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

structural style and metamorphic conditions of the jinshajiang metamorphic belt nature of the paleo jinshajiang orogenic belt in the eastern tibetan plateau abstract the jinshajiang metamorphic belt is a barrovian sequence distributed within the jinshajiang suture zone jsz which was formed in the thickened paleo jinshajiang orogenic belt after closure of the jinshajiang ocean we have identified three metamorphic phases m 1 m 2 and m 3 corresponding to deformation stages d 1 d 2 and d 3 in the barrovian sequences the metamorphic belt exhibits a metamorphic field gradient from chlorite to biotite garnet staurolite kyanite and sillimanite grades inclusions in garnet and staurolite chlorite mica quartz feldspar ilmenite and graphite indicate that m 1 reflects greenschist facies metamorphism pervasive m 2 metamorphism formed a dominant s 1 schistosity within the barrovian sequence peak metamorphic conditions for metapelites of the garnet staurolite and staurolite kyanite grade were 580 c and 0 65 gpa according to petrogenetic grids peak metamorphism was in conditions of 635 c and 0 50 gpa for the metapelites and 650 c and 0 61 gpa for amphibolites in sillimanite grade greenschist facies retrograde metamorphism m 3 followed d 2 deformation a top down to southeast shear in the jsz the d 3 deformation is characterized by well developed brittle faults with fault gouge and breccia zircon grains from an amphibolite sample have cores with igneous oscillatory zoning and metamorphic rims however the metamorphic rims are too narrow to analyze laser ablation inductively coupled plasma mass spectrometry la icpms analyses of the igneous cores of zircons yielded a crystallization age of 242 ma 40 ar 39 ar dating of white mica from a garnet schist gave a plateau age of 224 ma the peak metamorphism is thus limited to be between 242 and 224 ma we thus suggest a tectonic shift from collision to extension of for the permian to triassic paleo jinshajiang orogenic belt

double layer structure of the crust beneath the zhongdian arc sw china u pb geochronology and hf isotope evidence abstract u pb ages and hf isotopes of zircons in late triassic and cretaceous intrusive rocks from the zhongdian arc sw china are used to decipher the tectonic magmatic and metallogenic processes that occurred during this period new u pb dating of zircons from late triassic porphyries yielded ages of ca 216 ma and ehf t values of 2 1 to 6 1 combined with previous results the data indicate that these late triassic rocks were most likely derived from a juvenile mafic lower crust with minor old crust material however the cretaceous granites 80 ma have lower ehf t values 7 6 to 2 4 than the late triassic rocks indicating that the former originated from old crust based on the new data and previous studies of mesozoic magmatic activity a plausible model for the tectono magmatism and metallogenesis of the zhongdian arc is proposed the westwards subduction of the ganzi litang oceanic crust began before 230 ma resulting in the formation of a juvenile lower crust beneath the zhongdian arc due to the underplating of mafic arc magmas during ca 230 216 ma at ca 216 ma break off or slab tearing of the west dipping ganzi litang oceanic slab led to partial melting of the juvenile lower crust which gave rise to cu bearing porphyries in the late cretaceous the zhongdian arc probably underwent post collision extension triggering the partial melting of the old middle upper crustal materials and producing various granites and related mo cu deposits according to this model the crust beneath the zhongdian arc probably has a double layer structure with older crust at shallow levels and juvenile crust at deeper levels

kibaran a type granitoids and mafic rocks generated by two mantle sources in a late orogenic setting burundi in the mesoproterozoic northeastern kibaran belt of burundi central africa two distinct late kibaran magmatic suites coexist both including a type granitoids they are located along the boundary zone between the kibaran mobile belt western internal domain and the archaean tanzanian craton overlain by mesoproterozoic foreland deposits eastern external domain intense deformation high temperature metamorphism and intrusion of abundant peraluminous anatectic crustal granites occur only in the former domain whereas the mesoproterozoic sedimentary cover of the latter is much less or even nearly undeformed nor metamorphosed the first late kibaran magmatism 350 km long kabanga musongati with an emplacement age of 1275 ma u pb on zircon is mainly composed of mafic and ultramafic layered rocks with subordinate a type acidic differentiates moderately enriched in incompatible elements initial isotopic ratios srlr 0 708 nd 8 indicate an old continental lithospheric mantle origin the emplacement of these late kibaran magmatic rocks was controlled by late lateral shear possibly contemporaneous with the latest intrusions of the kibaran peraluminous synkinematic granites of the western internal domain 1330 1260 ma the second late kibaran magmatism 40 km long gitega makebuko and bukirasazi alignment with an emplacement age of 1249 s ma u pb on zircon is limited in volume it is mainly granitic in composition a type can be strongly enriched in incompatible elements and comprises both syenites and mafic rocks initial isotopic ratios srlr 0 702 na 4 5 to 1 4 point to an oib type asthenospheric lower continental lithospheric mantle origin with only slight contamination by the lower crust during differentiation this group was also intruded during the late lateral shear in both groups liquid lines of descent can be reconstructed although some of the rocks have been strongly albitized this indicates that the granites are produced by differentiation of less evolved magmas and not by crustal anatexis upwelling of the asthenosphere along the tanzanian craton can generate by adiabatic pressure release the oibtype basic melts and provide the heat necessary to melt the continental lithospheric mantle sources this mechanism assigns a major role to a lithosphere scale late kibaran shear event occurring at the end of the regional compressive deformation between two rheologically contrasted domains ascent of the asthenosphere continental lithospheric mantle delamination and late orogenic extensional collapse of the western internal domain are suggested as a possible geodynamic model for the entire northeastern kibaran belt additional work is however necessary to test this model finally our results indicate that in the northeastern kibaran belt the kibaran orogeny ended at 1250 ma despite various reactivation events occurring later e g at 1137 ma

