the life cycle of thunderstorm gust fronts as viewed with doppler radar and rawinsonde data abstract this paper presents the time dependent analysis of the thunderstorm gust front with the use of project nimrod data rhi cross sections of reflectivity and doppler velocity are constructed to determine the entire vertical structure the life cycle of the gust front is divided into four stages 1 the formative stage 2 the early mature stage 3 the late mature stage and 4 the dissipation stage a new finding is a horizontal roll detected in the reflectivity pattern resulting from airflow that is deflected upward by the ground while carrying some of the smaller precipitation ahead of the main echo core of the squall line this feature is called a precipitation roll as determined from rawinsonde data the cold air behind the gust front accounts for the observed surface pressure rise calculations confirm that the collision of two fluids produce a nonhydrostatic pressure at the leading edge of the outflow the equation governing the propagation speed of a density current accurately predicts t

mineral chemical and isotopic studies of namaqualand granulites south africa a grenville analogue the northwestern part of south africa and southern south west africa namibia is amongst the most extensive granulite terranes in africa this work reports the results of electron microprobe studies of minerals from two pyroxene cordieriteorthopyroxene gedrite sapphirine and garnet and or cordierite parageneses from namaqualand in the n w cape province of south africa determined pt conditions of prograde metamorphism based on thermodynamic calculations are 800 900 c and ca 6 7 kb and it is argued that rocks of unusual composition notably cordierite orthopyroxene rocks are restites after the extraction of granitic liquid from former argillites this interpretation is consistent with previously published data on similar rocks and with mccarthy s 1976 suggestion of extensive partial melting in the quartzofeldspathic rocks in the area

the 11 august 2006 squall line system as observed from mit doppler radar during the amma sop on the evening of 9 august 2006 a mesoscale convective system mcs having a north south oriented squall line organization formed over the border between chad and nigeria it propagated westward intensified over nigeria on 10 august and reached niamey niger at 0320 utc on 11 august its passage over niamey was accompanied by dust lifting and was well tracked by the massachusetts institute of technology mit doppler radar the three dimensional structure of the airflow and precipitation pattern is investigated from regular radar volume scans performed every ten minutes between 0200 and 0321 utc the 3d wind components are deduced from the multiple doppler synthesis and continuity adjustment technique muscat applied to a set of three volume scans obtained over a time period of one hour which are equivalent to a three radar observation of the squall line when considering a reference frame moving with the system and the hypothesis of a stationary field results of the wind synthesis reveal several features commonly observed in tropical squall lines such as the deep convective cells in front of the system fed by the monsoon air and extending up to 15 km altitude and the well marked stratiform rain region at the rear associated with mesoscale vertical motions forward and trailing anvils are clearly identified as resulting from the outflow of air reaching the tropopause and transported to this level by the sloping convective updraughts occurring in a sheared environment in the northern part a deeper and stronger front to rear flow at mid levels is found to contribute to the rearward deflection of the leading line and to promote a broader over 300 km stratiform cloud region eddy vertical transports of the cross line momentum mainly accounts for the mid level flow acceleration due to a momentum redistribution from low to higher levels the height distribution of hydrometeors and their associated production terms derived from a one dimensional microphysical retrieval model indicate the distinct roles of the convective and stratiform regions in the formation of graupel and rain and the respective contributions of cold riming and warm coalescence melting processes cooling from melting and heating cooling from condensation evaporation processes yield a net decrease and increase of the potential temperature at low and mid to upper levels respectively with respect to an environmental thermodynamic profile taken three hours ahead of the analysis finally the upper level rearward flow could convey the non negligible proportion of ice particles farther from the leading deep convection to the trailing stratiform region thereby favouring the extent of this region

a theory for strong long lived squall lines abstract we study herein the mechanics of long lived line oriented precipitating cumulus convection squall lines using two and three dimensional numerical models of moist convection these models used in juxtaposition enable us to address the important theoretical issue of whether a squall line is a system of special long lived cells or whether it is a long lived system of ordinary short lived cells our review of the observational literature indicates that the latter is the most consistent paradigm for the vast majority of cases but on occasion a squall line may be composed of essentially steady supercell thunderstorms the numerical experiments presented herein show that either type of squall line may develop from an initial line like disturbance depending on the magnitude and orientation of the environmental shear with respect to the line with shallow shear oriented perpendicular to the line a long lived line evolves containing individually short lived cells our analysis of this type o

u pb zircon calendar for namaquan grenville crustal events in the granulite facies terrane of the o okiep copper district of south africa s xvi geokongres geological society of south africa stellenbosch pp 160 163 van zyl d 1978 a petrological approach towards the ore bearing potentialities of the okiep basic intrusives in namaqualand in verwoed w j ed mineralization in metamorphic terranes geological society of south africa special publication 4 323 329 vellet v 1958 the geology of the copper district namaqualand cape province south africa unpublished report of the o okiep copper company 110 pp waters d j 1989 metamorphic evidence for the heating and cooling path of namaqualand granulites in daly j s cliff r a yardley b w d eds evolution of metamorphic belts geological society london special publication 43 357 363 williams i s claesson s 1987 isotopic evidence for the precambrian provenance and caledonian metamorphism of high grade paragneisses from the seve nappes scandinavian caledonides ii ion microprobe zircon u th pb contributions to mineralogy and petrology 97 205 217 clifford et al crustal events in o okiep district

