result of crustal growth from about 900 ma to 400 ma and formed by accretion of island arcs associated accretionary prisms and fore arc basin sediments within the iapetus ocean

an association between organic matter and localised prehnite pumpellyite alteration at builth wells wales u k metcalfe r banks d and bottrell s h 1992 an association between organic matter and localised prehnite pumpellyite alteration at builth wells wales u k chem geol 102 1 21 the builth volcanic group of the builth wells ordovician inlier wales is a sequence of basic intermediate lavas and volcaniclastics which shows a unique association between pyrobitumen and localised domainal calc silicate alteration the pyrobitumens occur in calcite veins and disseminated throughout alteration domains which form haloes to these veins the pyrobitumens were emplaced by ordovician hydrothermal fluids which were also responsible for the domainal alteration combined petrographic gas chromatography and stable isotopic studies show that light organic gases c2 c5 were liberated into the hydrothermal fluids by devolatilisation of liquid petroleum to form the pyrobitumens these light organic gases mixed with methane characterized by 13c 13 o which was already present in the fluids this methane was most likely mantle derived or produced from inorganic synthesis the organic content of the fluids exerted an important control on the inorganic secondary assemblages a similar control of inorganic assemblages by organic matter might possibly occur deep within modern petroleum bearing hydrothermal systems such as the guaymas basin and escanaba trough hydrothermal systems

rb sr whole rock isochron ages δ18o values and geochemical data for the sarn igneous complex and the parwyd gneisses of the mona complex of llŷn n wales the mona complex exposed on the ll n peninsula includes the sarn complex which is a suite of variably deformed plutonic igneous rocks originally mapped partly as gneisses ranging from gabbro to granite and an inlier of high grade paragneisses geochemical data for samples from the sarn complex may be divided into two distinct populations the rb sr data for samples from these geochemical groups define significantly different ages the older age 549 19 ma initial 87 sr 86 sr 0 7096 3 is identified as the age of intrusion in late precambrian to cambrian times while a younger age of 458 16 ma may approximate to the age of a resetting event the parwyd paragneisses yield an rb sr whole rock isochron age of 542 17 ma concordant with the age of intrusion of the sarn complex with an initial 87 sr 86 sr of 0 7069 4 for the sarn complex 18 o values range from 9 5 to 12 0 per mil smow and in the parwyd metasediments from 9 4 to 13 3 per mil we conclude that the earliest magmatism and metamorphism in the mona complex of ll n occurred during late precambrian to cambrian times broadly contemporaneous with similar events in anglesey the high initial 87 sr 86 sr and high 18 o values for the sarn complex are most simply interpreted as reflecting either a crustal source region or incorporation of a significant proportion of crustal material such as the parwyd gneisses into a mantle derived magma

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

the pontesford lineament welsh borderland the ne trending pontesford lineament centres on the pontesford linley and adjacent faults in the shelve longmynd inlier to the ne it is usually drawn along the se edge of the cheshire basin and to the sw along a variety of lines parallel to or converging with the church stretton fault new mapping and reassessment of published data suggest that sw of the pontesford linley fault the lineament is logically drawn along the clun forest disturbance a post pridoli late silurian fold fault belt then across the nw side of the llandrindod inlier to flank the tywi lineament the lineament was intermittently active at least from mid ordovician through to triassic time with both strike slip and dip slip displacements that involved the upper part of the precambrian basement late ordovician and early silurian strike slip was particularly important the lineament controlled the sites of several silurian sedimentary basins ordovician contrasts across the lineament can be explained either by contemporary fault control or by subsequent large displacements juxtaposing unlike terranes the long history of reactivation and basement control on the pontesford lineament is typical of lineaments throughout wales and the borderland it suggests that both caledonian and variscan structural trends n of the variscan front may be inherited from older pre silurian or even precambrian basement structures

sedimentation across the tywi lineament mid wales along the southeast edge of the welsh basin faults control sedimentation throughout the lower palaeozoic two important fault zones in the llandovery area are the myddfai steep belt and the tywi lineament both of which show several phases of movement that are reflected by subtle changes of sedimentary facies the myddfai steep belt consists of a relatively simple fault system that defined a palaeoshoreline in ashgill time only becoming fully overstepped by a basal wenlock transgression the tywi lineament which lies further offshore is a more complex structure facies variations across the lineament were controlled by a fine balance between the sedimentation and fault displacement rates with the fault tip lines concealed beneath a blanket of shelf sediment coarse elastics were resedimented locally in response to rapid displacement on fault strands less dram atic fault movements are shown by in situ liquefaction of mudstones together with a variety of small scale soft sediment deformation features the tywi lineament was active from at least ashgill to pridoli time and was probably originally an extensional fault reactivated in a transpressive stress regime evidence for possible oblique slip during ashgill time is found elsewhere along the lineament

the significance of cordierite hypersthene assemblages from the beitbridge region of the central limpopo belt evidence for rapid decompression in the archean

