Origin of the Rio Grande Rise–Walvis Ridge reviewed integrating palaeogeographic reconstruction, isotope geochemistry and flexural modelling
Origin of the Rio Grande Rise–Walvis Ridge reviewed integrating palaeogeographic reconstruction, isotope geochemistry and flexural modelling
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结合古地理重建、同位素地球化学和弯曲建模回顾里奥格兰德海隆-沃尔维斯海岭的起源
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发表时间：
2012
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通讯作者：
M. Ernesto
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作者：
N. Ussami;C. Chaves;L. Marques;M. Ernesto
南美和非洲大陆的古地理恢复到异常C34(84 Ma)，将里奥格兰德隆起(RGR)和沃尔维斯岭(WR)的中心部分结合在一起，因此RGR-WR无震脊可能具有共同的起源。如果RGR-WR玄武岩高原的建造主要发生在89~78 Ma之间，那么玄武岩岩浆就是山脊火山作用的结果。一旦分离，RGR和WR海脊的正常海底扩张和热沉降一直持续到大约47 Ma，那时RGR发生了始新世岩浆作用。在西海，在盖奥特省观察到了一场更年轻的火山活动。现有的WR-RGR玄武岩的地球化学和同位素数据表明，没有大陆地壳熔融成分的参与。RGR-WR山脊玄武岩的不相容微量元素比值和同位素特征与现今特里斯坦-达库尼亚碱性岩不同，与高钛帕拉那岩浆省(PMP)拉斑玄武岩(133-132 Ma)几乎相同。高钛PMP玄武岩和WR-RGR玄武岩具有中等的初始~(87)Sr/~(86)Sr和低的~(206)Pb/~(204)Pb同位素比值[富集地幔I(EMI)组分]，表明熔融来自共同的来源，大陆岩石圈地幔(SCLM)的显著参与。使用ETOPO1数字地形/测深和EGM2008衍生的自由空气异常作为约束，对RGR和WR进行了三维(3D)弯曲模拟。对于弹性板厚度(Te)小于5公里的弹性板，观测到的自由空气异常与计算的自由空气异常之间的最佳拟合度与RGR-WR的“山脊上”初始建造一致。对地壳-地幔界面深度的模拟表明，RGR-WR的地壳总厚度可达30公里。弯曲分析加强了RGR形成于两个主要岩浆阶段的地质证据，即桑托纪-锥期拉斑玄武岩和始新世碱性岩浆作用。文中提出的地球化学和地球物理证据排除了经典的深地幔热柱模型解释这些火山岩省玄武岩成因的可能性。最后，评述了解释RGR-WR玄武岩地球化学和同位素特征的三种模式：(1)边缘驱动对流导致的SCLM的热侵蚀；(2)碎裂或分离的SCLM与下地壳的熔融；(3)SCLM底部的热侵蚀和地幔流动对富集组分的横向输送。
Abstract A palaeogeographical reconstruction of the South American and African continents back to anomaly C34 (84 Ma) brings together the Rio Grande Rise (RGR) and the central portion of the Walvis Ridge (WR), thus the RGR–WR aseismic ridges may have a common origin. If the construction of the RGR–WR basaltic plateau took place mainly between 89 and 78 Ma, as indicated by the ages of the basalts sampled by DSDP wells, then the basaltic magmas are the result of an ‘on-ridge’ volcanism. Once separated, the normal sea-floor spreading and thermal subsidence of the RGR and WR ridges continued until approximately 47 Ma when an Eocene magmatism took place in the RGR. In the WR, a younger volcanism is observed in the Guyot Province. The available geochemical and isotope data of the WR–RGR basalts do not indicate the participation of the continental crust melting component. Incompatible trace element ratios and isotope signatures of the basalts from the RGR–WR ridges are distinct from the present-day Tristan da Cunha alkaline rocks, and are nearly identical to the high-Ti Paraná Magmatic Province (PMP) tholeiites (133–132 Ma). Both the high-Ti PMP and the WR–RGR basalts are characterized by moderate initial 87Sr/86Sr and low 206Pb/204Pb isotope ratios [Enriched Mantle I (EMI) mantle component], suggesting melting from a common source, with significant participation of sub-continental lithospheric mantle (SCLM). A three-dimensional (3D) flexural modelling of the RGR and WR was conducted using ETOPO1 digital topography/bathymetry and EGM2008-derived free-air anomalies as a constraint. The best fit between the observed and calculated free-air anomalies was obtained for an elastic plate with elastic plate thickness (Te) of less than 5 km, consistent with an ‘on-ridge’ initial construction of the RGR–WR. The modelling of the crust–mantle interface depths indicates a total crustal thickness of up to 30 km in the RGR–WR. Flexural analysis reinforces the geological evidence that RGR was constructed during two main magmatic episodes, the tholeiitic basalts in the Santonian–Conician times and the alkaline magmatism in the Eocene. Geochemical and geophysical evidence, which rules out the classical deep-mantle plume model in explaining the generation of basalts of these volcanic provinces, is presented. Finally, three models to explain the geochemical and isotope signatures of RGR–WR basalts are reviewed: (1) thermal erosion of SCLM owing to edge-driven convection; (2) melting of fragmented or detached SCLM and lower crust; and (3) thermal erosion at the base of the SCLM with lateral transport of enriched components by mantle flow.