The Western Branch of the East African Rift System has experienced multiple episodes of basin development and intraplate alkaline volcanism since the Jurassic, however the geodynamic processes and lithospheric evolution involved in this protracted geological history remains poorly defined. Here, we present Sm-Nd, Lu-Hf, and Rb-Sr isotopic data of igneous minerals for three magmatic episodes that coincide with basin reactivation in the Rukwa Rift of southwestern Tanzania, respectively represented by: (1) the Jurassic to Cretaceous Panda Hill carbonatite; (2) the late Oligocene phonolitic‑carbonatitic Nsungwe Formation tuffs; and (3) the Miocene–recent bi-modal volcanism of the Rungwe Volcanic Province. Of these, the Nsungwe Formation tuffs offer a discrete record of the early phases of lithospheric disturbance associated with modern rifting of the Western Branch.

Alkaline magmas erupted prior to the Miocene are isotopically distinct (ɛNd +0.5 to +1.5 and ⁸⁷Sr/⁸⁶Sr 0.7041) from modern Rungwe lavas and likely originate from a lithospheric mantle that experienced enrichment during the Pan-African Orogeny (~550–700 Ma). Consistent with previous studies, our results indicate that Rungwe Volcanic Province magmas were generated from a mixed, isotopically-enriched component of the lithospheric mantle that may have developed during the ca. 1.0 Ga Irumide Orogeny. The observed change in the source region of rift lavas though time is consistent with progressing melting of a compositionally layered lithosphere. Combined with existing geophysical and sedimentological data, we hypothesise that the lithosphere beneath the Rukwa-Malawi-Usangu junction of the Western Branch experienced an episode of destabilisation and foundering (lithospheric drip), during the early stages of East African Rift development.

东非裂谷系西支自侏罗纪以来经历了多次盆地发育和板内碱性火山作用，但在这段漫长的地质历史中所涉及的地球动力学过程和岩石圈演化仍不清楚。在这里，我们展示了与坦桑尼亚西南部鲁夸裂谷盆地重新激活相吻合的三个岩浆事件的火成矿物 Sm-Nd、Lu-Hf 和 Rb-Sr 同位素数据，分别代表：（1）侏罗纪到白垩纪熊猫希尔碳酸岩；(2) 晚渐新世音岩-碳酸盐质Nsungwe组凝灰岩；(3) Rungwe 火山省的中新世-近期双峰火山活动。这些，

中新世之前喷发的碱性岩浆在同位素上是不同的（ɛNd +0.5 至 +1.5 和⁸⁷ Sr/ ⁸⁶Sr 0.7041）来自现代 Rungwe 熔岩，可能起源于在泛非造山运动（~550-700 Ma）期间经历过富集的岩石圈地幔。与之前的研究一致，我们的研究结果表明，Rungwe 火山省的岩浆是由岩石圈地幔的一种混合的、富含同位素的成分产生的，该成分可能在大约 100 年前形成。1.0 Ga Irumide 造山运动。随着时间的推移，观察到的裂谷熔岩源区的变化与组成层状岩石圈的熔融过程是一致的。结合现有的地球物理和沉积学数据，我们假设在东非裂谷发育的早期阶段，西支鲁夸-马拉维-乌桑古交界处下方的岩石圈经历了一次不稳定和沉降（岩石圈滴灌）。