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

chitinozoan reflectance in the lower palaeozoic of the welsh basin vitrinite woody matter and spores are generally uncommon or absent in pre devonian sediments to determine organic maturity in the lower palaeozoic alternative methods such as chitinozoan reflectance must be employed chitinozoa are marine vase shaped organic walled microfossils of unknown affinity the reflectance of incident light from polished chitinozoa walls can provide reliable and accurate maturity data and due to their relative facies independency a high degree of resolution to regional maturity studies calibration of the chitinozoan reflectance scale with vitrinite reflectance enables palaeotemperature estimates to be made from pre devonian sediments the range of chitinozoan reflectance for the welsh basin is from under 1 to 8 this corresponds to estimated palaeotemperatures of under 100 c to over 300 c the regional spatial distribution of organic maturation recorded by chitinozoan reflectance in the welsh basin is attributed to palaeobasin form the increase in reflectance with age in the rocks of the myddfai steep belt and associated areas allows an estimation of a palaeogeothermal gradient in the silurian sequences around llandovery the gradient is calculated to be in the region of 50 c km 1 similar gradients are derived from other studies clay minerals metabasite mineral assemblages conodont colours the results from myddfai steep belt suggest the thermal peak occurred during the acadian event chitinozoan reflectivity demonstrates that with respect to hydrocarbon generation mature conditions occur on the palaeoplatform where the sedimentary sequences are relatively thin overmature conditions are prevalent in the palaeobasin where there is a thick sedimentary pile which has been subsequently deformed

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

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

a new high precision u pb date from the oldest known rocks in southern britain a new high precision u pb zircon age of 710 8 1 5 ma for granophyric granitic rock from the stanner hanter complex of the welsh borderland lies just within error of an older rb sr isochron age nd values of 0 3 and 1 2 combined with t dm of 1394 ma and 1468 ma indicate that the magma incorporated an older crustal source component the nd data highlight differences with western avalonia the widely considered late neoproterozoic north american counterpart to southern britain and point toward a closer similarity with other peri gondwanan terranes that incorporate older cratonic source material

potential low grade iron ore deposits in metamorphosed banded iron formations northern province south africa exploitation of low grade iron ore would be quite unique in a south african context as south africa is well endowed with high grade iron ore resources low grade iron ore defined as containing between 20 and 47 iron is thought to be the primary iron bearing lithology from which most high grade ore deposits formed through different processes of enrichment the low grade iron ores in the northern province represent meta banded iron formations bifs with an average iron content of about 36 the main iron bearing mineral is magnetite the northern province ores have to be milled to sizes smaller than 150 m in order to liberate the iron minerals from the host rock and beneficiation is accomplished through a series of magnetic separation processes irrespective of the in situ quality of the ore final concentrates of exceptionally good quality with more than 69 iron and very low contaminant levels can be produced this combined with mass yields of between 40 and 50 and iron recoveries greater than 80 are excellent for this type of iron ore deposit the beneficiation products are suitable for use in iron and steel making processes

a modern assessment of ordovician chitinozoans from the shelve and caradoc areas shropshire and their significance for correlation new chitinozoan data are presented from the classical section along the onny river in the type caradoc area and from the deeper water sections in the shelve area including the former british candidate gssp for the base of the upper ordovician series the rich and well preserved chitinozoan fauna of the onny river has been a standard for 40 years but new data revise some of the identifications the assemblages are now attributed to biozones that are more readily applicable for international correlation the main part of the section can be interpreted as belonging to the originally baltoscandian spinachitina cervicornis biozone although this is uncertain in the lower part within this biozone the fungochitina actonica subzone has been defined the onny formation at the top of the section is equated with the acanthochitina latebrosa ancyrochitina onniensis biozone contrary to earlier reports acanthochitina barbata is absent the lower wood brook and spy wood brook section from the shelve inlier yielded a great number of moderately to well preserved chitinozoans but a low diversity assemblage their ranges have been neatly positioned against the well known graptolite stratigraphy in the area a local eisenackitina rhenana biozone has been recognized allowing us to suggest some international correlations

