C.G. Kingsbury, W. Altermann, J. Kramers, R.E. Ernst, U. Söderlund, M. Klausen
A new 130 to 140 Ma mafic dyke swarm, is identified in western South Africa. It consists of the previously undated Cederberg dyke swarm (CDS), for which we report U-Pb ID-TIMS baddeleyite ages of 131.4 ± 4.5 Ma (Knersvlake subswarm) and 133.0 ± 1.5 Ma (Doring-Tanqua subswarm). 40Ar/39Ar dates on these two samples and two additional dates of the Doring – Tanqua subswarm cluster between 128.5 ± 1.4 Ma and 132.2 ± 1.5 Ma. We also report 40Ar/39Ar ages of 139.3 ± 3.5 Ma for an east-west trending dyke located further north: 27 km south of Kleinsee, and 140.3 ± 1.2 Ma for an east-west dyke near Garies. Together, these eight ages robustly date the emplacement of the northern part of a Greater Cederberg-False Bay Dyke Swarm (GCFDS) at ca. 130 to 140 Ma. Trace and rare earth element datareported herein suggest these dykes are compositionally E-MORB basalts that underwent modification either via subduction-modified lithospheric mantle, or by continental crust, or a combination thereof, and are petrogenetically similar to the ca. 132 Ma False Bay dykes around Cape Town. Therefore, we propose to unify all these coeval and compositionally similar dykes into one large igneous province (LIP) termed the Greater Cederberg-False Bay Large Igneous Province (GCF-LIP).
{"title":"The Greater Cederberg–False Bay Large Igneous Province in South Africa: A southern node of widespread magmatism associated with South Atlantic rifting","authors":"C.G. Kingsbury, W. Altermann, J. Kramers, R.E. Ernst, U. Söderlund, M. Klausen","doi":"10.25131/sajg.126.0023","DOIUrl":"https://doi.org/10.25131/sajg.126.0023","url":null,"abstract":"\u0000 A new 130 to 140 Ma mafic dyke swarm, is identified in western South Africa. It consists of the previously undated Cederberg dyke swarm (CDS), for which we report U-Pb ID-TIMS baddeleyite ages of 131.4 ± 4.5 Ma (Knersvlake subswarm) and 133.0 ± 1.5 Ma (Doring-Tanqua subswarm). 40Ar/39Ar dates on these two samples and two additional dates of the Doring – Tanqua subswarm cluster between 128.5 ± 1.4 Ma and 132.2 ± 1.5 Ma. We also report 40Ar/39Ar ages of 139.3 ± 3.5 Ma for an east-west trending dyke located further north: 27 km south of Kleinsee, and 140.3 ± 1.2 Ma for an east-west dyke near Garies. Together, these eight ages robustly date the emplacement of the northern part of a Greater Cederberg-False Bay Dyke Swarm (GCFDS) at ca. 130 to 140 Ma. Trace and rare earth element datareported herein suggest these dykes are compositionally E-MORB basalts that underwent modification either via subduction-modified lithospheric mantle, or by continental crust, or a combination thereof, and are petrogenetically similar to the ca. 132 Ma False Bay dykes around Cape Town. Therefore, we propose to unify all these coeval and compositionally similar dykes into one large igneous province (LIP) termed the Greater Cederberg-False Bay Large Igneous Province (GCF-LIP).","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":"60 20","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138985928","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
E. Zhimulev, I. Gryaznov, A. Chepurov, V. Sonin, A. Chepurov
The results on dissolution of flat-faced synthetic diamond crystals of octahedral habit in an Fe-Ni-S melt at 4.0 GPa and 1 400°C are presented. It has been established that the resulting diamond morphology is similar to some natural kimberlitic diamonds and follows the particular sequence: flat-faced octahedron – laminar octahedron – trisoctahedroid with parallel striations in the <110> direction (“O1-D1”). Comparing the obtained results with earlier experimental works it is concluded that oxidisation of octahedral diamonds by means of ditrigonal etching layers and formation of tetrahexahedroid form is a result of diamond interaction with the fluidised kimberlite magma. We suggest that formation of octahedral diamonds with trigonal etching layers does not occur in kimberlite magma, and that diamonds of the O1-D1 morphological series avoided natural oxidation in kimberlite magma, but, like flat-faced octahedrons, were enclosed within xenoliths. Most probably, this dissolution process took place in the mantle prior to their capture by kimberlite. The results support an idea that metal-sulphide melts could be considered not only as a growth place for the world’s largest Cullinan-like diamonds found in South Africa (CLIPPIR type), but also as a mantle agent producing crystal morphologies typical for common kimberlitic diamonds: it depends upon carbon content in metal-sulphide melt – with supersaturation of the melt, the growth of diamond occurs while at the undersaturation conditions the dissolution begins.