characterization of the diurnal cycle of the west african monsoon around the monsoon onset abstract this study investigates the diurnal cycle of the west african monsoon and its seasonal modulation with particular focus on the monsoon onset period a composite analysis around the monsoon onset date is applied to the 1979 2000 ncep doe reanalysis and 40 yr ecmwf re analysis era 40 at 0000 0600 1200 and 1800 utc this study points out two independent modes describing the space time variability of the diurnal cycle of low level wind and temperature while the first mode appears to belong to a gradual and seasonal pattern linked with the northward migration of the whole monsoon system the second mode is characterized by more rapid time variations with a peak of both temperature and wind anomalies around the monsoon onset date this latter mode is connected with the time pattern of a nocturnal jet reaching its highest values around the onset date the diurnal cycle of dry and deep convection is also investigated through the same method a distinct diurnal cycle of deep convection in the itcz i

plate tectonics and orogenic research after 25 years a tethyan perspective abstract orogeny the process by which the earth s prominent mountain ranges are constructed has been a central topic of interest in the earth sciences since at least the end of the 18th century the recognition that strains and displacements of very considerable magnitude occur along all of the three dimensions within an orogenic belt during its evolution has grown gradually during the last two centuries emphasis on primary vertical movements dominated the ideas on the nature of orogeny during the first half of the 19th century whereas compression and consequent uplift across mountain belts were believed to be the main cause for their origin during the subsequent one hundred years or so that were spent under the dominance of the fixist contraction theory mobilist tectonicians realised that continental drift in places also required motion along the trend of orogenic belts but this view did not gain general acceptance recognition of significant strike slip motion parallel or subparallel with mountain ranges evolved independently and mostly within the fixist camp by the 1960 s presence of important motions both along and across mountain belts had become common knowledge but no theoretical basis existed to account for them all

secondary initiation of multiple bands of cumulonimbus over southern britain ii dynamics of secondary initiation summary the convective storm initiation project csip pilot field programme took place in the summer of 2004 in southern england during this field campaign a case of a cold pool outflow initiating an arc of convection downwind of the generating storm the primary storm was observed three further arcs were initiated further downwind of this first arc these arcs all later gave significant rainfall over south east england results from the modelling studies described in this paper show that gravity waves generated by the primary storm may have been responsible for initiating the further three arcs of convective showers that were observed the modelled primary storm generates waves with a range of vertical wavelengths these are separated by dispersion with the higher order modes with the largest vertical wavelengths travelling fastest approximately 30 m s 1 the fastest two modes suppressed convection and later modes increased boundary layer depth and so initiated convection the multiple peaks from non hydrostatic effects in the third mode may have been responsible for the two main arcs observed and the fourth mode may have been responsible for the third arc the amplitude of the modelled waves at low levels was much larger downwind of the primary storm than upwind and only initiated convection there which agrees with the observations of the arcs forming downwind of the primary storm this asymmetry appears to be caused by the moving wave source and asymmetric wave trapping finally we discuss the implications for numerical weather prediction nwp reducing the resolution of the model and varying the magnitude of the wind speed used shows that a non hydrostatic high resolution 1k m nwp model should be able to capture such wave processes if the generating storm is forecast or analysed correctly

rare earth element geochemistry of an anorthosite diorite suite namaqua mobile belt south africa abstract a swarm of small dyke like plutons ranging in composition from anorthosite through diorite and norite to hypersthenite and glimmerite occurs in the western part of the namaqua mobile belt south africa known as the koperberg suite these rocks host important magmatic sulphide deposits zircon ages of approximately 1100 ma suggest that the suite was emplaced after the peak of the regional high grade metamorphism dated at 1200 ma a postulated comagmatic origin for the various rock types is confirmed by the ree distributions of anorthosite leucodiorite diorite hypersthenite and glimmerite the koperberg suite resembles the well known massif anorthosite complexes typified by the adirondack suite in most properties except for the dispersed mode of occurrence it is now shown that the similarity also holds for the ree spectra thus the predominant anorthosite and leucodiorite of the koperberg suite are comparable to similar rocks of the adirondacks and the burwash area ontario albeit more enriched in lree more mafic rock types such as norite mica diorite and glimmerite are substantially enriched in lree compared to mafic differentiates of massif anorthosites apatite is abundant in the mafic rocks and stores large proportions of the lanthanides the occurrence of apatite and mica rich assemblages indicates source magmas enriched in k ree and p a few intrusives exhibit excessively enriched ree patterns which are attributed to late hydrothermal alteration or metasomatism the ree data for syenite and two pyroxene assemblages from the copper district argue against a genetic relationship to the koperberg suite in spite of close spatial association