the deep structure of the limpopo belt from geophysical studies abstract geophysical studies in the limpopo belt and adjoining cratons contributed to the fundamental understanding of the deep structure in these tectonic provinces the seismic and geoelectrical data sets prove that the terrane boundaries of the limpopo belt dip in towards the belt to give it the appearance of a pop up structure the gravity data show that high grade metamorphic rock with densities that are high compared to the normal low grade upper crustal granitoids form the upper crust in the northern and southern marginal zones of the limpopo belt these rocks also show a low magnetic susceptibility the geophysical data clearly defined the position of the southern boundary of the limpopo belt the northern boundary of the central zone can be related to a southward dipping zone of seismic reflectors while the southern boundary of this terrane as formed by the palala shear zone seems to be near vertical the greenstone belts of the craton have a limited depth extent and are allochtonous the present position of these structures could be related to thin skinned tectonics that affected the upper crust on the kaapvaal craton and which were revealed by seismic reflection data

sedimentology structural evolution and tectonic setting of the late precambrian longmyndian supergroup of the welsh borderland uk abstract the longmyndian supergroup is an approximately 6500 m thick sedimentary sequence of late precambrian age major unconformities within this sequence are lacking the longmyndian supergroup can be divided into four facies associations 1 turbidite 2 subaqueous delta 3 alluvial floodplain and 4 braided alluvial these are organized into a single broadly upwards coarsening progradational sequence with the turbidite association at the base and the braided alluvial association towards the top the sediments are interpreted to have been deposited in a forearc or possibly interarc basin which lay to the north of a contemporaneous magmatic arc of which the now adjacent uriconian volcanic complex is considered to have been a part the longmyndian supergroup has been deformed into a tight nne ssw trending syncline fault patterns and strain markers within the longmyndian indicate that it was deformed during sinistral transpression the adjacent pontesford linley and church stretton fault systems are considered to be major strike slip fault systems which were initiated at about the same time as the deformation of the longmyndian supergroup strike slip movements along these fault systems resulted in the juxtaposition of the uriconian volcanic complex with the longmyndian supergroup late precambrian early cambrian deformation is indicated by some stratigraphic and radiometric age dating evidence since there is evidence that the fault systems have suffered strike slip displacements during the palaeozoic it is difficult to be certain of the magnitude and style of the late precambrian early cambrian deformation however the recognition of late precambrian early cambrian sinistral transpressional deformation in northwest wales suggests that the precambrian of the welsh borderland was similarly deformed during the late precambrian early cambrian

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

is the welsh borderland fault system a terrane boundary the welsh borderland fault system forms one of the most prominent tectonic lines in southern britain and along with the menai straits and malverns fault systems is a candidate for a terrane boundry a review of stratigraphic linkage across the system reveals significant contrasts in pre ashgill sequences these contrasts occur particularly across its north west edge the pontesford and tywi lineaments rather than across the church stretton lineament the ashgill event produced strike slip dominated structure and an unconformity that expands from the welsh basin towards the platform an important linkage sequence was produced by the subsequent late ashgill to llandovery marine transgression across the fault system silurian facies changes are marked but gradational and no more than expected across a basin margin from stable to thinned crust devonian acadian sinistral transpression and later phanerozoic movements reactivated the fault system but did not engineer major terrane amalgamation across it this reactivation has obscured the early history of the fault system the remaining evidence admits though does not prove major ashgill or earlier transcurrent movements broader considerations suggest that the welsh borderland fault system originated either as a cambro ordovician extensional system on the south eastern margin of the welsh basin or during late precambrian early cambrian transcurrent movements occurring within avalonia inboard from the menai strait line

the woolhope and usk basins silurian rift basins revealed by subsurface mapping of the southern welsh borderland subsurface mapping of the southern welsh borderland has utilized over 600 km of seismic data and three exploration boreholes to investigate the subsurface of this classic geological area the survey reveals two silurian basins each about 400 km 2 in area in the western part of the midland platform the woolhope basin underlies the silurian inlier of the woolhope anticline the basin fill is bounded to the northwest by faults of the neath disturbance and was also influenced by wnw striking extensional growth faults the fill thins eastward towards the malvern line lower silurian sequences thin southward before thickening again particularly across wnw striking faults the resulting usk basin underlying the usk inlier has ill defined margins but some additional n s fault control the fownhope 1 and usk 1 wells indicate that rapid subsidence occurred in both basins during rhuddanian and aeronian early to mid llandovery time modelling of this subsidence assuming the finite duration lithospheric extension model yields stretching factors of between 1 3 and 1 6 the later part of the fill to both basins is marine but red conglomerates sandstones and mudstones in their early llandovery intervals suggest continental fluvial environments in the initial stages of basin development the woolhope and usk basins show that the western sector of the midland platform rifted during early to mid llandovery time perhaps during regional dextral transtension the rifting is synchronous with and perhaps genetically related to an episode of mafic volcanism in the southwest part of the platform however this rifting apparently predates the late llandovery telychian fault controlled subsidence seen in the welsh basin to the northwest

