The making of an underplate: Pyroxenites from the Ethiopian lithosphere
The making of an underplate: Pyroxenites from the Ethiopian lithosphere
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DOI：
10.1016/j.chemgeo.2016.09.011
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发表时间：
2017-04
期刊：
影响因子：
3.9
通讯作者：
T. Rooney;A. Lavigne;C. Svoboda;G. Girard;G. Yirgu;D. Ayalew;J. Kappelman
中科院分区：
文献类型：
--
作者：
T. Rooney;A. Lavigne;C. Svoboda;G. Girard;G. Yirgu;D. Ayalew;J. Kappelman
与正常下地壳相比，岩浆底板具有高地震速度和异常高密度的特征，是裂谷岩石圈中最重要的成分不均匀性之一。岩浆与岩石圈地幔的相互作用形成辉石岩被认为是一个可行的机制，为创造一个底板，虽然这种相互作用的细节是难以约束的地球物理技术。在这里，我们对埃塞俄比亚西部高原上沿着Yerer Tullul Wellel火山构造线性构造（YTVL）喷发的辉石岩捕虏体进行了岩相学和地球化学研究。捕虏体记录的平衡压力（0.88-1.49 GPa）与埃塞俄比亚西部高原下存在的地震和重力定义的底板内的起源一致。已鉴定出三组辉石岩：第I组-以相对富集斜方辉石和单斜辉石为特征，具有未分馏的重稀有元素（HREE; TbN/YbN= 1.2-1.6）;第II类-以相对富集单斜辉石为特征，其已分馏HREE（TbN/YbN= 1.6-2.0）;第III组-过渡捕虏体，含有第I组的辉石特征，但也含有第II组的单斜辉石特征。第I组捕虏体形成的岩浆可能是亚碱性的组成和相对未分馏的稀土元素配置的基础上，从尖晶石橄榄岩衍生。这种岩浆可能是渐新世低钛（LT）型洪水玄武岩与阿法尔羽，或俯冲相关的岩浆活动与岛弧活动在新元古代泛非洲造山运动的区域岩石圈的建设。第二类捕虏体形成的岩浆成分可能是碱性的，并且来源于一个在重稀土分馏基础上含有石榴子石的源岩。这种岩浆可能是渐新世高钛（HT）型洪水玄武岩与阿法尔羽或中新世-最近的岩浆与裂谷发展过程中的YTVL的重新激活。我们的研究结果表明，YTVL下的岩石圈地幔的影响，至少有两个交代事件，并表现出更相似的特征，从裂谷捕虏体套件比其他地方的西部埃塞俄比亚高原。我们认为，西埃塞俄比亚高原下的岩浆底板是由多个事件的岩浆岩石圈相互作用，可能随着时间的推移，而不是在一个单一的事件。
Magmatic underplates, which are characterized by high seismic velocity and anomalously high density in comparison to normal lower crust, are among the most significant compositional heterogeneities identified in the rift lithosphere. The interaction of a magma with the lithospheric mantle to form pyroxenites is considered a viable mechanism for the creation of an underplate, though the details of such interaction are difficult to constrain with geophysical techniques alone. Here we present a petrographic and geochemical study of pyroxenite xenoliths erupted on the western Ethiopian plateau from along the Yerer Tullul Wellel Volcano-Tectonic lineament (YTVL). The equilibration pressures recorded by the xenoliths (0.88–1.49 GPa) are consistent with an origin within a seismically and gravitationally defined underplate that exists beneath the western Ethiopian plateau. Three groups of pyroxenite have been identified: Group I – characterized by a relative enrichment of orthopyroxene, and clinopyroxene with unfractionated heavy rare elements (HREE; TbN/YbN= 1.2–1.6); Group II – characterized by a relative enrichment in clinopyroxene, which have fractionated HREE (TbN/YbN= 1.6–2.0); Group III – transitional xenoliths that contain pyroxene characteristic of Group I but also contain clinopyroxene characteristic of Group II. The magma responsible for the formation of Group I xenoliths was likely of sub-alkaline composition and derived from a spinel peridotite on the basis of relatively unfractionated REE profiles. This magma may have been the Oligocene Low Titanium (LT)-type flood basalts associated with the Afar plume, or subduction-related magmatism associated with island arc activity during the construction of the regional lithosphere during the Neoproterozoic Pan African Orogeny. The magma responsible for the formation of Group II xenoliths was likely of alkaline composition and derived from a source with some garnet on the basis of fractionated HREE. This magma may have been the Oligocene High Titanium (HT)-type flood basalts associated with the Afar plume or Miocene-recent magmas associated with the reactivation of the YTVL during rift development. Our results show that the lithospheric mantle beneath the YTVL was impacted by at least two metasomatic events and exhibits characteristics more similar to the xenolith suites from the rift valley than elsewhere on the western Ethiopian plateau. We propose that the magmatic underplate beneath the western Ethiopian plateau was formed by multiple episodes of magma-lithosphere interaction, likely developing over time rather than in a single event.