geochemical constraints of the petrogenesis of the o okiep koperberg suite and granitic plutons in namaqualand south africa a crustal source in namaquan grenville times the namaquan grenville orogeny late mesoproterozoic in the o okiep district is characterized by two tectono magmatic episodes the o okiepian episode 1210 1180 ma with the intrusion of batholitic granites and the klondikean episode 1040 1020 ma which includes the intrusion of the copper bearing koperberg suite and the rietberg granite this study focuses on the geochemistry major and trace elements sr and nd isotopes of intrusive rocks of the o okiep terrane to provide better constraints on their source rock characteristics and on their petrogenesis the o okiepian granites belong to the k rich granite kindred with shoshonitic affinities the concordia granite results from dehydration melting of a pelitic source the rietberg granite shows geochemical similarities with post collisional magmatic series the anorthosites and related rocks of the koperberg suite are cumulates their ree distribution is controlled by their apatite content a new rock type jotunite has been identified in the koperberg suite it is analogous to the rogaland chilled jotunite a characteristic which gives strong evidence that the koperberg rocks belong to the massif type anorthosite suite inversion modelling of plagioclase ree compositions from anorthosite permits the reconstruction of melt compositions and places constraints on the melting process and on the characteristics of the source rocks the occurrence of jotunite in the koperberg suite is strong additional evidence for a crustal source because jotunite is produced by remelting of gabbronorite under dry conditions the various intrusions in the koperberg suite show distinct isotopic signatures which resulted from isotopic heterogeneities of the crustal source and from minor contamination with the country rocks the characteristic negative end 1030 ma values 5t o 11 can be explained by remelting at 1030 ma of a 1900 ma old oceanic crust protolith with an enriched morb ree distribution the large range in sr initial ratios 0 709 0 748 may reflect hydrothermal alteration of the oceanic crust a process which may also explain the cu enrichment the koperberg intrusions were produced by forceful injection of cumulate crystal mush which were differentiated in deeper magma chambers or in conduits

geochemical and isotopic characteristics of different types of anorthosite in the namaqua mobile belt south africa abstract anorthosite genesis in the proterozoic namaqua mobile belt of south africa encompasses a range of intrusive histories including massif type as in the occurrences of plat sjambok and possibly koperberg suite and layered type as at oranjekom different compositional ranges at all levels of analytical resolution mineralogical geochemical and isotopical are revealed there is a spectrum from orthopyroxene dominance and extremely high lree koperberg suite to clinopyroxene horn blende dominance with normal ree patterns oranjekom plat sjambok and low iron oranjekom isotopic data for the various complexes rb sr sm nd indicate differences in magma evolution the oranjekom complex represents a depleted mantle origin with low to moderate contamination sr 6 to 9 nd 7 to 9 while the plat sjambok complex reflects a depleted mantle origin with a noticeable contribution from a crustal component sr 40 to 150 nd 5 to 1 in contrast the koperberg suite sr 300 nd 9 if derived from the mantle would require mixing between a highly enriched mantle source and an extremely enriched crustal contaminant high sr 700 to 800 nd 2 resulting in an unusual isotopic trend with a positive slope the waterkop pluton that has in the past been regarded as related to the plat sjambok complex represents a quite different syenitic magma from a slightly enriched mantle source with very little contamination and is not a cogenetic with the latter collectively the analytical data support original magma generation from different mantle sources followed by various intrusive histories ranging from rapid transit between source and emplacement in the upper crust oranjekom to prolonged travel and differentiation involving considerable crustal contamination koperberg suite the fact that the different intrusive types occur in different suspect terranes of the mobile belt may reflect originally contrasting tectonic settings during a late proterozoic continental collision that played a role during the assembly of rodinia

convection initiation by density currents role of convergence shear and dynamical organization abstract steady state analytic models establish two key points concerning the impact of vertical shear on density currents and the implication for convection initiation first shear decreases the horizontal convergence and therefore the mean ascent associated with downshear propagating currents second shear has a basic effect on the dynamical organization if the downshear current travels at the speed of the ambient flow at a critical steering level an overturning circulation provides deep lifting although mean ascent is increased by shear in the case of upshear propagating currents the lifting is comparatively shallow because jumplike ascent occurs rather than deep overturning the convection initiation mechanism involving the downshear current is therefore very different from the upshear case these basic principles are borne out in two dimensional numerical simulations density currents generated by a stationary cold source imposed on an initially horizontally homogeneous sheared and neutra

life cycle of a mesoscale circular gust front observed by a c band doppler radar in west africa abstracton 10 july 2006 during the special observation period sop of the african monsoon multidisciplinary analysis amma campaign a small convective system initiated over niamey and propagated westward in the vicinity of several instruments activated in the area including the massachusetts institute of technology mit c band doppler radar and the atmospheric radiation measurement arm mobile facility the system started after a typical convective development of the planetary boundary layer it grew and propagated within the scope of the radar range so that its entire life cycle is documented from the precluding shallow convection to its traveling gust front the analysis of the observations during the transitions from organized dry convection to shallow convection and from shallow convection to deep convection lends support to the significant role played by surface temperature heterogeneities and boundary layer processes in the initiation of deep convection in semiarid conditions the analysis