{"title":"Dissolution of synthetic diamonds to produce morphologies similar to natural diamonds: an experimental study","authors":"E. Zhimulev, I. Gryaznov, A. Chepurov, V. Sonin, A. Chepurov","doi":"10.25131/sajg.126.0025","DOIUrl":"https://doi.org/10.25131/sajg.126.0025","url":null,"abstract":"\u0000 The results on dissolution of flat-faced synthetic diamond crystals of octahedral habit in an Fe-Ni-S melt at 4.0 GPa and 1 400°C are presented. It has been established that the resulting diamond morphology is similar to some natural kimberlitic diamonds and follows the particular sequence: flat-faced octahedron – laminar octahedron – trisoctahedroid with parallel striations in the <110> direction (“O1-D1”). Comparing the obtained results with earlier experimental works it is concluded that oxidisation of octahedral diamonds by means of ditrigonal etching layers and formation of tetrahexahedroid form is a result of diamond interaction with the fluidised kimberlite magma. We suggest that formation of octahedral diamonds with trigonal etching layers does not occur in kimberlite magma, and that diamonds of the O1-D1 morphological series avoided natural oxidation in kimberlite magma, but, like flat-faced octahedrons, were enclosed within xenoliths. Most probably, this dissolution process took place in the mantle prior to their capture by kimberlite. The results support an idea that metal-sulphide melts could be considered not only as a growth place for the world’s largest Cullinan-like diamonds found in South Africa (CLIPPIR type), but also as a mantle agent producing crystal morphologies typical for common kimberlitic diamonds: it depends upon carbon content in metal-sulphide melt – with supersaturation of the melt, the growth of diamond occurs while at the undersaturation conditions the dissolution begins.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":"467 ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138992509","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
M.A.G. Andreoli, J. Knight, R.J. Durrheim, M. Evans
The safe disposal of radioactive waste products remains a challenging and controversial issue, yet their burial in a stable geological environment is the option favoured by most countries. Past investigations of the Vaalputs facility in South Africa’s semi-arid Bushmanland Plateau demonstrated its suitability for the disposal of low-level, short-lived (<300 years) radioactive waste, but also the need for more data on hazardous seismotectonic and climatic processes for the potential storage of longer-lived spent nuclear fuel. We review and then provide new information on the regional geomorphological and morphotectonic features of the Bushmanland Plateau, focusing on pedogenic features and compressive shear fractures in the trenches excavated over the past two decades. The fractures, provisionally dated between ~75 ka and ~46 ka, resemble east-verging thrust faults of limited horizontal displacement (≤1 m) and die out at depths of 4 m. Having discounted a pedogenic origin, the shear fractures likely represent stress relief structures triggered by the seismogenic reactivation of one of the Quaternary active faults in the area. The most likely candidate is the Santab Fault Zone (SFZ), which has a strike length ≥20 km and a throw of up to ~5 m at its closest point (8 km) to the Vaalputs site. Regional remote sensing and off-site investigations in the Bushmanland Plateau relate the neotectonic episodes at Vaalputs and environs to a late Pleistocene climax of the Namaqualand seismic source zone. This zone is characterised by the Wegener Stress Anomaly, a long-lived ~northwest-southeast oriented compression that, at its apex in the Late Santonian, caused large scale folding and thrusting not only in Bushmanland and parts of central-southern Namibia, but also swept across the entire African plate.
放射性废品的安全处置仍然是一个具有挑战性和争议性的问题，但将其掩埋在稳定的地质环境中是大多数国家所青睐的选择。过去对南非半干旱布什曼兰高原瓦尔普茨设施的调查表明，该设施适合处理低放射性、短寿命（小于 300 年）的废料，但同时也表明，需要更多有关危险地震构造和气候过程的数据，以潜在地储存寿命较长的乏核燃料。我们回顾了布什曼兰高原的区域地貌和形态构造特征，然后提供了有关这些特征的新信息，重点是过去二十年挖掘的沟槽中的成土特征和压缩剪切断裂。这些断裂的临时年代介于约 75 ka 和约 46 ka 之间，类似于水平位移有限（≤1 米）的东倾推力断层，并在 4 米深处消亡。最有可能的候选断层是桑塔布断层带（SFZ），其走向长度≥20 千米，在距离瓦尔普茨遗址最近处（8 千米）的走向可达 5 米。布什曼兰高原的区域遥感和非现场调查将瓦尔普茨及其周边地区的新构造事件与纳马夸兰德震源区的晚更新世高潮联系起来。该区的特点是韦格纳应力异常，这是一种长期存在的~西北-东南向压缩，在晚山顿时期达到顶峰，不仅在布什曼兰和纳米比亚中南部部分地区造成了大规模的褶皱和推力，而且还席卷了整个非洲板块。
{"title":"The geomorphology and neotectonics of the Vaalputs Radioactive Waste Disposal Facility site, Namaqualand, South Africa: Palaeoseismological and geodynamic implications","authors":"M.A.G. Andreoli, J. Knight, R.J. Durrheim, M. Evans","doi":"10.25131/sajg.126.0022","DOIUrl":"https://doi.org/10.25131/sajg.126.0022","url":null,"abstract":"\u0000 The safe disposal of radioactive waste products remains a challenging and controversial issue, yet their burial in a stable geological environment is the option favoured by most countries. Past investigations of the Vaalputs facility in South Africa’s semi-arid Bushmanland Plateau demonstrated its suitability for the disposal of low-level, short-lived (<300 years) radioactive waste, but also the need for more data on hazardous seismotectonic and climatic processes for the potential storage of longer-lived spent nuclear fuel. We review and then provide new information on the regional geomorphological and morphotectonic features of the Bushmanland Plateau, focusing on pedogenic features and compressive shear fractures in the trenches excavated over the past two decades. The fractures, provisionally