Magmatic rifting in the Main Ethiopian Rift began in thick continental lithosphere; the case of the Galema Range
Magmatic rifting in the Main Ethiopian Rift began in thick continental lithosphere; the case of the Galema Range
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DOI：
10.1016/j.lithos.2021.106494
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发表时间：
2021-10
期刊：
影响因子：
3.5
通讯作者：
B. Chiasera;T. Rooney;I. Bastow;G. Yirgu;E. Grosfils;D. Ayalew;P. Mohr;J. Zimbelman;M. Ramsey
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文献类型：
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作者：
B. Chiasera;T. Rooney;I. Bastow;G. Yirgu;E. Grosfils;D. Ayalew;P. Mohr;J. Zimbelman;M. Ramsey
东非的北方主埃塞俄比亚裂谷（MER）被认为是一个早期海洋扩张的区域，中新世边界断层现在基本上被放弃了，而在元吉断层带（WFB）的岩浆延伸。然而，岩浆伸展是否开始时埃塞俄比亚岩石圈仍然厚，或严重拉伸，是未知的。Galema范围，一个线性上新世岩墙群平行于今天的中央MER的东部边缘，是一个理想的研究场所，以限制熔融深度，并推断岩石圈的厚度，在早期岩浆裂谷。为了解决这个问题，我们提出了77个样品的Galema范围岩浆的全岩，微量元素数据。我们解释两种建模方法之间的对比结果作为证据的岩浆积水熔体生成。熔体生成的微量元素模型揭示了在2.9-3.2 GPa下TP= 1418-1450 °C的熔化条件，比环境温度高约68-100 °C。相比之下，Si/Mg活性温压法探测了这些岩浆最后与地幔重新平衡的点，得出了大致相似的温度（1435-1474 °C），但压力较低（2.1-2.6 ± 0.2 GPa：78-89 km深度）;这些结果与Akaki岩浆带西部裂谷边缘的同期岩浆活动大致平行。我们解释这些结果作为证据的岩浆停滞在热-机械边界上升：岩石圈-软流圈边界。因此，埃塞俄比亚大陆岩石圈在裂陷过程的后期仍然相对较厚，这对海底扩张开始之前的后期减压熔融具有重要的潜在影响。
The northern Main Ethiopian Rift (MER) in East Africa is considered a region of incipient oceanic spreading, with Miocene border faulting now largely abandoned at the expense of magmatic extension in the Wonji Fault Belt (WFB). However, whether magmatic extension began when the Ethiopian lithosphere was still-thick, or heavily stretched, is unknown. The Galema range, a linear Pliocene dike swarm parallel to the eastern margin of the present-day central MER, is an ideal study locale to constrain melting depths, and by inference the thickness of the lithosphere, during early magmatic rifting. To address this issue, we present whole-rock, trace element data on 77 samples of Galema range magmas. We interpret contrasting results between two modeling approaches as evidence for magma ponding subsequent to melt generation. Trace element models of melt generation reveal melting conditions of TP= 1418–1450 °C at 2.9–3.2 GPa, some ~68–100 °C above ambient. In contrast, Si/Mg activity thermobarometry, which probes the point at which these magmas last re-equilibrated with the mantle, yielded broadly similar temperatures (1435–1474 °C) but at lower pressures (2.1–2.6 ± 0.2 GPa: 78–89 km depth); these results are broadly parallel to contemporaneous magmatism on the western rift margin in the Akaki Magmatic Zone. We interpret these results as evidence for magma stalling at a thermo-mechanical boundary to ascent: the lithosphere-asthenosphere boundary. The Ethiopian continental lithosphere has therefore remained relatively thick late into the rifting process, with important potential implications for late-stage decompression melting prior to the onset of seafloor spreading.