The Cenozoic magmatism of East Africa: Part V – Magma sources and processes in the East African Rift
The Cenozoic magmatism of East Africa: Part V – Magma sources and processes in the East African Rift
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DOI：
10.1016/j.lithos.2019.105296
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发表时间：
2020-05
期刊：
影响因子：
3.5
通讯作者：
T. Rooney
中科院分区：
文献类型：
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作者：
T. Rooney
东非裂谷系（EARS）岩浆的形成主要是两种原因之一：（A）岩石圈温压扰动导致岩石圈地幔中易熔成分熔融;（B）伸展过程中板块减薄引起的减压导致对流上地幔熔融。熔融角闪石或金云母轴承交代体岩石圈地幔内产生碱性，硅不饱和的熔岩，而更多的硅饱和的熔岩主要是对流上地幔内的熔融物质的功能。来源的岩石圈地幔内的二氧化硅不饱和熔体是一致的，在任何给定的区域内的初始熔体观察到的趋势，表现出微量元素的特征与岩石圈交代体熔融一致，可能反映了初始的不稳定和变薄的岩石圈地幔。随着岩石圈的不断减薄，硅饱和成分的趋势与对流上地幔熔融成分的转变相吻合。来自这两个来源的贡献可能会在伸展是脉冲的地方振荡-对流上地幔的熔融在板块变薄期间是有利的;现有的或最近形成的交代体的熔融可能在相对伸展静止期间是有利的。东非岩浆作用的同位素系统学揭示了可能参与上述熔融过程的特定储层的显著复杂性。东非岩石圈地幔下经历了富集通过渗透的岩石圈下衍生的熔体和流体在一个延长的时间间隔，这接近坦桑尼亚克拉通，导致层状岩石圈地幔表现出极端的同位素比值。在其他地方，岩石圈地幔也经历了富集，但由于这个岩石圈的年龄更年轻，不太极端的同位素值已经开发。从非洲大低剪切速度区（LLSVP）上升的物质也交代了岩石圈地幔，因此表现出与岩石圈地幔内熔融有关的微量元素特征的熔岩可能以任何数量的储层或混合物的形式存在。来自对流上地幔的物质包括阿法尔地幔柱端元、亏损地幔端元和某种形式的岩石圈端元。来自整个地区的岩浆套件的同位素特征形成阵列，广泛汇聚在阿法尔羽的组成，尽管有一些复杂性的羽流材料已经形成了混合羽-岩石圈组件。这些阵列的收敛有力地支持了一个模型，即普遍组成的材料上升，从非洲LLSVP下的EARS大致相当于组成的阿法尔羽。
The generation of magmas in the East African Rift System (EARS) is largely the result of either: (A) melting of easily fusible compositions located within the lithospheric mantle due to thermobaric perturbations of the lithosphere, or (B) melting of the convecting upper mantle due to decompression caused by thinning of the plate during extension. Melt generated from amphibole- or phlogopite-bearing metasomes within the lithospheric mantle yields alkaline, silica-undersaturated lavas, while more silica-saturated lavas are primarily a function of melting material within the convecting upper mantle. Sourcing of silica-undersaturated melts within the lithospheric mantle is consistent with the observed tendency for initial melts within any given region to exhibit trace element characteristics consistent with melting of lithospheric metasomes, likely reflecting the initial destabilization and thinning of the lithospheric mantle. With continued lithospheric thinning, the trend towards more silica-saturated compositions coincides with a shift towards compositions interpreted as melting of the convecting upper mantle. Contributions from these two sources may oscillate where extension is pulsed – melts of the convecting upper mantle are favored during periods of plate thinning; melting of either existing or recently formed metasomes may be favored during periods of relative extensional quiescence. The isotopic systematics of East African magmatism reveals significant complexity as to the specific reservoirs that may participate in the melting processes noted above. The lithospheric mantle beneath East Africa has undergone enrichment through the percolation of sub-lithospheric derived melts and fluids over an extended interval, which close to the Tanzania craton has resulted in a layered lithospheric mantle exhibiting extreme isotopic ratios. Elsewhere, the lithospheric mantle has also undergone enrichment but given the more juvenile age of this lithosphere, less extreme isotopic values have developed. Material rising from the African Large Low Shear Velocity Province (LLSVP) has also metasomatized the lithospheric mantle, and thus lavas exhibiting a trace element signature linked to melting within the lithospheric mantle may exist as any number of reservoirs or mixtures of the same. Material derived from the convecting upper mantle incorporates the Afar Plume endmember, a depleted mantle endmember, and some form of lithospheric endmember. The isotopic characteristics of magma suites from throughout the region form arrays that broadly converge on the composition of the Afar Plume, despite some complexity where the plume material has formed a hybrid plume-lithosphere component. The convergence of these arrays strongly supports a model whereby the prevalent composition of material rising from the African LLSVP beneath the EARS is broadly equivalent to the composition of the Afar Plume.