dated between ~75 ka and ~46 ka, resemble east-verging thrust faults of limited horizontal displacement (≤1 m) and die out at depths of 4 m. Having discounted a pedogenic origin, the shear fractures likely represent stress relief structures triggered by the seismogenic reactivation of one of the Quaternary active faults in the area. The most likely candidate is the Santab Fault Zone (SFZ), which has a strike length ≥20 km and a throw of up to ~5 m at its closest point (8 km) to the Vaalputs site. Regional remote sensing and off-site investigations in the Bushmanland Plateau relate the neotectonic episodes at Vaalputs and environs to a late Pleistocene climax of the Namaqualand seismic source zone. This zone is characterised by the Wegener Stress Anomaly, a long-lived ~northwest-southeast oriented compression that, at its apex in the Late Santonian, caused large scale folding and thrusting not only in Bushmanland and parts of central-southern Namibia, but also swept across the entire African plate.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":"108 ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139014186","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The 3D modelling of basic bodies of the Koperberg Suite (1060 to 1030 Ma) and their wall rocks from Narrap Mine illustrates the distribution, geometries and, by implication, processes that determined the ascent and emplacement of the mantle-derived mafic magmas into the partially-molten, mid-crustal granite-gneiss sequence of the Okiep Copper District in Namaqualand. The lens-like, discontinuous geometry of basic bodies suggests the transfer of the mafic magmas as self-contained, buoyancy-driven hydrofractures. The presence of both shallowly-dipping, foliation-parallel sills and subvertical lenses in zones of steep foliation development, so-called steep structures, indicates an emplacement of the mafic magmas under low deviatoric stresses and irrespective of the regional stress field. Instead, the emplacement of the mafic magmas parallel to pre-existing anisotropies (tectonic fabrics or lithological contacts) highlights those differences in tensile strength and fracture toughness parallel to or across anisotropies determined the propagation of the magmas. This also accounts for the common occurrence of basic bodies in steep structures in which the vertical gneissosity promoted the buoyancy-driven ascent of the mafic magmas. On a regional scale, the mechanical stratification of the subhorizontal, sheet-like granite gneisses and interlayered metasediments exerted important controls on the ascent of Koperberg Suite magmas. The preferential emplacement of basic bodies in schist and gneiss units suggests that the lower rigidity of the ductile wall rocks facilitated magma emplacement through a combination of viscous wall-rock deformation and fracture blunting that led to the arrest of the magma-filled hydrofractures. Multiple intrusive relationships of successively emplaced magma batches suggest that later magmas reutilised earlier established magma pathways, particularly in steep structures. High-rigidity lithologies, such as the massive Springbok Quartzite, in contrast, only allowed for smaller fracture apertures and limited dilation, resulting in the pinching of basic bodies and rather stringer-like geometries. It is conceivable that the higher fracture toughness of the quartzite may also have prevented propagation of the mafic magmas through the Springbok Quartzite and, instead, led to the ponding of basic bodies below the metasediments. The geometry and structural and lithological controls of basic bodies at Narrap Mine are similar to Koperberg Suite intrusions documented from many of the other mine workings in the Okiep Copper District. This suggests similar underlying emplacement controls of the cupriferous rocks, which can be extrapolated on a regional scale and that may guide exploration.
{"title":"Ascent and emplacement controls of mafic magmas in the mid crust − evidence from 3D modelling of basic bodies of the Koperberg Suite, Namaqualand","authors":"A. Bester, A. Kisters","doi":"10.25131/sajg.126.0024","DOIUrl":"https://doi.org/10.25131/sajg.126.0024","url":null,"abstract":"\u0000 The 3D modelling of basic bodies of the Koperberg Suite (1060 to 1030 Ma) and their wall rocks from Narrap Mine illustrates the distribution, geometries and, by implication, processes that determined the ascent and emplacement of the mantle-derived mafic magmas into the partially-molten, mid-crustal granite-gneiss sequence of the Okiep Copper District in Namaqualand. The lens-like, discontinuous geometry of basic bodies suggests the transfer of the mafic magmas as self-contained, buoyancy-driven hydrofractures. The presence of both shallowly-dipping, foliation-parallel sills and subvertical lenses in zones of steep foliation development, so-called steep structures, indicates an emplacement of the mafic magmas under low deviatoric stresses and irrespective of the regional stress field. Instead, the emplacement of the mafic magmas parallel to pre-existing anisotropies (tectonic fabrics or lithological contacts) highlights those differences in tensile strength and fracture toughness parallel to or across anisotropies determined the propagation of the magmas. This also accounts for the common occurrence of basic bodies in steep structures in which the vertical gneissosity promoted the buoyancy-driven ascent of the mafic magmas.