{"title":"中地壳钙质岩浆的上升和形成控制--纳马夸兰德科珀贝格岩组基本岩体三维建模的证据","authors":"A. Bester, A. Kisters","doi":"10.25131/sajg.126.0024","DOIUrl":null,"url":null,"abstract":"\n 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.\n 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.\n 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":1.2000,"publicationDate":"2023-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"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\":null,\"url\":null,\"abstract\":\"\\n 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.\\n 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.\\n 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\":1.2000,\"publicationDate\":\"2023-12-01\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"South African Journal of Geology\",\"FirstCategoryId\":\"93\",\"ListUrlMain\":\"https://doi.org/10.25131/sajg.126.0024\",\"RegionNum\":4,\"RegionCategory\":\"地球科学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q2\",\"JCRName\":\"GEOLOGY\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"South African Journal of Geology","FirstCategoryId":"93","ListUrlMain":"https://doi.org/10.25131/sajg.126.0024","RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"GEOLOGY","Score":null,"Total":0}
Ascent and emplacement controls of mafic magmas in the mid crust − evidence from 3D modelling of basic bodies of the Koperberg Suite, Namaqualand
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.
期刊介绍:
The South African Journal of Geology publishes scientific papers, notes, stratigraphic descriptions and discussions in the broadly defined fields of geoscience that are related directly or indirectly to the geology of Africa. Contributions relevant to former supercontinental entities such as Gondwana and Rodinia are also welcome as are topical studies on any geoscience-related discipline. Review papers are welcome as long as they represent original, new syntheses. Special issues are also encouraged but terms for these must be negotiated with the Editors.