unusual micro and nano inclusions in diamonds from the juina area brazil abstract a series of uncommon micro and nano inclusions has been identified in diamonds from the rio soriso placer deposit in mato grosso state brazil the micro inclusions are variable in size from 1 to 300 m usually they are polymineralic being formed predominantly by intergrowths of carbonates silicates and other minerals carbonates are represented mostly by dolomite and occasionally by calcite silicates found are coesite wollastonite ii cuspidine and monticellite sulphides and ilmenite form micro inclusions as well nano inclusions are different from micro inclusions not only in size not exceeding 200 nm they are usually included in micro inclusions among nano inclusions halides nacl kcl cacl2 and pbcl2 anhydrite spinel phlogopite pbo2 tio2 with an pbo2 structure native fe and other phases are identified all these minerals are of the eclogitic association they are either associated with coesite or are included in a diamond with a light organic carbon isotopic composition with 13c from 14 to 25 pdb this confirms our earlier conclusion that diamonds from the juina area may have formed as a result of subduction of the crustal material to depths of at least the lower transition zone or even the lower mantle the pressure estimates for the investigated diamonds vary in a range from 3 to 10 gpa an interesting assemblage of calcite cuspidine wollastonite monticellite fluid identified in one of the studied diamonds is considered as a product of a reaction of wollastonite fluid forming cuspidine monticellite the presence of numerous pores cavities and bubbles in mineral inclusions and identification of an association of volatile containing mineral inclusions such as halides nacl kcl cacl2 and pbcl2 fluorine containing silicate cuspidine and phlogopite emphasize the important role of volatiles particularly chlorine and fluorine in the formation of the diamonds

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

depositional setting of the lower old red sandstone at pantymaes quarry central south wales new perspectives on the significance and occurrence of senni beds facies

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

strike slip terranes and a model for the evolution of the british and irish caledonides evidence is presented that many of the major strike faults in the british and irish caledonides were active as sinistral strike slip zones in the end silurian to pre mid devonian period some such as the highland boundary fault moved in this way at an earlier stage in the ordovician these data allow the caledonian rocks lying between the laurentian miogeocline whose basement is represented by the lewisian moine and possibly the dalradian and the gondwanaland miogeocline midland platform and welsh basin to be re analysed as a group of disorganized terranes which originated to the southwest in north america and southwest europe africa prior to the silurian the highland border terrane and northern belt terrane are interpreted as duplicated pieces of a mid ordovician sequence which was a back are to northwest subduction the midland valley terrane is interpreted as a slice of laurentian foreland onto which ophiolites were obducted in the lower ordovician but which became the basement of a continental margin arc to northwest subduction in the mid ordovician the cockburnland terrane is inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip the connemara terrane is regarded as an allochthonous piece of the dalradian miogeocline and the south mayo terrane as a remnant of an early ordovician arc and fore arc which in mid ordovician times became a back arc marginal basin to northwest subduction the lake district wexford terrane is part of an arc to southeast subduction under gondwanaland whose activity climaxed in the mid ordovician the central terrane is interpreted as a silurian overstep assemblage which blankets the junction between laurentian and gondwanaland derived oceanic terranes and therefore iapetus is regarded as an ordovician ocean which closed prior to the silurian the model suggests that at the end of the silurian a clockwise rotating gondwanaland having carried laurentia into collision with baltica broke free and created a major sinistral strike slip zone which disrupted the ordovician palaeogeography in the british isles north american sector of iapetus