\u0000 On a regional scale, the mechanical stratification of the subhorizontal, sheet-like granite gneisses and interlayered metasediments exerted important controls on the ascent of Koperberg Suite magmas. The preferential emplacement of basic bodies in schist and gneiss units suggests that the lower rigidity of the ductile wall rocks facilitated magma emplacement through a combination of viscous wall-rock deformation and fracture blunting that led to the arrest of the magma-filled hydrofractures. Multiple intrusive relationships of successively emplaced magma batches suggest that later magmas reutilised earlier established magma pathways, particularly in steep structures. High-rigidity lithologies, such as the massive Springbok Quartzite, in contrast, only allowed for smaller fracture apertures and limited dilation, resulting in the pinching of basic bodies and rather stringer-like geometries. It is conceivable that the higher fracture toughness of the quartzite may also have prevented propagation of the mafic magmas through the Springbok Quartzite and, instead, led to the ponding of basic bodies below the metasediments.\u0000 The geometry and structural and lithological controls of basic bodies at Narrap Mine are similar to Koperberg Suite intrusions documented from many of the other mine workings in the Okiep Copper District. This suggests similar underlying emplacement controls of the cupriferous rocks, which can be extrapolated on a regional scale and that may guide exploration.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":"115 ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139017734","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
D. van REENEN, C. Smit, J.M. Huizenga, T. Tsunogae, O. Safonov
Combined geophysical, structural geological, metamorphic, geochronological, and stable isotope information is employed to elucidate the Neoarchaean thermo-tectonic evolution of the Southern Marginal Zone (SMZ) within the Limpopo Complex (South Africa) during the Limpopo orogeny (2.72 to 2.62 Ga). The complex evolutionary history of the SMZ was controlled by an allochthonous SMZ granulite nappe that was extruded from a rising granulite diapir through a process of mid-crustal heterogeneous channel flow. This granulite nappe with its embedded structures (steeply plunging reclined folds and steep shear zones) was formed during emplacement of the diapir to mid-crustal level (6 kbar, 20 km depth) from where it was thrust south-westwards along the Hout River shear zone (HRSZ) sole thrust against the Kaapvaal Craton (KVC) at 2.72 to 2.69 Ga. Evidence for the thermo-tectonic interaction of the granulite nappe with the KVC includes (1) thrust complexes (referred to as hot-iron zones) that are developed at the frontal ramp sections of the HRSZ juxtaposed against the granite-greenstone belts of the KVC, and (2) strike-slip shear deformation associated with the lateral ramp section of the HRSZ, which developed against the KVC devoid of greenstone belts. The emplacement of the post-tectonic Matok granitic pluton at ~2.68 Ga into the SMZ signified the end of the thermo-tectonic event that established the regional fold- and shear deformational framework of the granulite facies SMZ. Post-Matok secondary shear zones reflect evidence for HRSZ-linked tectonism that continued intermittently to 2.65 to 2.62 Ga. Low H2O-activity fluids (H2O activity of 0.1 to 0.3) released from devolatilisation of underthrust greenstone material passively infiltrated and interacted with the overlying cooling granulites. This established a retrograde anthophyllite-in isograd at ~6 kbar and ~620°C that subdivides the SMZ into a northern granulite domain and a southern retrograde hydrated granulite domain. Simultaneously, gold-bearing fluids focused into these minor shear zones established shear zone-hosted orogenic gold mineralisation at 2.65 to 2.62 Ga. Emplacement of the post-tectonic Palmietfontein granite at ~2.46 Ga and associated sub-volcanic granitic dykes into both the retrograde hydrated granulite domain and the granulite domain signifies the end of all thermo-tectonic activity in the SMZ. A Palaeoproterozoic thermal overprint at ~2.1 Ga is recorded by Rb-Sr biotite and phlogopite ages derived from various rocks from the SMZ and adjacent KVC. This thermal event is not associated with deformation and did not result in the formation of new mineral assemblages. Integrated data presented and discussed in this paper contradict the interpretation of age and petrological data utilised to support alternative models for the evolution of the SMZ, including a proposed ~2.1 Ga Palaeoproterozoic polymetamorphic amphibolite-grade thermo-tectonic event.
{"title":"Thermo-tectonic evolution of the Neoarchaean Southern Marginal Zone of the Limpopo granulite Complex (South Africa)","authors":"D. van REENEN, C. Smit, J.M. Huizenga, T. Tsunogae, O. Safonov","doi":"10.25131/sajg.126.0027","DOIUrl":"https://doi.org/10.25131/sajg.126.0027","url":null,"abstract":"\u0000 Combined geophysical, structural geological, metamorphic, geochronological, and stable isotope information is employed to elucidate the Neoarchaean thermo-tectonic evolution of the Southern Marginal Zone (SMZ) within the Limpopo Complex (South Africa) during the Limpopo orogeny (2.72 to 2.62 Ga). The complex evolutionary history of the SMZ was controlled by an allochthonous SMZ granulite nappe that was extruded from a rising granulite diapir through a process of mid-crustal heterogeneous channel flow. This granulite nappe with its embedded