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

petrochemistry of the okiep copper district basic intrusive bodies northwestern cape province south africa copper mineralization is associated with noritic and pyroxenitic rocks in the granulite facies metamorphic terrane in the okiep district of namaqualand in the northwestern cape province of south africa these bodies have been interpreted to be postmetamorphic and the mineralization has been inferred to be magmatic however the sulfide mineralogy of many of the orebodies is dominated by bornitc whereas immiscible magmatic sulfide ores contain pyrrhotite and chalcopyrite hence the okiep ores cannot be primary magmatic sulfides geochemical evidence is presented to demonstrate that the ores have undergone an extensive oxidative event in which pyrrhotite and chalcopyrite were converted to magnetite and bornite up to 90 percent of the sulfur has been lost in the case of some samples from the carolusberg mine the high fe 2 o 3 tio 2 and cu s ratios the low s se ratio of whole rock samples and their variation in different orebodies provide quantitative evidence for this process it is possible to recalculate the original sulfide from these data and a cu content of approximately 10 percent is indicated however these ores only contain on the order of hundreds of parts per million of ni and so the cu ni ratio is extremely high compared to that of most sulfide ores derived from a differentiated basic magma the textural and mineralogieal evidence for this oxidation is best seen in samples from the carolusberg mine in many of the other mines a later low temperature hydrothermal alteration has obliterated the evidence for these reactions

multiple crust reworking in the french armorican variscan belt implication for the genesis of uranium fertile leucogranites muscovite peraluminous granites mpgs form by partial melting of the continental crust and can be related to metalliferous deposits such as tin tungsten and uranium u metal enrichment in mpgs commonly results from fractional crystallization but the metal contents of the source play a major role for their fertility between ca 320 and 300 ma late carboniferous the french armorican variscan belt was intruded by numerous u fertile mpgs that contain inherited zircon grains with a wide range of ages from archean to carboniferous u pb and hf isotopic data of zircon grains from brioverian to carboniferous sediments cambrian to early carboniferous granitoids and late carboniferous mpgs indicate that the crust of the armorican massif is made up by detritus mainly derived from the west african craton 3500 1600 ma tdm 3 8 2 3 ga grenvillian belt 1200 900 ma tdm 2 7 1 2 ga and avalonian cadomian belt 800 550 ma tdm 2 5 0 8 ga and that the crust was affected by magmatic events at 510 470 ma tdm 1 6 0 6 ga 410 330 ma tdm 1 6 1 ga and 320 300 ma furthermore they reveal that the late carboniferous mpgs were mainly formed by partial melting of brioverian sediments with cambro ordovician and devonian carboniferous granitoids which are all genetically linked with each other and characterized by th u 4 the new data suggest that the u fertile mpgs result from multiple reworking of u rich brioverian sediments deposited ca 550 ma ago on the northern margin of gondwana and partially molten during several paleozoic events causing a successive increase in u content in the middle upper crust

implications of the distribution age and origins of the granites of the mesoproterozoic spektakel suite for the timing of the namaqua orogeny in the bushmanland subprovince of the namaqua natal metamorphic province south africa abstract the spektakel suite comprises voluminous late to post tectonic granitoids that intruded the pre tectonic gneisses of the bushmanland subprovince bsp of the mesoproterozoic namaqua natal metamorphic province nnmp based on new u pb zircon geochronology and geochemical data we redefine the suite as consisting of 22 plutons of porphyritic monzogranite granodiorite and charnockite that intruded the granulitic core of the bsp between 1097 and 1033 ma the new data show that some granites previously incorporated into the spektakel suite should be excluded a garnetiferous leucogranites concordia hangsfontein and ibequas granites are excluded on petrological grounds and genetic considerations and b the porphyritic nuwefontein granite 783 ma is younger and is now placed within the richtersveld suite the spektakel suite has geochemical and isotopic compositions suggesting that it was derived from recycled palaeoproterozoic crust with mesoproterozoic mantle input the main namaqua orogeny d2 in the bsp comprises at least two phases of compressional tectonics at 1200 ma and 1110 ma separated by a period of extension the new ages for the spektakel suite tightly constrain the end of d2 to between 1109 ma the age of the youngest pre tectonic gneiss and 1097 ma the age of the oldest spektakel suite granite peak low p high t metamorphism 1050 1020 ma occurred well after d2 and its close spatial and temporal relationship to the spektakel suite suggests this prolonged period of granite magmatism was responsible for the transfer of the heat into the bsp several other late to post tectonic megacrystic granitic suites intruded into other tectonic domains across the nnmp coevally with the spektakel suite indicating widespread granite magmatism at this time probably due to mafic under and intra plating in a back arc setting as previously suggested

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

40ar39ar thermochronological constraints on the structural evolution of the mesoproterozoic natal metamorphic province se africa abstract ten 40 ar 39 ar age spectra are presented for hornblende grains separated from mylonitized and weakly deformed amphibolites from the mesoproterozoic 1 2 1 0 ga natal metamorphic province nmp since the proposal of the sweat hypothesis the nmp has become a crucial area in which to study grenville aged locally termed namaquan accretion in this part of rodinia gondwana unlike many segments of the world s 1 1 ga belts the nmp is particularly suited to such studies since it is devoid of a high temperature pan african 500 ma overprint in this study we have dated hornblendes from within the earliest d1 ne directed thrust dominated structural domains of the belt the d1 domains are characterized by ne directed recumbent fold and thrust nappes and a pervasive generally s to sw dipping metamorphic foliation with down dip stretching lineations the new data constrain the minimum age of this early event which includes obduction of the northernmost tugela terrane onto the kaapvaal craton to 1135 9 ma in the d1 domains of the mzumbe terrane to the south cooling to below 550 c had only been attained by 1005 ma we also dated amphibolites from the later d2 structural domains which are characterized by sub vertical sinistral mylonite belts with sub horizontal to oblique stretching lineations our data show that d2 oblique shearing commenced at 1050 1035 ma in the mzumbe terrane and only terminated at 980 ma the youngest movements also included reactivation of major d1 structures such as the melville thrust mzumbe margate terrane boundary at 990 ma finally indications of minor pan african resetting was detected during the first 5 of the ar ar spectra from mylonites of the lilani matigulu shear zone tugela mzumbe terrane boundary indicating the polyphase nature of this major structure it has been suggested that the d1 event records the early ne sw directed arc continent collision history of the belt whilst the d2 event represents a continuation of essentially the same convergence vectors after extensive crustal thickening the new data place important time constraints on the major tectonic events in the nmp which appear comparable in many ways to other parts of the global grenvillian orogen

physical modelling of deformation in the tasman orogenic zone abstract structural vergence within the western subprovince of the lachlan fold belt is towards the hinterland rather than the foreland in contrast to many well known orogenic belts high angle reverse faults and upright folds verge eastwards away from the australian craton towards the inferred centre of orogenic and magmatic activity we designed a series of analogue models to test the anomalous vergence in the western lachlan fold belt particularly the interaction of a stable australian craton with tasman line geometry interacting with weaker oceanic or transitional lithospheric material we found consistently that vergence direction in the models was towards the hinterland not the foreland as in the western lachlan fold belt irrespective of the way the model was deformed strength gradients between the oceanic and cratonic lithosphere control the deformation patterns an important result of the models is that they demonstrate that fold belts with different vergences can be generated without the requirement of subducting oceanic lithosphere

