Paleogene volcanic ash layers in the Danish Basin: compositions and source areas in the North Atlantic Igneous Province☆
Paleogene volcanic ash layers in the Danish Basin: compositions and source areas in the North Atlantic Igneous Province☆
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丹麦盆地古近纪火山灰层：北大西洋火成岩省的成分和来源区☆
DOI：
10.1016/j.lithos.2003.07.001
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发表时间：
2003
期刊：
影响因子：
3.5
通讯作者：
A. Pedersen
中科院分区：
文献类型：
--
作者：
L. Larsen;J. Fitton;A. Pedersen
丹麦的古近纪粘土盆地含有约。 200 层大部分保存完好的火山灰。其中，约。通过 X 射线荧光光谱法 (XRF) 分析了 77 个层的主量元素和微量元素，其中 22 个层为稀土元素 (REE)，11 个层为 Sr 和 Nd 同位素。 “负”灰系（层-39至-1）包括拉斑玄武岩、地壳污染的粗面岩和流纹岩；碱性玄武岩、粗面玄武岩、粗面岩和流纹岩；以及富钛霞石和响沸石。 “正”灰系列（+1 至+140 层）包含富集拉斑铁玄武岩和两个流纹岩层。铁玄武岩形成一个彗岩浆群；然而，如 Zr/Nb、REE 含量和同位素组成的变化所示，在上剖面中可以看到富集程度较低和富集程度较高的成分之间存在振荡，这表明地幔来源存在异质性。来自德国北部格赖夫斯瓦尔德奥伊的两个始新世火山灰样本与正系列火山灰相同。通过将火山灰成分与北大西洋火成岩省的其他岩石进行比较，可以确定可能的来源区域。可以区分四个沉积阶段。在第一阶段（-39 至-22 层），玄武岩和流纹岩源自欧洲西北部大陆架的中心。在第2阶段（-21b层至-15层），粗面岩和流纹岩仍然源自陆架中心，而强碱性层可能全部起源于东格陵兰的加德纳杂岩。在第3阶段，碱性玄武岩（层-13至-11）可能是开放的海洋裂谷的失败或扩展部分的产物。在第 4 阶段（+1 至 +140 层），我们认为大量的玄武岩共岩浆组源自一个巨大的火山系统，代表了开放海洋中新生的原始冰岛。如果与原冰岛地幔柱相关的岩浆产量极高的区域（在此之前在格陵兰岛产生了大型地下溢流玄武岩）此时远离大陆并进入被海洋覆盖的开口裂谷，从而将大部分火山活动从喷发性转变为爆炸性，那么第四阶段的灾难性特征就可以理解。当原始冰岛最终出现时，爆炸活动再次减弱。
The Paleogene mo clay basin in Denmark contains ca. 200 layers of mostly well-preserved volcanic ash. Of these, ca. 77 layers have been analysed for major and trace elements by X-ray fluorescence spectrometry (XRF), 22 for rare-earth elements (REE), and 11 for Sr and Nd isotopes. The ‘negative’ ash series (layers −39 to −1) comprise tholeiitic basalt, crustally contaminated trachyte, and rhyolite; alkaline basalt, trachybasalt, trachyte, and rhyolite; and Ti-rich nephelinite and phonolite. The ‘positive’ ash series (layers +1 to +140) comprise enriched tholeiitic ferrobasalt and two rhyolite layers. The ferrobasalts form one comagmatic group; however, oscillations are seen up-section between less-enriched and more-enriched compositions, as indicated by variations in Zr/Nb, REE contents, and isotope compositions, suggesting heterogeneities in the mantle source. Two samples of Eocene ash from Greifswalder Oie in northern Germany are identical to the positive series ashes. By comparison of the ash compositions with other rocks from the North Atlantic Igneous Province, probable source areas can be identified. Four stages of deposition can be distinguished. In stage 1 (layers −39 to −22), basalts and rhyolites were sourced from centres on the NW European shelf. In stage 2 (layers −21b to −15), trachytes and rhyolites were still sourced from centres on the shelf, whereas the strongly alkaline layers could all have originated in the Gardiner Complex in East Greenland. In stage 3, alkali basalts (layers −13 to −11) may be the products of a failed or propagating part of the opening oceanic rift. In stage 4 (layers +1 to +140), we suggest that the comagmatic suite of voluminous ferrobasalts were sourced from a gigantic volcanic system representing the nascent Proto-Iceland within the opening ocean. The cataclysmic character of stage 4 can be understood if the areas of extremely high magma production associated with the Proto-Icelandic mantle plume, which had until then produced the large subaerial flood basalts in Greenland, at this time moved away from the continent and into the sea-covered opening rift, thus switching the bulk of volcanism from effusive to explosive. When Proto-Iceland finally emerged, the explosive activity abated again.