structures (steeply plunging reclined folds and steep shear zones) was formed during emplacement of the diapir to mid-crustal level (6 kbar, 20 km depth) from where it was thrust south-westwards along the Hout River shear zone (HRSZ) sole thrust against the Kaapvaal Craton (KVC) at 2.72 to 2.69 Ga. Evidence for the thermo-tectonic interaction of the granulite nappe with the KVC includes (1) thrust complexes (referred to as hot-iron zones) that are developed at the frontal ramp sections of the HRSZ juxtaposed against the granite-greenstone belts of the KVC, and (2) strike-slip shear deformation associated with the lateral ramp section of the HRSZ, which developed against the KVC devoid of greenstone belts. The emplacement of the post-tectonic Matok granitic pluton at ~2.68 Ga into the SMZ signified the end of the thermo-tectonic event that established the regional fold- and shear deformational framework of the granulite facies SMZ. Post-Matok secondary shear zones reflect evidence for HRSZ-linked tectonism that continued intermittently to 2.65 to 2.62 Ga. Low H2O-activity fluids (H2O activity of 0.1 to 0.3) released from devolatilisation of underthrust greenstone material passively infiltrated and interacted with the overlying cooling granulites. This established a retrograde anthophyllite-in isograd at ~6 kbar and ~620°C that subdivides the SMZ into a northern granulite domain and a southern retrograde hydrated granulite domain. Simultaneously, gold-bearing fluids focused into these minor shear zones established shear zone-hosted orogenic gold mineralisation at 2.65 to 2.62 Ga. Emplacement of the post-tectonic Palmietfontein granite at ~2.46 Ga and associated sub-volcanic granitic dykes into both the retrograde hydrated granulite domain and the granulite domain signifies the end of all thermo-tectonic activity in the SMZ. A Palaeoproterozoic thermal overprint at ~2.1 Ga is recorded by Rb-Sr biotite and phlogopite ages derived from various rocks from the SMZ and adjacent KVC. This thermal event is not associated with deformation and did not result in the formation of new mineral assemblages. Integrated data presented and discussed in this paper contradict the interpretation of age and petrological data utilised to support alternative models for the evolution of the SMZ, including a proposed ~2.1 Ga Palaeoproterozoic polymetamorphic amphibolite-grade thermo-tectonic event.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":"996 1","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139019420","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The petrogenesis of the mafic-ultramafic layered rocks of the Bushveld Complex, i.e., the Rustenburg Layered Suite (RLS) remains uncertain despite more than a century of intensive research. An investigation of a 100 km strike section in the Eastern Limb has led us to propose that igneous layering in the lowermost part of the RLS developed from a succession of intrusive events. Zonal boundaries and geochemical discontinuities are explained by radical switches in the composition of parental magmas. Magmas were sourced from deep staging chambers, and we find no evidence for the existence of a large magma reservoir in the shallow crust. At the base of the RLS is the Marginal Sill Phase (MSP), a stacked sequence of noritic-gabbronoritic and pyroxenitic sills that form a continuum with the syn-Bushveld sills in the floor rocks. The MSP does not constitute the chilled carapace of a magma chamber and the postulated so-called “B1” (tholeiitic) parental magmas have little direct bearing on the overlying components of the RLS. The Lower Zone (LZ) reflects a switch to ultramafic parental magmas. Magmas were fed into isolated subchambers, each of which has a unique igneous stratigraphy. The primary component of the LZ is an adcumulate comprised of >98.5 modal % orthopyroxene (En87-83) and from which Cr-spinel is conspicuously absent. Chemical equilibrium was maintained as the orthopyroxene-saturated magmas were intruded, not as sills, but as thin magma laminae. The increments of magma laminae matched the rate of batch crystallisation, and there is no evidence of a fractionated residue having been ejected. In a later phase of magmatism, peridotite magmas were injected into the floor rocks, where they formed complexly layered peridotite intrusions with their anastomosing groups of sills. Formation of peridotite bodies such as the Aapiesdoorndraai Intrusion with distinct layers of dunite, harzburgite and olivine orthopyroxenite (Fo92-83) (+Cr-spinel) result from near-equilibrium crystallisation of the olivine- and Cr-spinel-saturated magmas. Some peridotite sills stepped up through the thermal aureole and intruded the earlier-formed MSP and LZ, crystallising as syn-magmatic sills with broadly similar compositions. Development of the trough-like subchambers with thick sequences of the MSP and LZ triggered extensive deformation and thermal metamorphism of the floor rocks. Sinking of subchambers was compensated by intervening domes and upwarps, the crests of which reveal tectonically thinned sequences of the MSP and from which the LZ is absent. This early phase of syn-Bushveld tectonism has resulted in the Lower Critical Zone (LCZ) discordantly overlying either the MSP (on the crests of domes and upwarps) or the LZ (in subchambers). Field relationships thus demonstrate that the LCZ developed entirely independently of the MSP and LZ, from a new series of intrusive events and a new lineage of parental magma.