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

the crustal tongue melting model and the origin of massive anorthosites recent detailed field studies in several anorthosite complexes have shown that anorthosites are frequently associated with weakness zones in the crust which may have favoured their emplacement at mid crust levels recent experimental data have shown that the parent magma compositions of various anorthosite massifs lie on thermal highs in the relevant phase diagrams at 10 13 kbar indicating that these magmas cannot be derived by fractionation of peridotitic mantle melts but by melting of gabbronoritic sources in the lower crust at 40 50 km depths in the sveconorwegian province terne boundaries have been traced in deep seismic profiles to moho offsets or to tongues of lower crustal material underthrust to depths higher than 40 km in southern norway we suggest that a lithospheric scale weakness zone the feda transition zone has channelled the rogaland anorthosites through linear delamination asthenospheric uprise and melting of a mafic lower crustal tongue

reflectivity and seismic properties of the deep continental crust the recovery of drill core from a section of upper amphibolite facies terrane located in the inner piedmont of south carolina provides a rare opportunity to study in detail the seismic character of rocks which were once part of the deep continental crust seismic reflection profiles over the inner piedmont in the vicinity of the drill hole reveal subhorizontal high amplitude reflections the origin of these reflections has been investigated by measuring compressional wave velocities as a function of confining pressure and densities in 150 samples obtained from 305 m of drill core synthetic seismograms generated from the velocity and density measurements demonstrate that reflections originate from subhorizontal metamotphic layering probably originating from lower crustal ductile flow fine tuning of the metamotphic layering which is temperature and pressure related can produce bright spots in the deep crust anisotropy resulting from preferred mineral orientation is another important propeny of this sectim velocities are low for vertical wave propagatim whereas horizontal velocities are relatively high horizontal velocities show mly a slight variatim with azimuth which is unlikely to be detected using standard refractim techniques reflectivity decreases with increasing anisotropy critical to this study has been the continuous coring of a major section of high grade metamotphic terrane which has provided the necessary stratigraphic and lithologic cmtrol for the generation of accurate synthetic seismograms

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

a multistage genetic model for the metamorphosed mesoproterozoic swartberg base metal deposit aggeneys gamsberg ore district south africa

the chronology and tectonic style of landscape evolution along the elevated atlantic continental margin of south africa resolved by joint apatite fission track and u th sm he thermochronology atlantic type continental margins have long been considered passive tectonic settings throughout the entire postrift phase recent studies question the long term stability of these margins and have shown that postrift uplift and reactivation of preexisting structures may be a common feature of a continental margin s evolution the namaqualand sector of the western continental margin of south africa is characterized by a ubiquitously faulted basement but lacks preservation of younger geological strata to constrain postrift tectonic fault activity here we present the first systematic study using joint apatite fission track and apatite u th sm he thermochronology to achieve a better understanding on the chronology and tectonic style of landscape evolution across this region apatite fission track ages range from 58 3 2 6 to 132 2 3 6ma with mean track lengths between 10 9 0 19 and 14 35 0 22 m and mean u th sm he sample ages range from 55 8 31 3 to 120 6 31 4ma joint inverse modeling of these data reveals two distinct episodes of cooling at approximately 150 130ma and 110 90ma with limited cooling during the cenozoic estimates of denudation based on these thermal histories predict approximately 1 3 km of denudation coinciding with two major tectonic events the first event during the early cretaceous was driven by continental rifting and the development and removal of synrift topography the second event during the late cretaceous includes localized reactivation of basement structures as well as regional mantle driven uplift relative tectonic stability prevailed during the cenozoic and regional denudation over this time is constrained to be less than 1 km

a seismic refraction investigation of the namaqualand metamorphic complex south africa the namaqualand metamorphic complex forms part of the precambrian shield of southern africa and hosts several major base metal deposits a long line seismic refraction experiment traversing the complex was conducted in order to complement geological and geophysical studies of the structure and evolution of this region seismograms were obtained at approximately 10 km intervals along the 290 km profile seismic ray tracing techniques were used to model the travel time and amplitude data it was found that the namaqualand lower crust is comprised of a substantial proportion of intermediate velocity 6 2 to 6 9 km s rocks these intermediate velocity rocks approach the surface near the midpoint of the profile moho was found to be at a depth of 42 km no correlation was found between the seismic and electrical resistivity models of the crust when the seismic velocity structure of the middle proterozoic namaqualand metamorphic complex is compared with that of adjacent geological provinces it is found to differ significantly from the archaean kaapvaal province but resemble the late proterozoic damara province these differences might be attributed to variations in the principal mechanisms of crustal growth between archaean and proterozoic times

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

sm nd data for granitoids across the namaqua sector of the namaqua natal province south africa sm nd data for rocks of granitic composition in an east west traverse across the namaqua sector of the mesoproterozoic c 1 2 ga namaqua natal province of southern africa provide new evidence about the timing of crustal extraction from the mantle recent ion probe zircon dating has shown that contrary to previous indications that pre namaqua basement had been preserved in parts of the namaqua sector the majority of the magmatic rocks were emplaced during the 1 4 1 0 ga namaqua tectogenesis and very little u pb evidence of older precursors remains sm nd model ages show that mesoproterozoic crustal extraction did occur but was also strong in the palaeoproterozoic with local evidence for the existence of archaean crust the distribution of model ages generally corresponds with the established terranes subdivided using lithostratigraphic and structural criteria the northern part of the bushmanland terrane shows early palaeoproterozoic to archaean model ages and is clearly different from the southern part which has mesoproterozoic model ages this supports the previously published results of a north south traverse and justifies the separation of the garies terrane from the bushmanland terrane though further subdivisions are not supported the continuation of the richtersveld province east of pofadder is supported by one sample and the terrane boundary between the bushmanland and kakamas terranes is marked by an abrupt change from palaeoproterozoic to mesoproterozoic model ages model ages for the kakamas and areachap terranes do not distinguish them well they suggest a mesoproterozoic to late palaeoproterozoic origin for both terranes neither of which has a purely juvenile character the influence of palaeoproterozoic crust forming events is clear in the kaaien terrane to the east possibly reflecting reworking of the kaapvaal craton the namaqua sector of the province thus has a history of crustal extraction and evolution which reaches back locally to the archaean with major palaeoproterozoic and mesoproterozoic crust forming events this differs from the natal sector which has a largely juvenile character related to a mesoproterozoic wilson cycle attempts to reconstruct the mesoproterozoic supercontinent rodinia will have to take into account the extensive nd isotopic evidence for older crustal events in the namaqua sector

