In the Kaapvaal craton of southern Africa, as well as other Archaean cratons worldwide, the progression from dominant tonalite-trondhjemite-granodiorite(TTG) to granite-monzogranite-syenogranite(GMS)rock types is inte...In the Kaapvaal craton of southern Africa, as well as other Archaean cratons worldwide, the progression from dominant tonalite-trondhjemite-granodiorite(TTG) to granite-monzogranite-syenogranite(GMS)rock types is interpreted to reflect progressive reworking and differentiation of the continental crust.Here we re-evaluate the early Archaean evolution of the Kaapvaal craton and propose a unified view of the plutonic and volcanic records based on elemental and isotopic(Nd, Hf) data and zircon U-Pb ages.We also report new whole-rock major and trace element analyses, zircon U-Pb ages and Hf-in-zircon analyses of igneous clasts from a conglomerate of the 3.2 Ga Moodies Group of the Barberton Greenstone Belt. Many of these clasts are derived from shallow intrusive rocks of granitic composition, which are scarcely represented in outcrop. Despite alteration, the volcanic rocks can be classified based on their trace element contents into two main groups by comparison with plutonic rocks. One group has characteristics resembling TTGs: relatively low and fractionated rare earth element concentrations with no Eu anomaly and relatively low concentrations of high field strength elements(Nb mostly ≤12 ppm). The second group has GMS-like characteristics: less fractionated REE, marked negative Eu anomalies and HFSE-increasing trends with progressing fractionation(Nb ≤ 50 ppm or more, Th up to 30-40 ppm). In addition, igneous clasts of Moodies Group conglomerate have chemical, mineralogical and isotopic characteristics that link them to GMS. New analyses of some of these clasts indicate elevated high field strength elements(Nb up to 20 ppm) and_(εHf)(t)of zircon down to -3.5. These rocks imply the presence of an already differentiated felsic crust at >3.5 Ga, which has Nd and Hf model ages indicating mantle extraction ages extending back to the Eoarchaean. The combined record of plutonic and volcanic rocks of the Kaapvaal craton provides a more complex scenario than previously suggested and indicates that TTG and GMS-like felsic magmas were emplaced broadly coevally in multiple pulses between ~3.5 Ga and 3.2 Ga.展开更多
Kaapvaal lamproites(aka orangeites)are a group of volatile-rich(H_(2)O,CO_(2)),micaceous,ultrapotassic igneous rocks that are unique to the Kaapvaal craton in southern Africa.However,the composition of the melts that ...Kaapvaal lamproites(aka orangeites)are a group of volatile-rich(H_(2)O,CO_(2)),micaceous,ultrapotassic igneous rocks that are unique to the Kaapvaal craton in southern Africa.However,the composition of the melts that give rise to these rocks remains poorly understood due to overprinting effects of contamination by mantle and crustal material,volatile exsolution,fractional crystallisation and post-magmatic alteration.Consequently,this lack of reliable data on the initial composition of the Kaapvaal lamproite melts hampers our understanding of their source,petrogenesis and ascent mechanisms.Olivine is a common mineral comprising the Kaapvaal lamproites that has both xenocrystic(i.e.,mantle)and magmatic origins.Multiphase inclusions(melt/fluid)entrapped within olivine have been consistently demonstrated as an effective tool for gaining fundamental insights into the composition and evolution of melts that produce both kimberlites and lamproites,prior to processes,such as eruption,devolatization and syn-/post-magmatic alteration.In this study,multiphase inclusions of both secondary and pseudosecondary origin hosted in olivine from a Kaapvaal lamproite(Silvery Home,South Africa)provide novel insights into the composition of the melt(s)that initially transported olivine to the surface and then crystallised after emplacement to form the lamproite groundmass.The inclusions in our study contain daughter mineral assemblages consisting of diverse Ca-Mg carbonates,including K-,Na-,Ba-,and Sr-bearing varieties,moderate K-rich silicates(phlogopite,tetraferriphlogopite),and subordinate oxides,phosphates,sulphides,sulphates,and halides.Based on these daughter mineral assemblages,we suggest that the composition of the melt entrapped by olivine was SiO_(2)-poor,Ca-Mg carbonate-rich and contained elevated concentrations of K,Na,Ba,Sr,P and Cl.The mineral and reconstructed melt compositions are in stark contrast to the mineral association of the groundmass and the bulk-rock composition of the Silvery Home lamproite,respectively.We suggest that alkali-/alkali-earth carbonates,phosphates,sulphides,sulphates,and halides represented a potentially significant,or even dominant,component of the melt that crystallised the Silvery Home lamproite but were likely removed by degassing and/or interaction with syn-/post-magmatic fluids.We show that olivine-hosted multiphase inclusions from the Silvery Home lamproite share many compositional similarities to melt inclusions hosted in olivine from kimberlites but are distinct from‘classic’cratonic olivine lamproites worldwide.展开更多