{"title":"Petrogenesis of the Marginal Sill Phase and Lower Zone in the Eastern Limb of the Bushveld Complex: Incremental development of igneous layering and syn-magmatic emplacement of peridotite intrusions and sills","authors":"R. N. Scoon, A.A. Mitchell","doi":"10.25131/sajg.126.0021","DOIUrl":"https://doi.org/10.25131/sajg.126.0021","url":null,"abstract":"\u0000 The petrogenesis of the mafic-ultramafic layered rocks of the Bushveld Complex, i.e., the Rustenburg Layered Suite (RLS) remains uncertain despite more than a century of intensive research. An investigation of a 100 km strike section in the Eastern Limb has led us to propose that igneous layering in the lowermost part of the RLS developed from a succession of intrusive events. Zonal boundaries and geochemical discontinuities are explained by radical switches in the composition of parental magmas. Magmas were sourced from deep staging chambers, and we find no evidence for the existence of a large magma reservoir in the shallow crust. At the base of the RLS is the Marginal Sill Phase (MSP), a stacked sequence of noritic-gabbronoritic and pyroxenitic sills that form a continuum with the syn-Bushveld sills in the floor rocks. The MSP does not constitute the chilled carapace of a magma chamber and the postulated so-called “B1” (tholeiitic) parental magmas have little direct bearing on the overlying components of the RLS. The Lower Zone (LZ) reflects a switch to ultramafic parental magmas. Magmas were fed into isolated subchambers, each of which has a unique igneous stratigraphy. The primary component of the LZ is an adcumulate comprised of >98.5 modal % orthopyroxene (En87-83) and from which Cr-spinel is conspicuously absent. Chemical equilibrium was maintained as the orthopyroxene-saturated magmas were intruded, not as sills, but as thin magma laminae. The increments of magma laminae matched the rate of batch crystallisation, and there is no evidence of a fractionated residue having been ejected. In a later phase of magmatism, peridotite magmas were injected into the floor rocks, where they formed complexly layered peridotite intrusions with their anastomosing groups of sills. Formation of peridotite bodies such as the Aapiesdoorndraai Intrusion with distinct layers of dunite, harzburgite and olivine orthopyroxenite (Fo92-83) (+Cr-spinel) result from near-equilibrium crystallisation of the olivine- and Cr-spinel-saturated magmas. Some peridotite sills stepped up through the thermal aureole and intruded the earlier-formed MSP and LZ, crystallising as syn-magmatic sills with broadly similar compositions. Development of the trough-like subchambers with thick sequences of the MSP and LZ triggered extensive deformation and thermal metamorphism of the floor rocks. Sinking of subchambers was compensated by intervening domes and upwarps, the crests of which reveal tectonically thinned sequences of the MSP and from which the LZ is absent. This early phase of syn-Bushveld tectonism has resulted in the Lower Critical Zone (LCZ) discordantly overlying either the MSP (on the crests of domes and upwarps) or the LZ (in subchambers). Field relationships thus demonstrate that the LCZ developed entirely independently of the MSP and LZ, from a new series of intrusive events and a new lineage of parental magma.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":"11 5","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139015067","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The O- and H-isotope composition of rainfall collected monthly at the University of Cape Town (UCT) between 2009 and 2022 has been added to a previously published data set from 1996 to 2008 to make a continuous 27 year record. Monthly rainfall over the 27 year period has a range in δD and δ18O values from -57 to +21‰ and -8.1 to +3.5‰, respectively, and shows limited but discernible temperature and amount effects. The 27 year rainfall record defines a Local Meteoric Water Line (LMWL) whose equation is δD = 5.96*δ18O + 7.00 (r = 0.88), a slight change from the LMWL of 1996 to 2009 (6.41*δ18O + 8.89). Annual rainfall at UCT has varied from ~992 mm (2017) to 1996 mm (2013), with no systematic change in annual rainfall amount (r = −0.16). However, from 2015 to 2022, the average annual rainfall of 1 145 mm has been below the 27 year average of 1 313 mm. Mean monthly temperature has increased from 1996 to 2022 (r = 0.53), and the weighted mean annual δD and δ18O values have increased by ~4‰ (r = 0.53) and ~0.5‰ (r = 0.64), respectively, over the 27 years. The UCT data and the data for 1961 to 1974 from Cape Town International Airport plot around the same LMWL, with an average deuterium excess (d-excess) of 13.78 and 12.95, respectively. Natural springs in the area plot close to the LMWL with an average d-excess of 14.15, whereas local well-point and borehole water samples generally plot below the LMWL with an average d-excess of 10.65. These differences can be explained by relatively rapid recharge of springs and slower recharge of groundwater, with the latter containing an additional component, that could either be rainwater that fell during a period of hotter drier climate or, more probably, municipal mains water. Long-term monitoring of groundwater and spring water as well as rainwater would be of great help in assessing the sustainability of groundwater use, among other important questions.
{"title":"O- and H-isotope record of Cape Town rainfall from 1996 to 2022: the effect of increasing temperature, and the ‘water crisis’ of 2015 to 2018","authors":"C. Harris","doi":"10.25131/sajg.126.0026","DOIUrl":"https://doi.org/10.25131/sajg.126.0026","url":null,"abstract":"\u0000 The O- and H-isotope composition of rainfall collected monthly at the University of Cape Town (UCT) between 2009 and 2022 has been added to a previously published data set from 1996 to 2008 to make a continuous 27 year record. Monthly rainfall over the 27 year period has a range in δD and δ18O values from -57 to +21‰ and -8.1 to +3.5‰, respectively, and shows limited but discernible temperature and amount effects. The 27 year rainfall record defines a Local Meteoric Water Line (LMWL) whose equation is δD = 5.96*δ18O + 7.00 (r = 0.88), a slight change from the LMWL of 1996 to 2009 (6.41*δ18O + 8.89). Annual rainfall at UCT has varied from ~992 mm (2017) to 1996 mm (2013), with no systematic