did bushmanland extensionally unroof namaqualand abstract the namaqua natal orogen evolved between about 2000 and 1000 ma as part of an orogenic system that assembled the rodinian supercontinent and wraps the archaean kaapvaal craton to the south and west the orogen includes the western ultra high temperature sapphirine hercynite granulite facies namaqualand terrane structurally overlain to the north by the low grade high level igneous richtersveld terrane 1900 1700 ma and to the east by the amphibolite facies granite gneiss pb zn rich supracrustal bushmanland terrane which is overthrust from the north along the groothoek thrust by the high pressure hom terrane west of the dextral tantalite valley pofadder shear zone the gently dipping contacts among these terranes have been considered to be thrusts with the exception of the groothoek thrust the contacts are low angle top down to the east or northeast folded extensional detachments with displacements of up to at least 100 km the namaqualand part of the namaqua natal orogen comprises two distinct event sequences a kibaran phase of crustal shortening and thickening and voluminous granitic sheets at about 1200 ma then a namaquan phase of mafic underplating ultra high temperature metamorphism granitic sheets dextral transtension constrictional fabrics and crustal thinning from about 1060 to 1030 ma a dominant constrictional ls fabric was developed in regionally sub horizontal layered pink granite gneisses and metasediments deformed by ene trending folds extensional detachments developed late in the namaquan phase when the ultra high temperature rocks were exhumed in the bushmanland terrane aggeneys metasediments have mainly constrictional fabrics and are deformed by giant upward facing downward closing sheath folds developed during transtension in which lie 1700 ma pb zn sedex deposits like those of broken hill and flin flon the namaqualand terrane may be the largest and oldest extensional core complex on earth the proterozoic era was characterized by transtensional ultra high temperature horizontally layered lineated pink gneiss terrains with corrugating fold hinges roughly parallel with the finite stretching direction

composition age and tectonic setting of amphibolites in the central bushmanland group western namaqua province southern africa abstract the western namaqua province wnp of southern africa is composed of several supracrustal assemblages and intrusive granitoid suites that were formed during the mid proterozoic 2 0 1 0 ga ago the bushmanland group bg has been the subject of intense study recently because of its considerable strategic metal potential hosting several giant stratiform base metal deposits e g gamsberg and aggeneys the bg contains amphibolites which represent metamorphosed tholeiitic basalts with mgo 9 4 and enriched in ni relative to modern basalts a feature reminiscent of archaean tholeiites post depositional mobility of elements is probably widespread and hampers recognition of parental magma composition and source region characteristics however the amphibolites have yielded a sm nd isochron age of 1649 90 ma nd t 0 48 0 89 which is interpreted as the time of extrusion of the basalt precursors and by inference a minimum sedimentation age for the underlying metalliferous bg sequence it follows that the bg sequence evolved during an intermediate period in the history of the namaqua province after major crustal stabilisation around 2 0 1 9 ga and before the last major tectonothermal event around 1 2 1 1 ga results of a parallel rb sr and u th pb study of the bg amphibolites indicate strong metamorphic resetting 1 2 1 1 ga ago thereby confirming earlier work on the wnp characteristics of the source region to the bg basalts have been inspected with morb normalised trace element abundances with the salient result being the bg basalts require derivation from a source that experienced variable geochemical modification similar to that proposed for the mantle wedge overlying modern subduction zones such a process is currently preferred over crustal contamination since the sm nd isochron for the bg amphibolites is remarkably linear but independent confirmation of the sm nd age is still required before this alternative model can be discounted to reconcile the requirement of subduction related metasomatism in the source to tholeiitic basalts erupted within what appears to be a cratonic sedimentary basin we suggest that back are continental extension could provide a satisfactory tectonic setting as such an environment is intrinsically linked to subduction

chronology and tectonic framework of turbidite hosted gold deposits in the western lachlan fold belt victoria results abstract 40 ar 39 ar data constrain the history of metamorphism deformation and mineralization in the western subprovince of the lachlan fold belt rocks in the mount stavely volcanic complex and the bushy creek pluton give 40 ar 39 ar ages of about 500 ma for hornblende and biotite the dates from the mount stavely volcanic complex limit the age of mineralization in the metavolcanic rocks of the stavely belt whereas the date for the bushy creek pluton extends the area of cambrian ordovician plutons of the delamerian orogeny to east of the grampians mountains and helps define the surface expression of the western border of the paleozoic lachlan fold belt 40 ar 39 ar dating of metamorphic mica growth phengite in cleaved slates and phyllites within the landsborough fault avoca fault zone heathcote fault zone and mount wellington fault zone gives ages of 453 2 440 2 426 4 and 410 390 ma respectively these data indicate a progression of deformation from west to east starting in late ordovician early silurian time in the stawell and bendigo ballarat structural zones the major phase of deformation in the western subprovince of the lachlan fold belt is therefore silurian and not devonian sericites from major gold deposits in the stawell zone and bendigo ballarat zone give 40 ar 39 ar dates of about 440 ma that are coincident with folding and thrusting a second episode of mineralization and fault reactivation at about 420 to 410 ma is found in some deposits in these zones based on data from a previous study in the melbourne zone including the woods point dike swarm and locally in the bendigo ballarat zone some gold deposits give sericite dates of about 380 360 ma these ages are consistent with the close association of many melbourne zone gold deposits with devonian dikes and plutons the results indicate that gold mineralization in the stawell zone bendigo ballarat zone and melbourne zone was episodic and both temporally and spatially associated with crustal heating linked to regional metamorphism and plutonism initial stages of mineralization in the province comprising many of the large deposits in western and central victoria e g bendigo ballarat stawell wattle gully tarnagulla accompanied metamorphism folding and thrusting subsequent mineralization is widespread but produced mainly smaller deposits

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

crustal structure beneath southern africa and its implications for the formation and evolution of the kaapvaal and zimbabwe cratons the formation of archean crust appears to in volve processes unique to early earth history initial results from receiver function analysis of crustal structure beneath 81 broadband stations deployed across southern africa re vealsignicantdierences inthenatureof thecrustandthe crust mantle boundary between archean and post archean geologic terranes with the notable exception of the colli sional limpopo belt where the crust is thick and the moho complex the crust beneath undisturbed archean craton is typically thin 35 40 km unlayered and characterized byastrongvelocitycontrastacrossarelativelysharpmoho thiscrustalstructurecontrastsmarkedlywiththatbeneath post archeanterranesandbeneatharcheanregionsaected bylarge scaleproterozoicevents thebushveldcomplexand the okwa magondi belts where the crust tends to be rel atively thick 45 50 km and the moho is complex