petrogenesis of the concordia granite gneiss and its relation to wmo mineralization in western namaqualand south africa abstract the 1 1 ga concordia granite gneiss cgg is part of the late to postorogenic spektakel suite in the western namaqualand metamorphic complex south africa it intruded synkinematically with respect to the main d2 deformation event into lower to middle crustal rocks and granite emplacement was more or less coeval with the peak of granulite facies metamorphism 800 c 5 kbar several genetically related rock types megacrystic garnet bearing granite minor aplitic leucogranites and pegmatites are distinguished all varieties are sio 2 rich 69 79 wt peraluminous granites and show subalkaline monzonitic magma characteristics geochemical differences in whole rock chemistry between megacrystic granite and aplitic leucogranites e g lower al 2 o 3 mgo cao ba zr higher k 2 o rb nb w rb sr ga al and the decrease of e g cao mgo fe 2 o 3 ba zr th u with increasing sio 2 in the megacrystic granites as well as the variation in fe mn of magmatic garnets are best explained with crystal fractionation processes fractional crystallization of plagioclase produced potassium and silica rich residual melts characterised by very high rb sr rb ba u th mn fe ratios and higher concentration of w cu and zn crystal fractionation processes also resulted in a relative lree depletion and hree enrichment megacrystic granite la lu cn 8 87 31 67 aplitic leucogranite and pegmatites la lu cn 0 71 1 44 and evolution of pronounced negative eu anomalies the crystallization sequence near solidus crystallization of biotite prior to alkali feldspar suggests that the cgg magmas were h 2 o undersaturated over a long period of their evolution water saturation during late stage crystallization is however indicated by coarse late stage eutectic mineral textures pegmatites and w mo bearing siliceous rocks furthermore the economic potential of the cgg is supported by its geochemical signature e g high u th contents which is similar to evolved high heat production hhp granites the granitic magmas are attributed to partial melting of peraluminous crustal source rocks and are tentatively interpreted as fractionated s type granites the w mo deposits represent vein type and pegmatitic deposits genetically related to a deep seated granitic system

summary of convective storm initiation and evolution during ihop observational and modeling perspective abstract the data rich international h2o project ihop 2002 experiment is used to study convective storm initiation and subsequent evolution for all days of the experiment initiation episodes were almost evenly divided between those triggered along surface based convergence lines and elevated initiation episodes that showed no associated surface convergence the elevated episodes occurred mostly at night and the surface based episodes occurred during the afternoon and evening surface based initiations were mostly associated with synoptic fronts and gust fronts and less so with drylines and bores elevated initiations were frequently associated with observable convergent or confluent features in the rapid update cycle ruc wind analysis fields between 900 and 600 hpa the ruc10 3 h forecast of the precipitation initiation episodes were correct 44 of the time allowing a tolerance of 250 km in space and for the forecast being early by one period however the accuracy was closely tied to the scale of

the role of boundary layer convergence zones and horizontal rolls in the initiation of thunderstorms a case study abstract the initiation of thunderstorms is examined through a combined observational and modeling case study the study is based on doppler radar aircraft mesonet balloon sounding and profiler and photographic data from the convection initiation and downburst experiment cinde conducted near denver colorado the study examines the initiation of a line of thunderstorms that developed along a preexisting quasi stationary boundary layer convergence line on 17 july 1987 the storms were triggered at the intersection of the convergence line with horizontal rolls where enhanced updrafts were present the primary effect of the convergence line was to deepen the moist layer locally and provide a region potentially favorable to deep convection the critical factor governing the time of storm development was apparently related to the attainment of a balance between horizontal vorticity in the opposing flows on either side of the convergence line the effect was to cause the updrafts in the convergence line

the role of strain localization in the segregation and ascent of anatectic melts namaqualand south africa granulite facies gneisses of the late proterozoic okiep copper district of the namaqua province in south africa preserve evidence of a range of anatectic melt features that reflect the initial stages of segregation and ascent of crustally derived magmas during high grade metamorphism these melt bodies include both in situ and sharply transgressive subvertical pipe like bodies that show vertical dimensions of several hundreds of metres and horizontal dimensions of tens to hundreds of metres migmatite bodies are spatially closely linked with narrow upright zones characterized by intense high strain fabrics locally referred to as steep structures they display a progressive textural development from diktyonitic textures in in situ bodies via schollen and raft textures to largely homogeneous intrusive granites that have migrated vertically for distances of several hundreds of metres or more the intimate association of the migmatite bodies with the steep structures reflects strain induced melt segregation facilitated by increased permeabilities in these ductile deformation zones and migration of melts into sites of strain incompatibility at the intersections between the regional subhorizontal gneissosity and the superimposed high strain zones subsequent melt migration was focused along the network of subvertical structural anisotropies provided by the steep structures melt migration was controlled by a combination of buoyancy shear enhanced melt compaction during ongoing deformation melt compaction due to the settling of wall rock fragments from higher stratigraphiclevels and subordinate brittle fracturing the unusual geometry of the steep structures and the intensely heterogeneous nature of the strain and the absence of similar strain features and voluminous melt bodies elsewhere in the granulite facies terrane suggest a positive feed back mechanism between melt generation and strain localization in steep structures the structural development of the migmatite bodies illustrates that the efficiency of melt segregation and migration in a mid crustal segment is dependent not only on the fertility of its lithologies but also on its deformational style