change in annual rainfall amount (r = −0.16). However, from 2015 to 2022, the average annual rainfall of 1 145 mm has been below the 27 year average of 1 313 mm. Mean monthly temperature has increased from 1996 to 2022 (r = 0.53), and the weighted mean annual δD and δ18O values have increased by ~4‰ (r = 0.53) and ~0.5‰ (r = 0.64), respectively, over the 27 years. The UCT data and the data for 1961 to 1974 from Cape Town International Airport plot around the same LMWL, with an average deuterium excess (d-excess) of 13.78 and 12.95, respectively. Natural springs in the area plot close to the LMWL with an average d-excess of 14.15, whereas local well-point and borehole water samples generally plot below the LMWL with an average d-excess of 10.65. These differences can be explained by relatively rapid recharge of springs and slower recharge of groundwater, with the latter containing an additional component, that could either be rainwater that fell during a period of hotter drier climate or, more probably, municipal mains water. Long-term monitoring of groundwater and spring water as well as rainwater would be of great help in assessing the sustainability of groundwater use, among other important questions.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":"91 3","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139017667","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
S. Wilson, R. Bailie, C. Harris, S. Dunn, V. Silinda, S.N. Lembede
Abstract The mafic to felsic rocks at Yzerfontein form part of a composite 535 Ma post-orogenic pluton belonging to the Cape Granite Suite (CGS) of the Pan-African Saldania Belt of southwestern Africa. The pluton ranges from olivine gabbros (mostly at the northern end) to quartz monzonites, with monzonites the most abundant rock type. Minor felsic rocks in the form of quartz microsyenites and monzogranite occur more toward the southern end. The comparatively unaltered gabbro/monzonite has pyroxene δ18O values of 6.4‰, consistent with a mildly crustal-contaminated mantle-derived magma. Published εNd values of -1.9 to -0.3 similarly reflect crustal contamination of these mafic to intermediate rocks. The pluton is crosscut by extensive hydrothermal veining. Early quartz veining was followed by the precipitation of pyrite, calcite, tourmaline, jasper, epidote and a final stage involving the formation of barren quartz veins. The veins contain low-salinity (3 to 11 wt.% NaCl eq.), three phase (liquid + vapour ± opaques) fluid inclusions with total homogenisation temperatures of 125 to 320°C. The δ18O values of the quartz veins range from 9.8 to 14.7‰ and the fluid inclusion δD values range from -9 to -17‰. These ranges, together with the low salinities, are consistent with a meteoric origin for the vein fluids. Bulk-rock δ18O values range from 7.3 to 13.3‰ consistent with interaction of near surface fluids at temperatures of <300°C with the magmatic rocks. Open-space filling textures in quartz veins suggest vein formation in a hydrostatic regime. These, together with fluid inclusion isochores, suggest temperature-pressure conditions of no more than 170 to 300°C at ~0.5 kbar and no more than 1.6 kbar. The mineralised veins represent a shallow, poorly developed and possibly low-grade epithermal system. The mineralisation is likely related to drawdown of meteoric fluids during sub-solidus cooling of the dominantly intermediate pluton with little to no magmatic fluid influence. The mineralisation is poorly developed, possibly due to a lack of mixing between different fluid types. The various magmatic rocks, varying from mafic through dominantly intermediate to minor felsic, represents a minor mantle-derived mafic component of the Cape Granite Suite and likely related to the heat source that gave rise to voluminous crustal melting that generated the granitic magmas of the majority of the CGS.
{"title":"The origin, nature and fluid characteristics of the hydrothermal veining crosscutting the Yzerfontein gabbro-diorite, Cape Granite Suite, South Africa","authors":"S. Wilson, R. Bailie, C. Harris, S. Dunn, V. Silinda, S.N. Lembede","doi":"10.25131/sajg.126.0018","DOIUrl":"https://doi.org/10.25131/sajg.126.0018","url":null,"abstract":"Abstract The mafic to felsic rocks at Yzerfontein form part of a composite 535 Ma post-orogenic pluton belonging to the Cape Granite Suite (CGS) of the Pan-African Saldania Belt of southwestern Africa. The pluton ranges from olivine gabbros (mostly at the northern end) to quartz monzonites, with monzonites the most abundant rock type. Minor felsic rocks in the form of quartz microsyenites and monzogranite occur more toward the southern end. The comparatively unaltered gabbro/monzonite has pyroxene δ18O values of 6.4‰, consistent with a mildly crustal-contaminated mantle-derived magma. Published εNd values of -1.9 to -0.3 similarly reflect crustal contamination of these mafic to intermediate rocks. The pluton is crosscut by extensive hydrothermal veining. Early quartz veining was followed by the precipitation of pyrite, calcite, tourmaline, jasper, epidote and a final stage involving the formation of barren quartz veins. The veins contain low-salinity (3 to 11 wt.% NaCl eq.), three phase (liquid + vapour ± opaques) fluid inclusions with total homogenisation temperatures of 125 to 320°C. The δ18O values of the quartz veins range from 9.8 to 14.7‰ and the fluid inclusion δD values range from -9 to -17‰. These ranges, together with the low salinities, are consistent with a meteoric origin for the vein fluids. Bulk-rock δ18O values range from 7.3 to 13.3‰ consistent with interaction of near surface fluids at temperatures of &lt;300°C with the magmatic rocks. Open-space filling textures in quartz veins suggest vein formation in a hydrostatic regime. These, together with fluid inclusion isochores, suggest temperature-pressure conditions of no more than 170 to 300°C at ~0.5 kbar and no more than 1.6 kbar. The mineralised veins represent a shallow, poorly developed and possibly low-grade epithermal system. The mineralisation is likely related to drawdown of meteoric fluids during sub-solidus cooling of the dominantly intermediate pluton with little to no magmatic fluid influence. The mineralisation is poorly developed, possibly due to a lack of mixing between different fluid types. The various magmatic rocks, varying from mafic through dominantly intermediate to minor felsic, represents a minor mantle-derived mafic component of the Cape Granite Suite and likely related to the heat source that gave rise to voluminous crustal melting that generated the granitic magmas of the majority of the CGS.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":"31 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135687656","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A. Zametzer, S. Reimann, C. Heubeck, T. Thomsen, S. Serre