the kibaran of southern africa tectonic evolution and metallogeny abstract the tectono magmatic and metallogenic evolution of the 1100 ma late kibaran high grade namaqua natal metamorphic belt is reviewed and compared with that of the grenville province and the kibaran belts of central and eastern africa in namaqualand 1300 1600 ma supracrustal rocks are recognised including arc related sequences in the east and extensional foreland basin sequences in the west these were laid down either on a heterogeneous basement of eburnian 2000 ma age or on juvenile oceanic crust prior to late kibaran deformation metamorphism and multiple plutonic intrusion the pre namaqua basement infrastructure has been recognised largely in the eastern and nw parts of the orogen adjacent to areas of exposed eburnian crust and probably only the areachap terrane in the east comprises juvenile kibaran crust in natal 1400 ma predominantly metavolcanic supracrustal gneisses were extensively intruded by voluminous plutonic batholiths during the 1050 ma tectonothermal event the northern terrane is interpreted as an ophiolitic assemblage obducted to the ne onto the kaapvaal craton whereas the southern domains are predominantly arc related no older basement is recognised and the entire belt comprises juvenile crustal material none of which is apparently older than 1400 ma the kinematic evolution of the entire namaqua natal belt is comparable in both areas early sw and ne d directed thrusting and nappe emplacement during collision were followed by pervasive steep transcurrent ductile shearing in natal the later shearing event gave rise to major wnw trending oblique sinistral transcurrent steep belts whereas the same event in namaqualand is manifested as major subvertical nw trending dextral transcurrent shear zones the difference in dominant sense of movement in the two regions can be attributed to changes in orientation of the adjacent archaean craton margin within a progressive consistently oriented ne sw directed zone of plate convergence in eastern namaqualand and namibia the transcurrent shearing gave rise to a north propagating stricke slip rift system in which the volcano sedimentary koras sinclair ghansi sequences were deposited mineral cooling ages of 950 ma reported from throughout the belt are considered to signal the end of the kibaran cycle in southern africa the metallogenic evolution is discussed in the light of the tectono magmatic model presented economically important stratiform volcanic and sedimentary exhalative base metal occurrences are extensively associated with early supracrustal sequences throughout the belt other metal deposits and occurrences are predominantly related to the voluminous granitoid and more restricted metabasic suites which characterise the younger rocks in both natal and namaqualand the metallogenesis of the dominantly convergent collisional namaqua natal belt is similar to that seen in the grenville province but distinct from and younger than the lower grade strike slip basin related kibaran belt of central and eastern africa

deep crustal reflection results from the central eromanga basin australia abstract from 1980 to 1982 deep seismic reflection profiles were recorded across the central eromanga basin in eastern australia to study the regional structure stratigraphy and geological history of the eromanga basin and infra basins the reflection data were recorded to 20 s to obtain additional information on the nature and structure of the crust below the sediments and their relationship to the development of the basins the seismic sections show good quality reflections from the deep crust as well as from the sedimentary layers based on the character strength coherence continuity and spatial distribution of the reflections the sections can be divided into four zones the tope zone between 0 and 2 5 s shows fairly uniform coherent and continuous events which correlate with the mesozoic and late palaeozoic sediments the zone from 2 5 to 8 s 4 to 22 km does not show any primary reflections and is interpreted as the highly deformed metasedimentary and metavolcanic rocks of the early palaeozoic thomson orogen underlying the sediments without any recognisable reflection or diffraction patterns in this zone it is difficult to say whether the faulting and folding observed in the sediments extend into the upper crustal basement the deeper zone of numerous reflection segments between 8 and 12 5 s 22 to 36 km is interpreted as thin laminae of alternating low and high velocity intermediate and basic rocks and correlates with the lower crust bounded by refraction velocity discontinuities the lowest zone of no reflections below 12 5 s corresponds with the upper mantle the reflection character and thickness as well as the refraction velocity structure of the crust under the central eromanga basin area are significantly different from those of the precambrian crust under the georgina basin to the northwest it is proposed that the crust under the eromanga basin is extensionally attenuated crust which had been intruded by sills of basaltic melt from the underlying asthenosphere

late cretaceous granitic intrusions and associated deposits in the yidun arc of the eastern tibetan plateau abstract the yidun arc is an important tectonic unit and polymetallic metallogenic belt in the sanjiang tectono magmatic belt in which many cretaceous 138 75 ma granites and associated polymetallic deposits are distributed along a nw trending belt the queershan cuomolong rongyicuo and genie granitic intrusions and the associated sn pb zn ag polymetallic deposits are distributed in the changtai and xiangcheng arcs whereas the xiuwacu relin hongshan and tongchanggou granitic intrusions and the associated cu mo w deposits are mainly distributed in the geza arc this study presents new zircon u pb hf o isotopic data for the queershan cuomolong rongyicuo genie xiuwacu and tongchanggou granitic intrusions combined with the structural analysis of the xiuwacu granitic complex to discuss the differences in the petrogenesis and metallogenesis of these granitic intrusions as well as the geodynamic processes that controlled the formation of the late cretaceous intrusions and related deposits hf o isotopic characteristics indicate that the highly fractionated granites hfg in the changtai and xiangcheng arcs were mainly derived from the partial melting of metasedimentary crustal materials with a minor input from mantle components while the i type granites ig in the geza arc were predominantly derived from the partial melting of ancient lower crustal material the spatial distributions of the different types of deposits in the yidun arc were probably controlled by the magma sources reduced magmas produced by the partial melting of the sedimentary crust in the changtai and xiangcheng arcs are favorable for w sn pb and zn mineralization whereas oxidized magmas produced by the partial melting of the lower crust in the geza arc are conducive to the enrichment of cu au and mo the geochronological data indicate that the emplacement ages of the late cretaceous granites in the yidun arc gradually decrease from north to south ranging from 106 to 73 ma reflecting the southward lateral tectonic migration the structural analysis of the xiuwacu w mo deposit in combination with previous studies demonstrated that the late cretaceous granites in the yidun arc were likely formed in a regional extensional setting which was probably related to the slab break off of the bangong nujiang oceanic slab or the subduction of the yarlung zangbo oceanic slab