petrographical characteristics of the koperberg suite south africa an analogy to massif type anorthosites a swarm of anorthositic dioritic intrusives collectively termed the koperberg suite is scattered over an area of about 3000 km2 in the northwestern cape province namaqualand south africa the intrusive suite is renowned for its associated magmatic copper sulphide deposits transgressive field relationships and the preservation of primary igneous features suggest emplacement of the suite after the main metamorphic event which has been dated at c 1200 ma the distribution of the intrusions seems to have been controlled by vertical shearing which is manifested by antiformal disruptions steep structures in the country rocks post intrusion movement along some of these structures is deemed responsible for metamorphic textures in some mafic bodies igneous breccias associated with some intrusions present evidence of explosive tectonics at the time of emplacement the rock types which compose the mafic suite include andesine anorthosite leucodiorite hypersthene and mica diorite hypersthenite and glimmerite the occurrence of composite intrusions with two or more mafic varieties and the modal gradation between the rock types suggest consanguinity the anorthosite appears to be the oldest member of the series followed by increasingly more mafic varieties such as leucodiorite hypersthene diorite norite and hypersthenite owing to the presence of virtually monomineralic rock types various differentiation trends are recognized series that are dominated by hypersthene brown mica iron titanium oxides and amphibole can thus be delineated microprobe analyses of the major silicates indicate plagioclase compositions of andesine en58 68 for hypersthene and brown mica compositions on the biotite phlogopite boundary the predominance of anorthosite and leucodiorite in the suite suggest an affinity with the massif type anorthosite kindred this relation is supported by other criteria such as the preponderance of hypersthene the age of emplacement the sequence of intrusion en an ratios of plagioclase and hypersthene and the association of syenitic rocks as well as iron titanium oxide and apatite rich assemblages it is provisionally proposed that the okiep district represents the lower levels of an anorthosite complex of the massif type

the copper bearing basic rocks of namaqualand south africa copper mineralisation is associated with rocks of basic intermediate composition which occur as a widespread swarm of relatively small irregularly shaped bodies within the gneissic terrain of the okiep district these basic rocks comprise the so called koperberg noritoid suite of namaqualand and constitute one of the three major copper producers in south africa while a metasomatic origin was briefly favoured for the noritoids most researchers have emphasized the cross cutting nature of the noritoids relative to their host rocks and prefer an origin involving magmatic injection high noritoid 87sr 86sr initial ratio values indicated to some workers that mobilisation of suitable crustal materials was more attractive than intrusion of mantle derived melts to account for the origin of the noritoid suite the present study has shown that the structure texture and mineralogy of the noritoids do not lend themselves to interpretation of the noritoids as pristine intrusive rocks and indicate that the noritoids have undergone high grade metamorphism after emplacement the geochemistry of the noritoid suite reveals a pronounced alkaline affinity and mineralogical control of the chemistry characterises the rocks as cumulates the dimensions of the noritoids appear unfavourable for development of cumulate rocks and it is considered possible that the present configuration of the noritoids results from dismemberment of larger intrusive bodies as a result of shear deformation during high grade metamorphism it has not been possible to identify a magma parent to the noritoid suite but we believe the parent to have been an alkaline mantle derived magma which suffered contamination by granitic anatects in the lower crust recognition of the metamorphic history of the namaqualand noritoids renders a comparison between them and the caraiba noritoids of interest

weathering dynamics and geosphere mixing with reference to the potassium cycle abstract continental weathering and erosion are discussed from a planetary perspective as important links in the chain of chemical and physical exchanges between surface geospheres atmosphere hydrosphere crust and the mantle the influence of weathering on geosphere chemistry is explored by considering potassium exchanges between geospheres k is transferred from the continents by weathering and erosion to the oceans from which dissolved k is removed during spilitization and by incorporation into sediments k carried to the mantle by subduction of spilites and sediments 8 10 13 g a 1 could balance the dissolved k added to the oceans from continental river runoff and from seafloor hydrothermal springs the largest k flux is that 2 7 10 14 g a 1 physically carried to the oceans in suspended sediments the weakest link in quantifying geosphere exchanges is the lack of understanding of the construction and restoration of continents and hence in defining the parameters involved in their chemical and physical renewal

