Depleted arc volcanism in the Alboran Sea and shoshonitic volcanism in Morocco: geochemical and isotopic constraints on Neogene tectonic processes
Depleted arc volcanism in the Alboran Sea and shoshonitic volcanism in Morocco: geochemical and isotopic constraints on Neogene tectonic processes
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DOI：
10.1016/j.lithos.2004.07.002
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发表时间：
2004-12
期刊：
影响因子：
3.5
通讯作者：
R. Gill;A. Aparicio;M. E. Azzouzi;Jean Hernandez;M. Thirlwall;J. Bourgois;G. F. Marriner
中科院分区：
文献类型：
--
作者：
R. Gill;A. Aparicio;M. E. Azzouzi;Jean Hernandez;M. Thirlwall;J. Bourgois;G. F. Marriner
对来自Alborán岛、阿尔博兰海底和摩洛哥北部Gourougou火山中心的火山岩样本进行了主要元素和微量元素以及Sr-Nd同位素的分析，以检验目前关于阿尔博兰海构造地球动力学演化的理论。Alborán岛样品为低钾拉斑玄武岩安山岩，其HFS元素（~ 0.5×N-MORB），特别是Nb （~ 0.2×N-MORB）含量的枯竭，与未成熟的海内弧和弧后盆地的火山岩具有明显的地球化学相似性。几个海底样品具有相似的成分，其中一个显示低钙硼铁矿亲和力。143Nd/144Nd比值与许多岛弧和弧后盆地样品在同一范围内下降，而87sr /86Sr比值（在浸出样品上）在某种程度上更具有放射性。我们的数据表明，中新世在阿尔伯拉罕地区下方发生了活跃的俯冲作用，并暗示存在一个相关的弧后扩张中心。我们的海底套件包括一些更进化的英安岩和流纹岩样品，（87Sr/86Sr）0至0.717，可能代表不同程度的地壳熔融。高钾玄武安山岩熔岩与大洋岛弧的高钾玄武安山岩火山岩具有类似的归一化不相容元素富集图和Ce/Y比值，但它们的Nb亏缺不那么明显。岩浆可能起源于经历了可忽略的俯冲输入的次大陆岩石圈地幔的熔融。Sr-Nd同位素组成表明明显的地壳污染，这似乎解释了小的Nb异常。Alboran玄武岩和玄武岩安山岩样品所显示的明显的超俯冲带（SSZ）特征驳斥了将Alboran域所有新近纪火山活动归因于过度增厚岩石圈对流变薄引起的上升流软流圈减压融化的地球动力学模型。我们的数据支持最近的模型，其中下沉是由地中海最西端下方向东倾斜的俯冲带向西回滚引起的。此外，在回滚板块边缘的岩石圈断裂为岩石圈地幔的局部融化提供了机会，这可能解释了在摩洛哥北部和西班牙东南部零星出现的shoshontic火山活动。
Samples of volcanic rocks from Alborán Island, the Alboran Sea floor and from the Gourougou volcanic centre in northern Morocco have been analyzed for major and trace elements and Sr–Nd isotopes to test current theories on the tectonic geodynamic evolution of the Alboran Sea. The Alborán Island samples are low-K tholeiitic basaltic andesites whose depleted contents of HFS elements (∼0.5×N-MORB), especially Nb (∼0.2×N-MORB), show marked geochemical parallels with volcanics from immature intra-oceanic arcs and back-arc basins. Several of the submarine samples have similar compositions, one showing low-Ca boninite affinity.143Nd/144Nd ratios fall in the same range as many island-arc and back-arc basin samples, whereas87Sr/86Sr ratios (on leached samples) are somewhat more radiogenic. Our data point to active subduction taking place beneath the Alboran region in Miocene times, and imply the presence of an associated back-arc spreading centre. Our sea floor suite includes a few more evolved dacite and rhyolite samples with (87Sr/86Sr)0up to 0.717 that probably represent varying degrees of crustal melting. The shoshonite and high-K basaltic andesite lavas from Gourougou have comparable normalized incompatible-element enrichment diagrams and Ce/Y ratios to shoshonitic volcanics from oceanic island arcs, though they have less pronounced Nb deficits. They are much less LIL- and LREE-enriched than continental arc analogues and post-collisional shoshonites from Tibet. The magmas probably originated by melting in subcontinental lithospheric mantle that had experienced negligible subduction input. Sr–Nd isotope compositions point to significant crustal contamination which appears to account for the small Nb anomalies. The unmistakable supra-subduction zone (SSZ) signature shown by our Alboran basalts and basaltic andesite samples refutes geodynamic models that attribute all Neogene volcanism in the Alboran domain to decompression melting of upwelling asthenosphere arising from convective thinning of over-thickened lithosphere. Our data support recent models in which subsidence is caused by westward rollback of an eastward-dipping subduction zone beneath the westernmost Mediterranean. Moreover, severance of the lithosphere at the edges of the rolling-back slab provides opportunities for locally melting lithospheric mantle, providing a possible explanation for the shoshonitic volcanism seen in northern Morocco and more sporadically in SE Spain.