petrography chemical composition and raman spectra of chrome spinel constraints on the diamond potential of the no 30 pipe kimberlite in wafangdian north china craton abstract conventional diamond exploration seldom searches directly for diamonds in rock and soil samples instead it focuses on the search for indicator minerals like chrome spinel which can be used to evaluate diamond potential chrome spinels are preserved as pristine minerals in the early paleozoic 465 ma hydrothermally altered group i no 30 pipe kimberlite that intruded the neoproterozoic qingbaikou strata in wafangdian north china craton ncc the characteristics of the chrome spinels were investigated by petrographic observation bse imaging quantitative chemical analysis epma and raman spectral analysis the results show that the chrome spinels are mostly sub rounded with extremely few grains being subhedral and these spinels are macrocrystic more than 500 m in size the chrome spinels also have compositional zones the cores are classified as magnesiochromite as they have distinctly chromium rich cr 2 o 3 up to 66 56 wt and titanium poor tio 2 3 cr 3 isomorphism which results in a strong raman spectra peak a 1g mode varying from 690 cm 1 to 702 9 cm 1 in the rims of chrome spinel compositional variations result in the a 1g peak varying from 660 cm 1 to 672 cm 1 the morphology and chemical compositions indicate that the chrome spinels are mantle xenocrysts the cores of the spinel are remnants of primary mantle xenocrysts that have been resorbed and the rims were formed during kimberlite magmatism the compositions of the cores are used to evaluate the diamond potential of this kimberlite through comparison with the compositions of chrome spinels from the changmazhuang and no 50 pipe kimberlites in the ncc in mgo al 2 o 3 and tio 2 versus cr 2 o 3 plots the chrome spinels from the changmazhuang and no 50 pipe kimberlites are mostly located in the diamond stability field however only a small proportion of chrome spinels from no 30 pipe kimberlite have same behavior which indicates that the diamond potential of the former two kimberlites is greater than that of the no 30 pipe kimberlite this is also supported by compositional zones in the spinel grains there is with an increase in fe 3 in the rims which suggests that the chrome spinels experienced highly oxidizing conditions oxidizing conditions may have been imparted by fluids melts that have a great influence on diamond destruction here we suggest that chrome spinel compositions can be a useful tool for identifying the target for diamond potential in the north china craton

silurian basin slope sedimentation and mass movement in the wigmore rolls area central welsh borderland in the wigmore rolls area of the central welsh borderland westward directed mass movements sliding and slumping on the silurian welsh basin slope removed at least 275 m of the wenlock and ludlow homerian to upper ludfordian succession of mudstones and calcilutites the resulting slide scar was more than 2 5 km wide lower leintwardine formation lower ludfordian flaggy siltstones rest with marked unconformity in this erosional hollow continuing instability is shown by a basal coarse boulder bed up to 10 m thick channellized in places that resulted from a debris flow coming to rest downslope and by widespread slumping during the accumulation of the lower leintwardine formation siltstones instability at this time outside the wigmore rolls area is confirmed by the formation of submarine canyon heads channelling and hardgrounds comparable mass movements leaving slide scars and channels on modern basin slopes are becoming better known but ancient examples such as those described here are rare the slide scar and channels were filled by the end of early leintwardine times and sandier siltstones of the upper leintwardine lower whitcliffe and upper whitcliffe formations prograded uniformly over the whole area in shallower water during the acadian orogeny the area was folded and faulted into a southwesterly continuation of the ludlow anticline

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

cambrian successions of the meguma terrane nova scotia and harlech dome north wales dispersed fragments of a peri gondwanan basin abstract the meguma terrane of nova scotia and the harlech dome of north wales preserve similar sedimentary successions of cambrian age both successions comprise a thick succession of early cambrian sandstone turbidites overlain by early to middle cambrian alternating mud rich and sand rich units in which manganese is concentrated in two stratigraphic intervals above these both successions comprise anoxic organic rich turbidites shallowing upward into paler more bioturbated tremadocian mudstone with rhabdinopora within the limited constraints of the available biostratigraphic and geochronological data major changes in environment occurred synchronously in the two successions both successions show much greater similarity to each other than to adjacent successions on avalonia9 a detrital zircon analysis from the rhinog formation low in the harlech dome succession reveals distinct clusters of ages around 537 ma and 2 0 2 1 ga a close similarity to analyses from the meguma terrane suggests proximity between the two terranes on the margin of gondwana during the cambrian period we suggest the term megumia for the palaeogeographical domain that included the two successions which was dispersed during subsequent appalachian and caledonian movements these observations suggest that megumia may have separated terranes previously regarded as parts of avalonia supplementary material u pb detrital zircon analytical data are available at http www geolsoc org uk sup18431