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

reos dating of molybdenites from ore deposits in japan implication for the closure temperature of the reos system for molybdenite and the cooling history of molybdenum ore deposits abstract rhenium and osmium abundances were determined on molybdenites from eighteen ore deposits in japan fifteen vein type a skarn type a greisen type and a pegmatite type deposit re os ages obtained for molybdenites from northeastern central and southwestern japan are 100 130 ma 50 75 ma and ca 10 ma respectively reflecting the ages of regional igneous activities in individual areas for vein type ore deposits re os ages for molybdenites agree with rb sr whole rock ages of host granitoids while they are 3 12 ma older than k ar mineral ages of the host rocks on the other hand re os ages of molybdenites from skarn pegmatite and greisen type ore deposits agree with k ar ages of the host rocks the comparison of re os ages for molybdenites with rb sr and k ar ages of host rocks in vein type deposits suggests that re os closure temperature for vein molybdenite is close to whole rock rb sr closure temperature for host granitic rocks i e roughly estimated to be around 500 c and higher than k ar closure temperature for host granodiorites i e 230 370 c if the thermal history of vein molybdenites is essentially equivalent to that of host rocks one possible explanation for age discrepancies of 3 12 ma between re os ages for molybdenites and k ar ages for the wall rocks observed in vein type molybdenum deposits is that later thermal events after formation of molybdenum ores may reset k ar ages hydrothermal alteration might have occurred in such a temperature condition as to disturb k ar mineral systems in wall rocks and made them younger but not the re os system in molybdenite the other explanation might be that the age discrepancies correspond to a cooling interval from ca 500 to 300 c of host granitoid on the other hand the agreement of re os ages of molybdenites and k ar ages of host rocks in skarn pegmatite and greisen type deposits in this study indicates no later disturbance for k ar system in the wall rock or the rapid cooling of the ore and wall rocks after the formation of molybdenite deposits

mid crustal detachments controlling basin deformation ramp synforms in southwestern queensland abstract deep seismic data from the adavale basin region of southwestern queensland suggest that westward carboniferous movement of the upper crust over mid crustal ramps was responsible for the present day geometry of two synforms containing devonian sediments we interpret the troughs as hanging wall synforms formed by sub vertical simple shear within the upper crust during down dip movement over the ramp one of these structures the quilpie trough in the southern adavale basin is an asymmetric trough interpreted to be an extensional ramp syncline it overlies a west dipping ramp in the boundary between the seismically non reflective upper crust and the seismically reflective lower crust formation of this basin by the relative movement of the upper crustal thomson fold belt over the mid crustal ramp is postulated the significant asymmetry of the trough suggests that lower crustal extension by pure shear may have occurred during upper crustal translation the other structure the westgate trough contains two palaeozoic sequences we propose that the devonian lower sequence is in a ramp syncline formed by folding of the sediments in a conveyor belt style mechanism during a translation of the upper crust the upper sequence is interpreted as a ramp basin in which sediments were deposited in the synform as it developed deep seismic reflection data from the central eromanga basin region indicate that relative horizontal translation can occur between the upper and lower crust on mid crustal detachments the geometry of the synforms suggests that above these detachments crustal deformation has a sub vertical simple shear style whereas the lower crust below the detachments seems to deform in a pure shear style recognition of relative movement of tens of kilometres on mid crustal detachments is important for our understanding of basin geometry and development in the central eromanga basin

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

new isotope data from a neoproterozoic porphyritic garnitoid charnockite suite from natal south africa abstract the tectonic setting and age of emplacement of the oribi gorge suite voluminous rapakivi textured granites and charnockites megacrystic granites from the 1 ga kibaran natal metamorphic province has been controversial isotopic dates of 1 0 ga have been obtained fromfive plutons various isotopic systems but two of these fafa and oribi gorge plutons have also given 0 89 ga rb sr whole rock isochrons the age problem has been exacerbated by apparently equivocal field and structural relationships in which both syn and post tectonic settings have been argued new u pb isotopic analyses for zircon fractions from a sample of the fafa pluton give a concordia data of 1029 1010ma single zircon evaporation analyses of selected grains from the oribi gorge pluton bomela locality give a data of 1092 2ma rb sr model dates of biotite separates from three plutons range between 970 and 880 ma generally similar to the rb sr whole rock dates it is significant that no pan african 500 ma dates were obtained the zircon data strongly support a 1 0 ga age for the emplacement of the oribi gorge suite an age similar to both the main regional tectogenesis and syntectonic granites of the margate suite the oribi gorge suite can thus be regarded as late syn tectonic and not partly post tectonic as previously envisaged consequently the need to invoke two periods of identical a type magmatism is removed the younger rb sr whole rock and biotite dates may represent the time when the various plutons or the entire terrane cooled through the relevant blocking temperatures though this was not a simple homogeneous process

a crustal progenitor for the intrusive anorthosite charnockite kindred of the cupriferous koperberg suite o okiep district namaqualand south africa new isotope data for the country rocks and the intrusives

the stratigraphy and deformation history of the tumut region implications for the development of the lachlan fold belt abstract the tumut region forms the southern part of the tumut synclinorial zone in the southeastern part of the lachlan fold belt lfb the area contains two major stratigraphie packages ordovician to early silurian quartz rich to quartz intermediate flysch and volcanics package a and overlying fossiliferous early late silurian volcanics and flysch package b rhyolite within the older package a yields a u pb zircon age of 428 6 ma both packages were meridionally folded during the siluro devonian bowning orogeny following intrusion of the gocup granite at 411 5 ma an earlier deformation characterised by thrust faulting e w recumbent folding and later local coaxial upright folding is present only in the older flysch and volcanics package a this earlier deformation is part of the benambran orogeny that affects the ordovician metamorphics of the wagga metamorphic belt and other parts of the lfb and is constrained to about 425 ma fold characteristics of this deformation are indicative of thin skinned intraplate transpressional deformation rather than classical collisional tectonics as envisaged by some workers for the benambran orogeny here and elsewhere in the lfb the composition and deformational history of the older flysch indicates that it is part of the ordovician to early silurian molong volcanic belt consequently the older concept of an early silurian tumut trough incorporating both packages of rocks is rejected instead it is proposed that up to 2500 m of early late silurian rocks formed in a pull apart basin the tumut basin thus the silurian depositional history of the tumut region previously considered unique in the lfb is little different from other basins of similar age throughout the lfb

