A new lithostratigraphic subdivision and geodynamic model for the Pan-African western Saldania Belt, South Africa
A new lithostratigraphic subdivision and geodynamic model for the Pan-African western Saldania Belt, South Africa
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DOI：
10.1016/j.precamres.2013.03.014
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发表时间：
2013-07
影响因子：
3.8
通讯作者：
H. Frimmel;M. Basei;Vinicius X. Correa;Ndawedapo Mbangula
中科院分区：
文献类型：
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作者：
H. Frimmel;M. Basei;Vinicius X. Correa;Ndawedapo Mbangula
通过碎屑锆石U-Pb和Lu-Hf同位素分析以及全岩地球化学、Rb-Sr、本文通过对泛太平洋主要构造地层单位中硅质变质沉积岩的Sm-Nd和Pb同位素分析，南非萨尔达尼亚带的非洲西部分支揭示的年代关系和源域与当前的地层方案和地球动力学模型的演变不兼容这条腰带。以前的三个分区不能维持。前泰格堡和斯瓦特兰地体的沉积物似乎具有相同的物源，但与前博兰地体的沉积物不同。后者被认为是晚中元古代纳马夸-纳塔尔带基底岩石顶部的准原地带。根据我们的新数据，建议将以前的泰格伯格和斯瓦特兰地体统一为一个地体，并建议将其命名为马尔梅斯伯里地体。科伦索断层，以前被认为是一个断层边界，被重新解释为从马尔梅斯伯里地体内的基底继承的重新激活的旧结构，而主要的断层边界是Piketberg-Wellington断层，它最有可能与更北的加里普带的Schakalsberge逆冲断层相连。马尔梅斯伯里地体的沉积年龄在557 ~ 552 Ma之间，博兰带的沉积年龄在609 ~ 532 Ma之间。没有证据表明，更古老的，Cryogenian沉积岩可以找到，除了火山为主的布里奇敦形成，这可能是一个构造银。寒武纪Klipheuwel群无疑比其他Saltenic沉积物更年轻，并且与其来源无关。所有地球化学证据都表明，活动大陆边缘是马尔梅斯伯里地体和博兰带沉积物的来源。前者碎屑岩的时代主要为新元古代，特别是低温纪，而博兰带碎屑岩的时代主要为早托尼世和晚中元古代。最可能的低温期碎屑源是巴西东南部和乌拉圭的Dionisio Cuchilla-Pelvilla-magmatic arc，它形成于卡拉哈里盆地的一个分裂部分，即Arachania。弧后位置相对于弧推断沉积盆地中重新定义的马尔梅斯伯里群，组成新元古代继承的马尔梅斯伯里地体，沉积。因此，我们的新数据支持这样的假设，即西Saldania带及其北方西Gariep带的新元古代继承并不反映完整的威尔逊旋回，而只是埃迪卡拉弧后盆地的衰退阶段。因此，喀拉哈里和拉普拉塔河之间的主缝合线一定位于Dionisio Cuchilla-Pelerican岩浆弧以西，可能沿着南美洲东南部的Major Gercino-Sierra Balena线性构造。这意味着现代南大西洋的开放在很大程度上遵循了假设的前埃迪卡拉弧后盆地的轴线。
A provenance study involving U–Pb and Lu–Hf isotope analyses of detrital zircon grains as well as whole rock geochemical, Rb–Sr, Sm–Nd and Pb isotope analyses of siliciclastic metasedimentary rocks from the principal tectonostratigraphic units of the Pan-African western branch of the Saldania Belt in South Africa revealed age relationships and source domains that are not compatible with current stratigraphic schemes and geodynamic models for the evolution of this belt. The previous subdivision into three terranes cannot be upheld. The sediments in both the former Tygerberg and Swartland terranes seemingly have the same provenance but differ from those in the former Boland Terrane. The latter is considered a para-autochthonous zone on top of basement rocks of the late Mesoproterozoic Namaqua-Natal Belt. On the basis of our new data it is recommended to unify the former Tygerberg and Swartland terranes into a single terrane for which the name Malmesbury Terrane is proposed. The Colenso Fault, previously regarded as a terrane boundary, is reinterpreted as reactivated old structure inherited from the basement within the Malmesbury Terrane, whereas the major terrane-bounding fault is the Piketberg–Wellington Fault, which can be linked most likely with the Schakalsberge Thrust further north in the Gariep Belt. The age of sedimentation is constrained within a narrow time window between 557 and 552Ma in the Malmesbury Terrane and between 609 and 532Ma in the Boland Zone. No evidence of older, Cryogenian sedimentary rocks could be found, except for the volcanic-dominated Bridgetown Formation, which is presumably a tectonic sliver. The Cambrian Klipheuwel Group is undoubtedly younger than, and unrelated with regard to its provenance to, the other Saldanian sediments. All geochemical evidence points to an active continental margin as source for the sediments in the Malmesbury Terrane and Boland Zone. While the ages of the detritus in the former are strongly dominated by Neoproterozoic, especially Cryogenian, ages, those in the Boland Zone are mainly early Tonian and late Mesoproterozoic. The most likely source of the Cryogenian detritus is the Dionisio Cuchilla-Pelotas magmatic arc in southeastern Brazil and Uruguay, which formed within a splinter of Kalahari Craton, i.e. Arachania. A back-arc position relative to that arc is inferred for the depo-basin in which the redefined Malmesbury Group, making up the Neoproterozoic succession of the Malmesbury Terrane, was deposited. Consequently, our new data support the hypothesis that the Neoproterozoic successions of the western Saldania Belt and their northern equivalents in the western Gariep Belt do not reflect a full Wilson Cycle but only the waning stages of an Ediacaran back-arc basin. The main suture between the Kalahari and Rio de la Plata cratons must be located, therefore, to the west of the Dionisio Cuchilla-Pelotas magmatic arc, probably along the Major Gercino-Sierra Balena Lineament within southeastern South America. This implies that the opening of the modern South Atlantic followed to a large extent the axis of the postulated former Ediacaran back-arc basin.