north atlantic hurricanes contributed by african easterly waves north and south of the african easterly jet abstract a majority of tropical cyclones in the north atlantic develop from african easterly waves aews which originate along both the southern and northern flanks of the midtropospheric african easterly jet aews and aewn respectively the purpose of this note is to identify the contribution of aewss and aewns to north atlantic tropical cyclones that develop from aews applying a manual backtracking approach to identify the genesis locations of aews it was found that the population ratio of tropical cyclones formed from aewss to those formed from aewns is 1 1 2 because the population ratio of aewss to aewns is 1 2 5 the conversion rate of the former aews to tropical cyclones is twice as effective as the latter waves in addition it was found that aewns travel farther and take longer to transform into tropical cyclones than aewss which is likely because the aewns are drier and shallower than aewss an analysis of various terms in the moisture and vorticity budgets reveals that the monsoon trough

stable isotopic evidence for crustal contamination and desulfidation of the cupriferous koperberg suite namaqualand south africa abstract the middle proterozoic koperberg suite occurs as a swarm of differentiated mafic dikes in a granulite facies terrane in the northwestern cape province of south africa rock types include pyroxenite norite anorthosite diorite and glimmerite and host an important repository of copper ore this paper reports petrographic and stable isotope data that places new constraints on the magmatic and metamorphic evolution of the koperberg suite and associated copper mineralization oxygen isotope ratios of the basic bodies are significantly enriched in 18 o 18 o plag 6 2 to 9 6 18 o opx 6 2 to 7 4 and 18 o rock 5 9 to 8 3 relative to mantle derived basic rocks this enrichment appears to be the result of crustal contamination granite gneiss country rocks exhibit 18 o values in the range from 7 8 8 7 the systematics of oxygen isotope data for coexisting minerals suggest that in addition to crustal contamination further modification took place due to low temperature post peak metamorphic exchange involving circulating fluids that had undergone isotopic exchange with the surrounding gneisses these fluids are probably related to the upper greenschist grade metamorphism associated with the regional pan african orogeny 500 550 ma this is indicated by the lack of temperature concordancy based on 18 o values of several mineral pairs involving orthopyroxene plagioclase biotite and magnetite sulfur isotope ratios from diorite and norite hosted sulfides are light relative to mantle values and co genetic mineral pairs show a lack of temperature concordancy the ranges in 34 s values for the sulfides are 34 s cpy 1 5 to 3 8 0 34 ss bo 1 9 to 4 1 0 and 34 s po 1 9 to 2 5 0 bulk ore samples show a systematic decrease in 34 s values with increasing cu s ratios this is seen as being diagnostic for an oxidation process and the degassing of a so 2 rich vapour phase petrographic observations and mineral chemistry data support this conclusion and indicate that the devolatilization event was caused by granulite facies metamorphism followed by a later lower temperature alteration event in our genetic model we envisage a mantle origin for the koperberg suite the original magma was modified through assimilation and fractional crystallization processes later modification occurred at the level of emplacement through differentiation and metamorphism crustal contamination can explain the origin of the enriched o isotope values the high levels of incompatible trace elements as well as the high initial 87 sr 86 sr ratios in the basic bodies

low p high t metamorphism in the okiep copper district western namaqualand south africa the okiep copper district part of the 1 2 1 0 ga high grade terrane in western namaqualand is composed of a mid proterozoic supracrustal sequence and several pre to post orogenic intrusive suites affected by two high grade events m2a m2b m3 of kibaran and one low grade event m4 of pan african age peak assemblages in quartz bearing pelites are characterized either by garnet cordierite coexisting with sillimanite biotite or by biotite sillimanite garnet a difference controlled by bulk composition and variation in water activities 0 1 0 7 during dehydration melting maximum p t conditions were reached during m2a coevally with the major deformational event d2a and are estimated at 750 820 c and 5 6 kbar a counterclockwise p t path is indicated by regionally occurring pseudomorphs of sillimanite after andalusite and by prograde reaction textures preserved as relics in m2a porphyroblasts two stages of retrograde metamorphism are distinguished m2a garnet cordierite bearing assemblages were retrogressed to biotite sillimanite quartz m2b along discontinuous foliation planes and shear zones d2b retrograde m3 corona assemblages formed at similar p t conditions 580 660 c and 5 8 0 5 kbar to the m2b assemblages but m3 crystallization postdates penetrative d2 deformation intrusion of 1 06 ga granitoids and formation of associated w mo deposits it is concluded that a kibaran high grade metamorphism in the okiep copper district is thermally punctuated and b reaction textures documenting apparent isobaric cooling of this low p high t terrane must be interpreted with caution