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

fe ti v p ore deposits associated with proterozoic massif type anorthosites and related rocks abstract magmatic rocks containing economic concentrations of iron titanium vanadium and phosphorous are commonly associated with massif type anorthosites and related rocks this rock association is part of the anorthosite mangerite charnockite rapakivi granite suites that are restricted to the proterozoic understanding the geochemistry and emplacement mechanisms of ilmenite magnetite and apatite ore deposits is crucial for exploration efficient mining operations and ore processing this review discusses the controlling factors on the grade of an ore its mineralogy and its major and trace element distribution we present petrogenetic models of currently mined deposits lac tio tellnes damiao and discuss the characteristics of minor ore bodies from anorthosite provinces worldwide grenville north china craton east european craton rogaland laramie models of formation of anorthosite and related rocks are presented as well as the nature of the possible parental magmas of the suite a mineralogical classification of fe ti ores is proposed 1 gabbro noritic ilmenite ore apatite magnetite 2 ti magnetite dominated ore 3 nelsonite fe ti oxides apatite and 4 rutile ilmenite ore the stability of ilmenite and magnetite is then critically reviewed and the influence of various factors particularly oxygen fugacity and crystallization pressure is examined we discuss liquidus compositions of fe ti oxides and the behavior of important trace elements such as cr and v both of which are sensitive to fo2 variations post cumulus evolution of both oxides can occur due to re equilibration with trapped liquid re equilibration with ferromagnesian silicates exsolution oxidation reaction between ilmenite and magnetite and metamorphic overprinting these various processes are described and their effects on the oxide geochemistry are emphasized several potential ore forming processes have been invoked and can explain the formation of huge concentration of ilmenite magnetite apatite fractional crystallization can be combined with crystal sorting and plagioclase buoyancy to produce relative enrichment of dense ore minerals silicate liquid immiscibility can segregate conjugate si rich and fe rich melts the latter being enriched in fe ti p magma mixing can produce hybrid magmas located in a single phase field of the phase diagram and precipitate a pure ilmenite cumulate alternative processes are also described such as ejection of fe ti enriched residual melts by filter pressing and compaction solid state remobilization of ilmenite in veins and hydrothermal transport of fe and ti from the host anorthosite followed by concentration in veins and lenticular ore bodies the magnetic properties of fe ti ore deposits present contrasting signatures depending on whether the natural remanent magnetization is dominated by hemo ilmenite or multi domain magnetite micro and macro scale deformation features of ore rocks are intimately correlated with magma emplacement and with ballooning of the anorthosite diapir associated with gravitational sagging of dense ore bodies exploration perspectives show that oxide apatite gabbronorites are interesting targets because ilmenite in these rocks is poorer in cr and mg and because the ti resource may be combined with apatite and vanadiferous magnetite

re os dating and sulfur isotope composition of molybdenite from tungsten deposits in western namaqualand south africa implications for ore genesis and the timing of metamorphism both stratiform stratabound and granite related models have been used to explain the genesis of w mo deposits in the okiep copper district in western namaqualand south africa apparently stratabound mineralization fe rich wolframite with accessory molybdenite occurs in foliation parallel quartz veins in high grade 750 c 5 6 kbar metapelites of the wolfram formation and less commonly in small bodies of silicified leucogranites and pegmatites six re os ages for molybdenites from four deposits nababeep tungsten far west kliphoog narrap tweedam range between 1000 4 and 1026 5 ma these molybdenites define a well constrained 187re 187os isochron with an age of 1019 6 ma which is interpreted as the age of w mo mineralization this age is significantly younger than proterozoic protolith ages for supracrustal rocks and the emplacement ages for the main intrusive suites but geologic evidence requires overlap with a period of high grade metamorphism we suggest that w mo mineralization is genetically linked to intra crustal magmatic processes at 1020 ma thereby precluding the 1060 ma concordia granite as the source for mineralizing fluids a narrow range of positive 34s compositions 3 6 to 4 5 for eight molybdenites from five w mo mines is consistent with a so2 rich fluid and a granite related genetic model post peak metamorphic deformation and metamorphism of w mo ores is most likely related to the retrograde stage of the namaquan orogeny which overlaps emplacement of late orogenic evolved granites and pegmatites and the formation of w mo deposits in western namaqualand therefore the effects of retrograde namaquan metamorphism extend at least to 1020 ma or alternatively these w mo veins were affected by a poorly constrained later event e g early pan african

magmatic hydrothermal evolution of rare metal pegmatites from the mesoproterozoic orange river pegmatite belt namaqualand south africa abstract the 450 km long orange river pegmatite belt intruded the namaqua sector of the mesoproterozoic namaqua natal province southern africa at ca 1 ga the western part of the belt is characterized by the occurrence of lct pegmatites locally mineralized in li ta nb be and bi most of these mineralized pegmatites display zonation with a quartz feldspar muscovite garnet beryl bearing aplite border and wall zone a quartz feldspar spodumene lepidolite bearing intermediate zone and a quartz k feldspar core the pegmatites and their granodioritic country rocks commonly show evidence of albitization and greisenization i e secondary muscovitization in this study detailed petrographic observations and the compositions of mica feldspar spodumene and nb ta oxide minerals are reported this information is combined with bulk country rock compositions and new u pb zircon and monazite ages plus sm nd isotope compositions of monazite to constrain the origin and magmatic hydrothermal evolution of weakly and strongly mineralized lct pegmatites the li ta be kokerboomrand i pegmatite emplaced at ca 985 ma during the main stage of pegmatite emplacement in the orange river belt and regional strike slip deformation it has end t monazite 14 3 and potentially formed by partial melting of paleoproterozoic rocks from the richtersveld magmatic arc alternatively mesoproterozoic sediments of the bushmanland subprovince may also provide a suitable magma source for the kokerboomrand i pegmatite the rare metal contents of muscovite and columbite group minerals cgm increase from the weakly mineralized to strongly mineralized pegmatites suggesting early stage fractional crystallization or variable partial melting conditions mica nb ta values of rare metal bearing minerals crystallized throughout the magmatic hydrothermal evolution of mineralized pegmatites during the magmatic dominated stage i constitutional zone refining likely induced a decrease of spodumene fe contents and increase of cgm ta contents toward the pegmatite core stage ii marks the pervasive circulation of an immiscible hydrosaline melt this stage is characterized by the replacement of the primary mineral assemblage by lamellar albite cleavelandite fluor calciomicrolite and cs ta rich li muscovite and zinnwaldite stage iii represents the progressive transition from a magmatic hydrothermal system dominated by a li cs ta rich hydrosaline melt toward a system mainly involving li cs ta bi rich aqueous fluids this stage is characterized by the crystallization of lepidolite and zero valence dominant pyrochlore as well as strong localized greisenization and bi mineralization magmatic hydrothermal alteration of the immediate country rock of the mineralized pegmatites is marked by the crystallization of albite li ta cs rich mica various nb ta oxide minerals and led to a strong decrease of the whole rock nb ta ratio plus significant rare metal enrichment our findings from the orange river belt demonstrate that an important role can be ascribed to melt melt and melt fluid immiscibility and metasomatism in the formation of pegmatite related rare metal deposits greisens and albitites associated with pegmatites and their immediate country rocks can be considered as favorable targets for li ta mineral exploration strong and pervasive metasomatism across the mineralized pegmatites and their country rocks suggests involvement of a parental medium that was highly enriched in water