The geological setting of the Bushveld Complex,the world’s largest layered igneous body containing most of the planet’s chromium,Platinum Group Elements(PGEs)and vanadium resources,is outlined.The complex is situate...The geological setting of the Bushveld Complex,the world’s largest layered igneous body containing most of the planet’s chromium,Platinum Group Elements(PGEs)and vanadium resources,is outlined.The complex is situated in a central position on the ancient Kaapvaal Craton of South Africa,with the mineralisation contained in three large arcuate mafic to ultramafic Limbs,comprised of rocks of the Rustenburg Layered Suite(RLS).The Limbs are overlain by co-magmatic felsic rocks while the floor consists mainly of sediments of the Transvaal Supergroup.展开更多
The Pilbara region of Western Australia is one of only two areas on Earth–the other being the Kaapvaal Craton of southern Africa–that contain well preserved,nearcontinuous geological records of crustal evolution fro...The Pilbara region of Western Australia is one of only two areas on Earth–the other being the Kaapvaal Craton of southern Africa–that contain well preserved,nearcontinuous geological records of crustal evolution from the Paleoarchean into the late Paleoproterozoic.The Pilbara is famous for hosting fossil evidence of early life(stromatolites and microfossils),and for containing a record of the early Archean atmosphere.The geological record extends from granite–greenstone terranes and overlying clastic basins of the 3.53–2.83 Ga Pilbara Craton,across a major unconformity,to a series of 2.78–1.79 Ga volcanic and sedimentary successions.Between 3.53–3.23 Ga,a succession of mantle plume events formed a thick volcanic plateau on older continental crust,remnants of which include enclaves of c.3.6 Ga granitic gneiss and abundant 3.8–3.6 Ga inherited and detrital zircons.During each of the plume events,the volcanic plateau was intruded by crustally-derived granitic rocks,leading to vertical deformation by partial convective overturn.By 3.23 Ga,these processes had established thick continental crust that was then rifted into three microplates separated by c.3.2 Ga basins of oceanic crust.Subsequent plate tectonic processes to 2.90 Ga included subduction,terrane accretion,and orogeny.From 2.78–2.63 Ga the northern Pilbara Craton was affected by minor rifting,followed by deposition of thick basaltic formations separated by felsic volcanic and sedimentary rocks(Fortescue Basin).Rifting in the southern Pilbara resulted in progressively deepening marginal basin sedimentation,including thick units of banded iron formation(Hamersley Basin:2.63–2.45 Ga).展开更多
On the 3rd of April 2017, an earthquake of moment magnitude 6.5 occurred near Moiyabana in central Botswana. This paper is aimed at studying the spatial distribution of the foreshock and aftershock sequences associate...On the 3rd of April 2017, an earthquake of moment magnitude 6.5 occurred near Moiyabana in central Botswana. This paper is aimed at studying the spatial distribution of the foreshock and aftershock sequences associated with the Moiyabana earthquake. The foreshocks and aftershocks data used were from the Botswana Geoscience Institute (BGI) and the Seisan software was used to analyze the foreshock and aftershock events. The analyses revealed nine epicenter locations of foreshocks which are spread out across the country and most of them are located in the central and southern parts of Botswana, while the aftershocks are clustered around the mainshock. Although five of the nine foreshocks occurred far from the boundaries of major tectonic units, the other four occurred near key features such as the Zoetfontein, Lecha and Chobe faults. The spatial distribution of aftershocks indicates that the stress released by the mainshock, re-activated the planes of weakness in the vicinity of the mainshock and farther away from the mainshock. Hence, this affected the Zoetfontein fault, the boundary between Passarge basin and Magondi belt and the boundary between the Kaapvaal craton and Limpopo mobile belt. The aftershocks also show a northwest-southeast trend, which probably indicates the rupture plane;and mainly lie within the Limpopo mobile belt that is sandwiched between the Kaapvaal craton to the south and Zimbabwe craton to the north. Furthermore, the aftershocks concentration to the south reveals a close relation in demarcating the boundary of the Kaapvaal craton and the Limpopo mobile belt.展开更多
The world’s greatest deposit of gold is hosted by the Archaean Witwatersrand sedimentary basin,situated in the central portion of the Kaapvaal Craton of South Africa.The geological setting of this remarkable clastic ...The world’s greatest deposit of gold is hosted by the Archaean Witwatersrand sedimentary basin,situated in the central portion of the Kaapvaal Craton of South Africa.The geological setting of this remarkable clastic sedimentary deposit,which has yielded more than one third of all the gold ever produced on the planet,is discussed.The stratigraphy and structure of the Witwatersrand Supergroup is reviewed together with the sedimentology,mineralogy and geochronology of the more important auriferous conglomerate(reef)horizons.展开更多