genesis of the precambrian copper rich caraiba hypersthenite norite complex brazil caraiba the largest brazilian copper deposit under exploitation consists mostly of disseminated and remobilised bornite and chalcopyrite hosted in early proterozoic norite and hypersthenite the mafic igneous complex comprises multiple intrusions of dykes veins and breccias of norites and hypersthenites with minor proportions of amphibolised gabbronorite and peridotite xenoliths transported by the magma from deeper levels in the lithosphere the country rocks are high grade gneisses granulites and metasediments compositions of plagioclase an60 40 and orthopyroxene en70 60 fall in a narrow range similar to the koperberg suite from the okiep copper district south africa and to that in many massif type anorthosites whole rock major and trace element geochemistry indicate a parental magma enriched in fe lree p k and cu negative nb anomalies on multi element plots and fractionated ree patterns along with sulphide sulphur isotopes in the range 34s 1 495 to 0 643 suggest a primary mantle lithosphere source although a lower crustal source for the gabbronorite and peridotite xenoliths cannot be excluded geochronological and field evidence indicate that both norite and hypersthenite are likely to have been emplaced during a major sinistral transcurrent partly transpressional shearing event associated with the waning stage of evolution of the early proterozoic salvador curaca orogen

the diurnal interaction between convection and peninsular scale forcing over south florida abstract surface wind measurements taken during the summers of 1973 and 1975 in the florida area cumulus experiments face fine mesh networks are used to calculate surface divergence on the convective scale and mesoscale examination of the daily time series of divergence averaged over a 30 km 25 km grid showed that on days with deep convective activity over and around the network a definable sequence in the network averaged surface wind divergence is observed the sequence consists of five stages persistent convergence peak convergence non divergence peak divergence and the return to a non divergent state use is made of observations of individual cases and of radar composites built around the above sequence in the network averaged divergence fields to demonstrate that a convective scale feedback mechanism consisting of outflows from previous storms triggering new growth is frequently observed during both experimental periods it is shown that the observations taken over the face 1975 network are

sulfide oxidation as a process for the formation of copper rich magmatic sulfides typical magmatic sulfides are dominated by pyrrhotite and pentlandite with minor chalcopyrite and the bulk atomic cu fe ratio of these sulfides is typically less than unity however there are rar

the o okiep copper district namaqualand south africa a review of the geology with emphasis on the petrogenesis of the cupriferous koperberg suite the o okiep copper district is the oldest formal mining area in south africa between 1852 and 2002 the 2 500 km2 area yielded two million tons of copper from 32 mines ranging in ore tonnages from 140 000 to 37 million tons this paper summarizes the calendar of events from the formation of the first primitive crust 1 700 2 000 ma ago to early cambrian times 500 ma ago with particular emphasis on the namaquan grenville orogeny notably the o okiepian episode 1 180 1 210 ma ago of alpine type folding regional granite plutonism and granulite facies metamorphism and the klondikean episode 1 020 1 040 ma ago of open and tight folding and the intrusion of the rietberg granite and the koperberg suite almost all of the copper in the o okiep district occurs in the koperberg suite of which there are 1 700 small bodies that constitute 0 7 of the outcrop area the suite comprises jotunite anorthosite biotite diorite and hypersthenic rocks ranging from leuconorite to hypersthenite and it is one of only two world examples of economic copper mineralization in rocks of the anorthosite charnockite kindred the second example is caraiba brazil high isr and low end for a 1 030 ma intrusion age and high 2 of 10 1 for koperberg rock types indicate a crustal progenitor for the suite and the presence of jotunite suggests a subducted crustal source at ca 40 50 km depth the magmatic sulphide paragenesis in the koperberg suite is chalcopyrite pyrrhotite narrap type ore that in a number of ore bodies has been inverted under upper amphibolite facies conditions to bornite ti free magnetite carolusberg type ore meteoric fluids resulted in supergene cu enrichment in koperberg bodies to 500 m below the pre nama peneplane and lower greenschist facies metamorphism 500 570 ma ago is reflected by inter alia hoits type ore bearing second generation bornite chalcopyrite covellite chalcocite

multi scale analysis of the 25 27 july 2006 convective period over niamey comparison between doppler radar observations and simulations the present study investigates the multi scale processes associated with a sequence of convective events that occurred over niamey during the period 25 26 july 2006 this period corresponds to the active stage of the first intense monsoon surge over sahel for 2006 during this two day period two successive sequences of mesoscale convective systems mcss were located ahead of and in phase with the trough of an african easterly wave aew they were followed by suppressed or isolated convection behind the trough and in the vicinity of the ridge the large amma sop dataset in particular the uhf radar and the mit doppler radar in niamey are used in combination with a low resolution 5 km cloud resolving model to understand the convection organization and its interaction with the environment several initial and boundary conditions have been tested but only the simulation starting with the ecmwf amma reanalysis succeeds in reproducing the observed features this emphasizes the importance of the initial state from the simulated mcss the along line component of the apparent source of momentum due to the convection is found to be up to 1 m s 1h 1 it seems that mcss globally reduce the monsoon flow and generate southerlies at mid levels which can reinforce the rotation of the wind at the passage of the trough during the afternoon of 26 july the local convection over niamey resulted from some favourable factors humidity cape cin that triggered convection while inhibiting factors mid level dry layer weaker low level wind shear pointing to the north anticyclonic curvature of the streamlines at 700 hpa prevented it organizing itself and propagating in particular the low level wind shear seems of critical importance