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

primordial and recycled helium isotope signatures in the mantle transition zone isotope compositions of basalts provide information about the chemical reservoirs in earth s interior and play a critical role in defining models of earth s structure however the helium isotope signature of the mantle below depths of a few hundred kilometers has been difficult to measure directly this information is a vital baseline for understanding helium isotopes in erupted basalts we measured he sr pb isotope ratios in superdeep diamond fluid inclusions from the transition zone depth of 410 to 660 kilometers unaffected by degassing and shallow crustal contamination we found extreme he c pb sr isotope variability with high 3he 4he ratios related to higher helium concentrations this indicates that a less degassed high 3he 4he deep mantle source infiltrates the transition zone where it interacts with recycled material creating the diverse compositions recorded in ocean island basalts

a new early silurian turbidite system in central wales insights into eustatic and tectonic controls on deposition in the southern welsh basin the newly recognized nant brianne turbidite system was a focus of laterally supplied coarse grade sediment deposition that along with the caban ystrad meurig system punctuated late hirnantian to early telychian mudstone dominated slope apron deposition along the se margin of the southern welsh basin geological mapping coupled with detailed biostratigraphy enable the depositional influence of sea floor topography an active llyn brianne fault and eustatic sea level changes to be tracked the latter may represent far field effects relating to the retreat and advance of contemporary gondwanan ice sheets slope apron mudstone facies reveal a strong response to high order eustatic events however the response of contemporary easterly sourced coarse grade turbidite systems was more complex with some periods of increased sand and gravel input coinciding with times of rising global sea level these anomalous relationships are explained by invoking a series of smaller scale movements in marine base level it was the interaction of these lower order events with the main eustatic cycles that appears to have been the primary control on sand and gravel input to the welsh basin an early telychian expansion of the nant brianne turbidite system records a marked increase in tectonically generated sediment at a time of palaeo plate collision between baltica avalonia and laurentia before intra and peri basinal faulting led to its abandonment as a supply path to the basin centre

palaeozoic palaeomagnetic studies in the welsh basin recent advances in the last two years new palaeomagnetic data fro m wale s have resulted i n radical revision of the ordovician palaeogeography of eastern avalonia part of the southern margin of the iapetus ocean combined with palaeozoic palaeomagnetic data from laurentia and gondwana these data suggest that eastern avalonia was a peri gondwanide high latitude continental fragment during at least part of ordovician time with a palaeolatitude of about 62 s and 51 s in arenig and llanvirn time respectively this implies a latitudinal width of the early ordovician iapetus ocean between eastern avalonia and laurentia of at least 30 geological evidence for the proximity of eastern avalonia and laurentia suggests that the intervening iapetus ocean closed during silurian time from late llandovery to early ludlow recent palaeolatitude data from the iapetus bordering continents are consistent wit h closure b y middle to late silurian time new pre acadian early devonian palaeomagnetic data from the old red sandstone places the welsh basin at about 17 s consistent with a palaeogeography in which laurentia baltica avalonia armorica and possibly gondwana wer e part o f a singl e supercontinent pervasiv e lat e carboniferous early permia n remagnetization affect s the welsh basin the remagnetization i s probably associated with fluids emanating from th e variscan thrust front w e do not observe remagnetization associate d with acadian orogen y and the remagnetizations which have been studied in more detail in north america appear to be a unique feature of the variscan hercynian alleghenian orogeny

diamonds in brazil and a proposed model for the origin and distribution of diamonds in the coromandel region minas gerais brazil the geology of diamonds in brazil is briefly summarized in relation to the known primary source localities and secondary host rocks emphasis is placed on the geotectonic control of mineralized primary diamond source rocks and it is shown that the distribution and occurrences in brazil of diamond bearing versus nondiamond bearing kimberlites and or lamproites are compatible with the geotectonic constraints observed worldwide in brazil the distribution of the majority of the diamonds can be correlated with three principal glaciations the first in the lower proterozoic is designated the jequitai glaciation the second probably in the cambrian is represented by the santa fe tillites and the third in the carboniferous is represented by the itarare subgroup these glaciations with the capacity to transport kimberlitic minerals and diamonds intact for extreme distances are considered the principal sources of diamonds in for example the coromandel region of western minas gerais cretaceous kimberlites and lamproites of unknown age in brazil were intruded along major arcs and lineaments along basin borders and on the amazonico craton upper cretaceous kimberlites in the alto paranaiba are noneconomic in respect to diamonds to date the only known economically viable primary diamond sources are cretaceous kimberlites in the guapore shield within a substratum that has remained tectonically and thermally stable for more than 1 500 ma it is postulated that a precambrian diamondiferous kimberlite event occurred in brazil which intruded principally into the sao francisco craton it is proposed that these are the primary diamond source s for the large diamonds 100 kt that are found in the alluvial deposits of the coromandel region of minas gerais