基金supported through the years by Innovation Fellowship funds of the National Research Foundation (Innovation Postdoctoral Fellowship grant number 80422)a Post-doctoral Fellowship of the University of Johannesburg (at UJ)+2 种基金SIEF funds(Grant No. RP04-063 at Curtin)to AAthe NRFNEP funded (grant#93208)LA-MC-ICPMSCIMERA for their further financial support of the laboratory
文摘In the Kaapvaal craton of southern Africa, as well as other Archaean cratons worldwide, the progression from dominant tonalite-trondhjemite-granodiorite(TTG) to granite-monzogranite-syenogranite(GMS)rock types is interpreted to reflect progressive reworking and differentiation of the continental crust.Here we re-evaluate the early Archaean evolution of the Kaapvaal craton and propose a unified view of the plutonic and volcanic records based on elemental and isotopic(Nd, Hf) data and zircon U-Pb ages.We also report new whole-rock major and trace element analyses, zircon U-Pb ages and Hf-in-zircon analyses of igneous clasts from a conglomerate of the 3.2 Ga Moodies Group of the Barberton Greenstone Belt. Many of these clasts are derived from shallow intrusive rocks of granitic composition, which are scarcely represented in outcrop. Despite alteration, the volcanic rocks can be classified based on their trace element contents into two main groups by comparison with plutonic rocks. One group has characteristics resembling TTGs: relatively low and fractionated rare earth element concentrations with no Eu anomaly and relatively low concentrations of high field strength elements(Nb mostly ≤12 ppm). The second group has GMS-like characteristics: less fractionated REE, marked negative Eu anomalies and HFSE-increasing trends with progressing fractionation(Nb ≤ 50 ppm or more, Th up to 30-40 ppm). In addition, igneous clasts of Moodies Group conglomerate have chemical, mineralogical and isotopic characteristics that link them to GMS. New analyses of some of these clasts indicate elevated high field strength elements(Nb up to 20 ppm) and_(εHf)(t)of zircon down to -3.5. These rocks imply the presence of an already differentiated felsic crust at >3.5 Ga, which has Nd and Hf model ages indicating mantle extraction ages extending back to the Eoarchaean. The combined record of plutonic and volcanic rocks of the Kaapvaal craton provides a more complex scenario than previously suggested and indicates that TTG and GMS-like felsic magmas were emplaced broadly coevally in multiple pulses between ~3.5 Ga and 3.2 Ga.
基金funded by the K.H.Renlund Foundation(Finland)and Future Making Fellowship(University of Adelaide)AVG and AAT were supported by the state assignment of IGM SB RAS(No.122041400157-9).
文摘Kaapvaal lamproites(aka orangeites)are a group of volatile-rich(H_(2)O,CO_(2)),micaceous,ultrapotassic igneous rocks that are unique to the Kaapvaal craton in southern Africa.However,the composition of the melts that give rise to these rocks remains poorly understood due to overprinting effects of contamination by mantle and crustal material,volatile exsolution,fractional crystallisation and post-magmatic alteration.Consequently,this lack of reliable data on the initial composition of the Kaapvaal lamproite melts hampers our understanding of their source,petrogenesis and ascent mechanisms.Olivine is a common mineral comprising the Kaapvaal lamproites that has both xenocrystic(i.e.,mantle)and magmatic origins.Multiphase inclusions(melt/fluid)entrapped within olivine have been consistently demonstrated as an effective tool for gaining fundamental insights into the composition and evolution of melts that produce both kimberlites and lamproites,prior to processes,such as eruption,devolatization and syn-/post-magmatic alteration.In this study,multiphase inclusions of both secondary and pseudosecondary origin hosted in olivine from a Kaapvaal lamproite(Silvery Home,South Africa)provide novel insights into the composition of the melt(s)that initially transported olivine to the surface and then crystallised after emplacement to form the lamproite groundmass.The inclusions in our study contain daughter mineral assemblages consisting of diverse Ca-Mg carbonates,including K-,Na-,Ba-,and Sr-bearing varieties,moderate K-rich silicates(phlogopite,tetraferriphlogopite),and subordinate oxides,phosphates,sulphides,sulphates,and halides.Based on these daughter mineral assemblages,we suggest that the composition of the melt entrapped by olivine was SiO_(2)-poor,Ca-Mg carbonate-rich and contained elevated concentrations of K,Na,Ba,Sr,P and Cl.The mineral and reconstructed melt compositions are in stark contrast to the mineral association of the groundmass and the bulk-rock composition of the Silvery Home lamproite,respectively.We suggest that alkali-/alkali-earth carbonates,phosphates,sulphides,sulphates,and halides represented a potentially significant,or even dominant,component of the melt that crystallised the Silvery Home lamproite but were likely removed by degassing and/or interaction with syn-/post-magmatic fluids.We show that olivine-hosted multiphase inclusions from the Silvery Home lamproite share many compositional similarities to melt inclusions hosted in olivine from kimberlites but are distinct from‘classic’cratonic olivine lamproites worldwide.
