Post-collisional volcanism with adakitic signatures in the Arabian-Nubian Shield: A case study of calc-alkaline Dokhan volcanics in the Eastern Desert of Egypt
Post-collisional volcanism with adakitic signatures in the Arabian-Nubian Shield: A case study of calc-alkaline Dokhan volcanics in the Eastern Desert of Egypt
复制标题
DOI：
10.1016/j.lithos.2021.106051
复制
发表时间：
2021-03-16
期刊：
影响因子：
3.5
通讯作者：
Shi, Qingshang
中科院分区：
文献类型：
--
作者：
Abuamarah, Bassam A.;Azer, Mokhles K.;Shi, Qingshang
埃及东北部沙漠的加巴尔乌姆古鲁夫（Gabal Um Guruf）出露一套火山 - 沉积层序，包括古鲁夫火山岩系。一系列中性到酸性火山岩流以及相关的火山碎屑沉积不整合地覆盖在石英闪长岩和花岗闪长岩深成岩体之上，并被高位碱长石花岗岩侵入。花岗闪长岩中的变火山岩捕虏体表明与古鲁夫火山岩无关。可以识别出两个火山旋回。下部单元是一个高钾钙碱性系列，含有少量玄武安山岩，主要是从安山岩到英安岩。上部单元由流纹岩和流纹英安岩组成，向碱性A型岩浆过渡。这两个单元都归属于多汉火山岩，并被解释为碰撞后产物。稀土元素（REE）模式的特征是在整个岩套中逐渐富集，重稀土元素显然比轻稀土元素更不相容；玄武安山岩中向上凹的REE模式表明源区存在残余角闪石。只有上部流纹岩显示出明显的负铕异常（Eu/Eu* = 0.35 - 0.73）。原生辉石的表观结晶温度为800 - 1000℃，原生角闪石为781 - 944℃。该岩套的主量和微量元素化学特征可以模拟为单一的液相演化线，随着岩浆冷却和演化压力降低，并且磷灰石在捕获CaO、P₂O₅和轻稀土元素方面起重要作用。尽管不能排除上地壳混染和岩浆混合的可能性，但数据并不需要这些过程。该岩套包括红钛锰矿，一种不寻常的富锰钛铁矿族固溶体，此前在阿拉伯 - 努比亚地盾（ANS）的火山岩中未被记录。古鲁夫火山岩系未变形且未变质，这与阿拉伯 - 努比亚地盾典型的与俯冲相关的火山单元不同。其地球化学特征与许多阿拉伯 - 努比亚地盾碰撞后岩石的特征一致。其主要的钙碱性特征以及先前被解释为指示活动弧环境的其他特征，更可能反映的是碰撞前阶段（850 - 740 Ma）与弧相关物质的重熔。下部层序的熔岩流具有埃达克岩特征，包括高锶、低钇、低镱、高锶/钇以及高（La/Yb）(n)。这可能反映了阿拉伯 - 努比亚地盾地壳中残余或分异角闪石的影响，而不是活动俯冲期间的板片熔融。一次碰撞后的拆沉和英云闪长质下地壳物质受热事件，随后与软流圈地幔混合，产生了一种具有古鲁夫玄武安山岩特征的原生岩浆，包括残余角闪石。该地区不需要晚期俯冲，也不需要在当地对流地幔中长期存在受俯冲影响的物质。（C）2021爱思唯尔公司。保留所有权利。
A volcano-sedimentary succession, including the Guruf volcanic series, is exposed at Gabal Um Guruf in the north Eastern Desert of Egypt. A series of intermediate to silicic volcanic flows and associated pyroclastic deposits unconformably overlie quartz-diorite and granodiorite plutons and are intruded by high-level alkali feldspar granites. The metavolcanic xenoliths in the granodiorite are shown to be unrelated to the Guruf volcanics. Two volcanic cycles can be recognized. The lower unit is a high-K calc-alkaline series with minor basaltic andesite and mostly andesite through dacite. The upper unit consists of rhyolite and rhyodacite that is transitional towards alkaline, A-type magma. Both units are assigned to the Dokhan volcanics and interpreted as post-collisional. REE patterns are characterized by slow progressive enrichment throughout the suite, with heavy REE apparently acting more incompatibly than light REE; concave-upwards REE patterns in basalt andesite suggest residual amphibole in the source. Only the upper rhyolite displays prominent negative Eu anomalies (Eu/Eu* = 0.35-0.73). Apparent crystallization temperatures of primary pyroxene are 800-1000 degrees C and of primary amphiboles 781-944 degrees C. The major and trace element chemistry of the suite can be modeled as a single liquid line of descent, with pressure decreasing as the magma cools and evolves, and a prominent role for apatite in sequestering CaO, P2O5, and light REE. Although upper crustal contamination and magma mixing cannot be ruled out, the data do not require them. The suite includes pyrophanite, an unusual Mn-rich ilmenite-group solid solution, not previously recorded in volcanic rocks in the Arabian-Nubian Shield (ANS). The Guruf volcanic series is undeformed and unmetamorphosed, unlike the typical subduction-related volcanic units of the ANS. The geochemical characteristics are consistent with those of many post-collisional ANS rocks. The mostly calc-alkaline character and other traits previously interpreted to indicate an active arc setting instead more likely reflect remelting of earlier arc-related material from the pre-collisional stage (850-740 Ma). The lava flows of the lower succession have adakitic characteristics, including high Sr, low Y, low Yb, high Sr/Y, and high (La/Yb)(n). This likely reflects the influence of residual or fractionated amphibole in the ANS crust, rather than slab melting during active subduction. A post-collisional episode of delamination and heating of tonalitic lower crustal material, followed by mixing with asthenospheric mantle, created a primary magma with the characteristics of the Guruf basaltic andesite, including residual amphibole. There is no need for either late subduction in the area or long-term persistence of subduction-influenced material in the local convecting mantle. (C) 2021 Elsevier B.V. All rights reserved.