Greenstone belt dynamics are a key to understanding the formation and evolution of continental crust but the scarcity, commonly poor exposure, strong deformation, and high metamorphic grades of many Archaean supracrustal rocks preserved in greenstone belts often hinder geological insights. Exceptionally, the largely siliciclastic Palaeoarchaean (ca. 3.22 Ga) Moodies Group, uppermost unit of the Barberton Greenstone Belt (BGB) in South Africa and Eswatini, allows a detailed understanding of regional depositional processes and stratigraphic architecture due to its low degree of deformation, considerable thickness, generally good exposure, and diverse sedimentary facies. We here investigate the Powerline Road Syncline (PRS) and the adjacent Maid-of-the-Mists Syncline (MMS), two east-west-trending, tightly to isoclinally folded, locally northwardly overturned synclines in Moodies Group strata in the central BGB. The northern and southeastern margins of the PRS and the southern to southwestern margin of MMS are bounded by the bedding-parallel 24-Hour-Camp Fault. Along this fault, Moodies Group strata and stratigraphically underlying units of the Auber Villiers Formation of the upper Fig Tree Group structurally overlie lower Fig Tree strata above Onverwacht Group strata. Stratigraphic architecture and palaeocurrent analysis of Moodies Group strata in both synclines document a northeastward (depositionally downdip) facies transition from proximal fan delta conglomerates to fluvial- to coastal-plain and estuarine sandstones in which thick foresets likely represent subtidal channel fills. The overall deepening- and fining-upward trend in this sequence, corresponding to an increasing mineralogical maturity of sandstones, is partially obscured by local hydrothermal alteration which was contemporaneous with sedimentation, minor volcanism, and sill intrusion. Moodies Group conglomerates and sandstones were largely shed from the rising Onverwacht Anticline to the southwest. Lithologies and stratigraphy in the PRS-MMS region closely resemble strata of the lower Moodies Group in the Sadddleback Syncline, located nearby to the north but across the Inyoka Fault. If the 24-Hour-Camp Fault was part of a group of extensional faults accompanying the rise of the Onverwacht Anticline and of tonalitic-trondhjemitic-granodioritic plutons around the BGB margins at ca. 3 224 Ma, it would have acted as one of numerous basin-bounding normal faults during early Moodies time. This would place the deposition of Moodies Group strata in the PRS-MMS region in a supradetachment fault environment which was subsequently tightly folded.
{"title":"Tectonic and sedimentary evolution of a potential supradetachment fault basin, Archaean Moodies Group (~3.22 Ga), central Barberton Greenstone Belt","authors":"A. Zametzer, S. Reimann, C. Heubeck, T. Thomsen, S. Serre","doi":"10.25131/sajg.126.0016","DOIUrl":"https://doi.org/10.25131/sajg.126.0016","url":null,"abstract":"\u0000 Greenstone belt dynamics are a key to understanding the formation and evolution of continental crust but the scarcity, commonly poor exposure, strong deformation, and high metamorphic grades of many Archaean supracrustal rocks preserved in greenstone belts often hinder geological insights. Exceptionally, the largely siliciclastic Palaeoarchaean (ca. 3.22 Ga) Moodies Group, uppermost unit of the Barberton Greenstone Belt (BGB) in South Africa and Eswatini, allows a detailed understanding of regional depositional processes and stratigraphic architecture due to its low degree of deformation, considerable thickness, generally good exposure, and diverse sedimentary facies. We here investigate the Powerline Road Syncline (PRS) and the adjacent Maid-of-the-Mists Syncline (MMS), two east-west-trending, tightly to isoclinally folded, locally northwardly overturned synclines in Moodies Group strata in the central BGB. The northern and southeastern margins of the PRS and the southern to southwestern margin of MMS are bounded by the bedding-parallel 24-Hour-Camp Fault. Along this fault, Moodies Group strata and stratigraphically underlying units of the Auber Villiers Formation of the upper Fig Tree Group structurally overlie lower Fig Tree strata above Onverwacht Group strata. Stratigraphic architecture and palaeocurrent analysis of Moodies Group strata in both synclines document a northeastward (depositionally downdip) facies transition from proximal fan delta conglomerates to fluvial- to coastal-plain and estuarine sandstones in which thick foresets likely represent subtidal channel fills. The overall deepening- and fining-upward trend in this sequence, corresponding to an increasing mineralogical maturity of sandstones, is partially obscured by local hydrothermal alteration which was contemporaneous with sedimentation, minor volcanism, and sill intrusion. Moodies Group conglomerates and sandstones were largely shed from the rising Onverwacht Anticline to the southwest. Lithologies and stratigraphy in the PRS-MMS region closely resemble strata of the lower Moodies Group in the Sadddleback Syncline, located nearby to the north but across the Inyoka Fault. If the 24-Hour-Camp Fault was part of a group of extensional faults accompanying the rise of the Onverwacht Anticline and of tonalitic-trondhjemitic-granodioritic plutons around the BGB margins at ca. 3 224 Ma, it would have acted as one of numerous basin-bounding normal faults during early Moodies time. This would place the deposition of Moodies Group strata in the PRS-MMS region in a supradetachment fault environment which was subsequently tightly folded.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.8,"publicationDate":"2023-08-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47865291","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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