a geophysical transect across the tumut synclinorial zone n s w abstract this integrated geophysical study across the tumut synclinorial zone n s w used seismic magnetic and gravity techniques to determine the structure underlying the zone and the faults which bound it the mooney mooney fault zone to the east and the gilmore fault zone to the west although reflections in the seismic section generally show poor lateral continuity we have recognised domains which display similar reflection character or seismic facies in the seismic data the resulting structural model has been used as the basis for both gravity and magnetic modelling seismic and the gravity interpretations suggest that the mooney mooney fault zone has steep dips to a depth of around 10 km magnetic data suggests that the coolac serpentinite is sub vertical above 7 km depth but declines to the east below this depth below 12 km depth the seismic reflections suggest that structures within the middle crust dip gently to the west the gravity profile is dominated by a positive anomaly west of the mooney mooney fault zone which is associated with the dense bullawyarra schist in the jindalee block the coolac serpentinite dominates the magnetic profiles but has almost no gravity expression a negative bouguer anomaly is associated with the gocup granite and to a lesser extent with the ordovician to early silurian sediments in the tumut block within the tumut synclinorial zone two domains of higher reflectivity are present beneath the jindalee block the top of the first domain is relatively shallow whereas in the tumut block farther to the west the second domain commences beneath the gocup granite at a depth greater than 7 km the tops of these domains are interpreted to be the base of the ordovician to early silurian strata the reflection seismic data indicate no major structural differences between the crust underlying the tumut synclinorial zone and that beneath the goobarragandra block to the east the seismic character of the middle and lower crust of the lachlan fold belt beneath the tumut region is relatively non reflective

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 new chronostratigraphic paradigm for the age and tectonic history of the mesoproterozoic bushmanland ore district south africa a discussion sir the authors are to be congratulated for their contribution to the greater understanding of the aggeneys gamsberg district and bushmanland group through the addition of new detrital zircon age data we trust that these data will ultimately contribute to a better understanding of the depositional age of the bushmanland group as well as the tectonic evolution for this portion of the namaqualand metamorphic province fig 1 while we concur with the authors interpretation that deposition of the bushmanland group and associated base metal sulfide deposits of the aggeneys gamsberg district must be younger than 1650 ma as previously concluded by mcclung 2006 and bailie 2007b we would like to raise some fundamental objections to the contribution by cornell et al 2009 our objections center on the poor and possibly erroneous lithostratigraphic position of the samples analyzed by the authors much of this uncertainty is due to the absence of proper geologic descriptions or convincing evidence to support the assumed stratigraphic position uncritical use of an inadequate lithostratigraphic column for the bushmanland group and the lack of appropriate referencing of recent contributions relevant to this region of the na maquland metamorphic province our most important objection to the communication by cornell et al 2009 relates to the stratigraphic column endorsed the authors present a modified version of the stratigraphic column that was originally described and proposed by praekelt et al 1997 and reiterated in several subsequent publications by the same group of authors e g colliston and schoch 1998 2002 2003 2006 colliston et al 2008 this stratigraphic column is however not widely accepted as it is in direct conflict with the results of regional and local field based investigations of the lithostratigraphy of the bushmanland group fig 2 joubert 1974 1986 rozendaal

mesozoic deposits of sw gondwana namibia unravelling gondwanan sedimentary dispersion drivers by detrital zircon the namibian mesozoic successions may be remnant of a high dynamic sedimentary system that is characterized by multiple stages of sediment accumulation and erosion with contemporaneous homogenization starting with the deposition of the permo carboniferous dwyka group strata and continues at least until the lower cretaceous the lower cretaceous sedimentary system is interpreted to have involved at least an area covering the whole sw gondwana documenting the sedimentary history during the evolution from an ice house environment to an arid desert to test the sediment homogenization hypothesis we applied a combination of isotopic and morphometric data on detrital zircon grains as well as whole rock geochemical data of selected mesozoic sandstones from namibia as a base for the interpretation of the detrital zircon age data we compiled a zircon age dataset with c 44 000 analyses for the southern african region all samples reveal a major detrital pan african zircon age peak of c 0 5 0 7 ga sourced from the pan african magmatic events occurring around the kalahari craton margin the lowermost triassic is characterized by the occurrence of additional mesoproterozoic and paleoproterozoic age peaks of c 1 0 1 2 ga and 1 8 2 0 ga with a majority of zircon grains showing angular shapes the protosource of these grains is interpreted to possibly be the namaqua metamorphic complex and other paleoproterozoic structural units deformed in course of the namaqua orogeny in contrast other samples show a prominent permo triassic age peak and completely rounded zircon grains putatively derived from within the gondwanides volcanic arc the disparity in the zircon age pattern may point towards a change in provenance and also a change in the whole system of zircon recycling during the mesozoic southern gondwana the lower triassic neu loore fm are constrained to more local bedrock sources and short zircon transport distance in contrast zircon grains of the middle triassic omingonde the jurassic etjo und the cretaceous twyfelfontein formations are an expression for a major recycling and sediment homogenization system the system was facilitated by an interplay between fluvial and eolian sedimentary transport systems

grenvillian aged orogenesis in the palaeoproterozoic gascoyne complex western australia 1030 950 ma reworking of the proterozoic capricorn orogen in situ shrimp u pb geochronology of monazite and xenotime in pelitic schists from the central gascoyne complex western australia shows that greenschist to amphibolite facies metamorphism occurred between c 1030 and c 990 ma monazite from an undeformed rare element pegmatite from the same belt gives a 207pb 206pb age of c 950 ma suggesting that peak metamorphism and deformation was followed by pegmatite intrusion and coeval granite magmatism metamorphism in the central gascoyne complex was previously interpreted as barrovian largely based on the identification of kyanite in peak metamorphic assemblages and has been attributed to intense crustal shortening and substantial tectonic thickening during palaeoproterozoic continent continent collision however the stable al2sio5 polymorph has been identified in this study as andalusite rather than kyanite and the prograde assemblages of staurolite garnet andalusite biotite muscovite quartz indicate temperatures of 500 550 c and pressures of 3 4 kbar these data show that the palaeoproterozoic gascoyne complex underwent an episode of grenvillian aged intracontinental reworking concentrated in a nw se striking corridor during the edmundian orogeny until now the edmundian orogeny was thought to have involved only reactivation of structures in the gascoyne complex along with deformation and very low to low grade metamorphism of mesoproterozoic cover rocks some time between 1070 and 755 ma however we suggest that it involved regional amphibolite facies metamorphism and deformation granite magmatism and pegmatite intrusion between c 1030 and c 950 ma therefore the capricorn orogen experienced a major phase of tectonic reworking c 600 myr later than previously recognized our results emphasize the importance of in situ geochronology integrated with petrological studies in order to link the metamorphic history of a terrane with causally related tectonic events