a density current parameterization coupled with emanuel s convection scheme part i the models abstract the aim of the present series of papers is to develop a density current parameterization for global circulation models this first paper is devoted to the presentation of this new wake parameterization coupled with emanuel s convective scheme the model represents a population of identical circular cold pools the wakes with vertical frontiers the wakes are cooled by the precipitating downdrafts while the outside area is warmed by the subsidence induced by the saturated drafts the budget equations for mass energy and water yield evolution equations for the prognostic variables the vertical profiles of the temperature and humidity differences between the wakes and their exterior they also provide additional terms for the equations of the mean variables the driving terms of the wake equations are the differential heating and drying due to convective drafts the action of the convection on the wakes is implemented by splitting the convective tendency and attributing the effect of the precip

steep structure formation in the okiep copper district south africa bulk inhomogeneous shortening of a high grade metamorphic granite gneiss sequence abstract subhorizontal gneissosities and lithologies of the 1200 1000 ma granulite facies granite gneiss terrane of the okiep copper district south africa are cut by narrow discontinuous e trending cusp like and or monoclinal structures in which the regional gneissosity has been rotated to subvertical attitudes closely associated migmatization and charnockitization indicates that these steep structures formed during high grade metamorphic conditions in this paper we present a model which relates the deformational style strain and orientation of the steep structures to bulk inhomogeneous shortening the development of steep structures can be described as a progression from initial folding via explosive fold amplification to the progressive obliteration of folds by a subvertical e trending intensely developed transposition fabric this fabric is parallel to the axial planes of regional scale open folds shortening in the structures is mainly accommodated by a vertical material extrusion resulting in the commonly upward pointing steep structure geometries and volume loss the strain compatibility with surrounding rocks is maintained by bounding shear zones the progressive development of steep structures illustrates the close relationship between buckle folding and the formation of internal induced anisotropies during deformation of the high grade metamorphic granitic gneisses the apparent discrepancy between large scale open folding and simultaneously developed steep structures recording large finite shortening strains is interpreted to be the result of a large component of internal layer parallel shortening and strain partitioning

a magmatic sheet origin for thin metagabbroic anorthosite units in the fishog subdomain of the southern central gneiss belt grenville province ontario thin tens of metres wide by tens of kilometres long sheets of metagabbroic anorthosite are a common and intriguing feature of the central gneiss belt cgb southwestern grenville province their

radar rain gauge comparisons on squall lines in niamey niger for the amma massachusetts institute of technology c band radar observations are integrated with rainfall measurements from an extensive network of gauges in niamey niger west africa for the african monsoon and multidisciplinary analysis amma the large number of gauges available enabled ze r power law relationships for the convective and stratiform regions of individual squall lines the ze r relationships based solely on radar measurements directly over the gauges were developed for the estimate of rainfall and attendant latent heat release by other amma investigators where gauges were unavailable the low prefactor values of the ze r power laws relative to like values for z r disdrometer power laws have contributions of order 1 2 db from the use of the lowest beam tilt 0 57 a nd 1 2 db by the radar reading low the sphere calibration and the tropical rainfall measuring mission trmm radar calibration are inconsistent at the 1 2 db level for unknown reasons radar gauge comparisons are also shown for individual storms accurate unbiased results for the convective regime require adjustment of the radar to gauge radials for attenuation beam filling problems and aliasing issues can often be identified in the case of outlier points copyright c 2010 royal meteorological society

mesoscale shallow convection in the atmosphere this paper is a review of the observational experimental theoretical and numerical studies of mesoscale shallow convection msc in the atmosphere typically msc is 1 to 2 km deep has a horizontal length scale of a few to a few tens of kilometers and takes distinctive planforms linear and hexagonal the former is called a cloud street roll or band while the latter is called mesoscale cellular convection mcc comprising three dimensional cells msc is characterized by its shape horizontal extent convective depth and aspect ratio the latter is the ratio of the horizontal extent to that in the vertical for cells the horizontal extent is their diameter whereas for rolls it is their spacing rolls usually align along or at angles of up to 10 from the mean horizontal wind of the convective layer with lengths from 20 to 200 km widths from 2 to 10 km and convective depths from 2 to 3 km the typical value of aspect ratio ranges from 2 to 20 rolls may occur over both water surface and land surfaces mesoscale convective cells may be divided into two types open and closed open cell circulation has downward motion and clear sky in the cell center surrounded by cloud associated with upward motion closed cells have the opposite circulation both types of cell have diameters ranging from 10 to 40 km and aspect ratios of 5 to 50 and both occur in a convective layer with a depth of about 1 to 3 km both the magnitude and direction of horizontal wind in the convective layer change little with height msc results from a complex and incompletely understood mix of processes these processes are outlined and their interplay is examined through a review of theoretical and laboratory analyses and numerical modeling of msc