乍得gusamra和Ouaddaï地块地球化学勘查:撒哈拉中部盾构元古代地壳演化

IF 1.2 4区 地球科学 Q2 GEOLOGY
M. Wit, S. Bowring, R. Buchwaldt, F. Dudás, D. MacPhee, G. Tagne-Kamga, N. Dunn, A. M. Salet, D. Nambatingar
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引用次数: 5

摘要

1964年,W.Q.Kennedy提出撒哈拉非洲的地壳与非洲其他地区不同。到目前为止,该地区的地质演化仍不清楚,因为撒哈拉大片地区的结晶基底的年龄和组成尚不清楚。非洲大部分地区包括被古元古代至中元古代造山带包围的太古宙克拉通,这些造山带共同构成了非洲的三个主要地盾(南非地盾、中非地盾和西非地盾),最终沿着泛非岩石带聚集。相比之下,撒哈拉以南非洲中部(5.3x106平方公里)的面积略高于欧洲的一半,被认为是由相对年轻的地壳(0.5至1.0 Ga)构成的新元古代区域,或者是在那段时间内重新激活和重新稳定的较老(北非)地盾,这一时期通常被称为“泛非”。在这里,使用U-Pb锆石年龄测定和Nd同位素数据,我们表明,乍得的偏远地区,即未注明日期的达尔富尔高原的一部分,横跨撒哈拉中部的¾万平方公里,由构造前辉长岩-英云闪长岩-花岗闪长岩和主要在构造后侵入约550至1050 Ma的碱长花岗岩和正长岩组成的广泛新元古代结晶基底。该基底沿其西部边缘两侧为新元古代大陆钙碱性岩浆弧,与从提贝斯蒂穿过达尔富尔西部进入喀麦隆的约2400公里的隐蔽缝合带相连。我们称之为撒哈拉中部地带。在冈瓦纳大陆的构架中,这是一个更大的弧结构的一部分,我们在这里称之为大中央冈瓦纳弧(GCGA)。继承锆石和Nd同位素比值表明,撒哈拉中部的新元古代岩浆主要来源于中元古代大陆岩石圈。613至623 Ma之间的区域变形标志着晚碱性花岗岩岩浆作用的开始,该岩浆作用在北非更大的地区广泛分布,直到550 Ma左右。在该岩浆作用期间,该区域被挖掘和侵蚀,留下了一个区域准平原,随后在该准平原上沉积了早古生代(下-中寒武纪)的硅碎屑沉积物,作为覆盖北非和阿拉伯半岛大部分地区的厚而广泛的覆盖层的一部分。这些覆盖层序列中的碎屑锆石提供了大量碎屑来源于撒哈拉中部地区的证据,因为这些序列包括迄今为止缺乏来源地形的“Kibaran年龄”锆石(约1000 Ma)。我们建议,与其将中撒哈拉称为“幽灵”或“元”克拉通,不如将其称为中撒哈拉地盾。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Geochemical reconnaissance of the Guéra and Ouaddaï Massifs in Chad: evolution of Proterozoic crust in the Central Sahara Shield
In 1964, W.Q. Kennedy suggested that the crust of Saharan Africa is different from the rest of Africa. To date, the geologic evolution of this region remains obscure because the age and composition of crystalline basement are unknown across large sectors of the Sahara. Most of Africa comprises Archaean cratons surrounded by Palaeo- to Mesoproterozoic orogenic belts, which together constitute Africa’s three major shields (the Southern, Central and West African Shields), finally assembled along belts of Pan-African rocks. By contrast, central Saharan Africa (5.3x106 km2), an area just over half the size of Europe, is considered either as a Neoproterozoic region constructed of relatively juvenile crust (0.5 to 1.0 Ga), or as an older (North African) shield that was reactivated and re-stabilized during that time, a period commonly referred to as “Pan African”. Here, using U-Pb zircon age determinations and Nd isotopic data, we show that remote areas in Chad, part of the undated Darfur Plateau stretching across ¾ million km2 of the central Sahara, comprise an extensive Neoproterozoic crystalline basement of pre-tectonic gabbro-tonalite-granodiorite and predominantly post-tectonic alkali feldspar granites and syenites that intruded between ca. 550 to 1050 Ma. This basement is flanked along its western margin by a Neoproterozoic continental calc-alkaline magmatic arc coupled to a cryptic suture zone that can be traced for ~2400 km from Tibesti through western Darfur into Cameroon. We refer to this as the Central Saharan Belt. This, in a Gondwana framework, is part of a greater arc structure, which we here term the Great Central Gondwana Arc (GCGA). Inherited zircons and Nd isotopic ratios indicate the Neoproterozoic magmas in the central Sahara were predominantly derived from Mesoproterozoic continental lithosphere. Regional deformation between 613 to 623 Ma marks the onset of late alkaline granite magmatism that was widespread across a much larger area of North Africa until about 550 Ma. During this magmatism, the region was exhumed and eroded, leaving a regional peneplain on which early Palaeozoic (Lower-Middle Cambrian) siliciclastic sediments were subsequently deposited, as part of a thick and widespread cover that stretched across much of North Africa and the Arabian Peninsula. Detrital zircons in these cover sequences provide evidence that a substantial volume of detritus was derived from the central Sahara region, because these sequences include ‘Kibaran-age’ zircons (ca. 1000 Ma) for which a source terrain has hitherto been lacking. We propose that, in preference to calling the central Sahara a “ghost” or “meta” craton, it should be called the Central Sahara Shield.
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来源期刊
CiteScore
3.60
自引率
5.60%
发文量
18
审稿时长
>12 weeks
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.
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