文摘The geological setting of the Bushveld Complex,the world’s largest layered igneous body containing most of the planet’s chromium,Platinum Group Elements(PGEs)and vanadium resources,is outlined.The complex is situated in a central position on the ancient Kaapvaal Craton of South Africa,with the mineralisation contained in three large arcuate mafic to ultramafic Limbs,comprised of rocks of the Rustenburg Layered Suite(RLS).The Limbs are overlain by co-magmatic felsic rocks while the floor consists mainly of sediments of the Transvaal Supergroup.
文摘The Pilbara region of Western Australia is one of only two areas on Earth–the other being the Kaapvaal Craton of southern Africa–that contain well preserved,nearcontinuous geological records of crustal evolution from the Paleoarchean into the late Paleoproterozoic.The Pilbara is famous for hosting fossil evidence of early life(stromatolites and microfossils),and for containing a record of the early Archean atmosphere.The geological record extends from granite–greenstone terranes and overlying clastic basins of the 3.53–2.83 Ga Pilbara Craton,across a major unconformity,to a series of 2.78–1.79 Ga volcanic and sedimentary successions.Between 3.53–3.23 Ga,a succession of mantle plume events formed a thick volcanic plateau on older continental crust,remnants of which include enclaves of c.3.6 Ga granitic gneiss and abundant 3.8–3.6 Ga inherited and detrital zircons.During each of the plume events,the volcanic plateau was intruded by crustally-derived granitic rocks,leading to vertical deformation by partial convective overturn.By 3.23 Ga,these processes had established thick continental crust that was then rifted into three microplates separated by c.3.2 Ga basins of oceanic crust.Subsequent plate tectonic processes to 2.90 Ga included subduction,terrane accretion,and orogeny.From 2.78–2.63 Ga the northern Pilbara Craton was affected by minor rifting,followed by deposition of thick basaltic formations separated by felsic volcanic and sedimentary rocks(Fortescue Basin).Rifting in the southern Pilbara resulted in progressively deepening marginal basin sedimentation,including thick units of banded iron formation(Hamersley Basin:2.63–2.45 Ga).
文摘On the 3rd of April 2017, an earthquake of moment magnitude 6.5 occurred near Moiyabana in central Botswana. This paper is aimed at studying the spatial distribution of the foreshock and aftershock sequences associated with the Moiyabana earthquake. The foreshocks and aftershocks data used were from the Botswana Geoscience Institute (BGI) and the Seisan software was used to analyze the foreshock and aftershock events. The analyses revealed nine epicenter locations of foreshocks which are spread out across the country and most of them are located in the central and southern parts of Botswana, while the aftershocks are clustered around the mainshock. Although five of the nine foreshocks occurred far from the boundaries of major tectonic units, the other four occurred near key features such as the Zoetfontein, Lecha and Chobe faults. The spatial distribution of aftershocks indicates that the stress released by the mainshock, re-activated the planes of weakness in the vicinity of the mainshock and farther away from the mainshock. Hence, this affected the Zoetfontein fault, the boundary between Passarge basin and Magondi belt and the boundary between the Kaapvaal craton and Limpopo mobile belt. The aftershocks also show a northwest-southeast trend, which probably indicates the rupture plane;and mainly lie within the Limpopo mobile belt that is sandwiched between the Kaapvaal craton to the south and Zimbabwe craton to the north. Furthermore, the aftershocks concentration to the south reveals a close relation in demarcating the boundary of the Kaapvaal craton and the Limpopo mobile belt.
文摘The world’s greatest deposit of gold is hosted by the Archaean Witwatersrand sedimentary basin,situated in the central portion of the Kaapvaal Craton of South Africa.The geological setting of this remarkable clastic sedimentary deposit,which has yielded more than one third of all the gold ever produced on the planet,is discussed.The stratigraphy and structure of the Witwatersrand Supergroup is reviewed together with the sedimentology,mineralogy and geochronology of the more important auriferous conglomerate(reef)horizons.
