Tectonic setting, origin, and obduction of the Oman ophiolite
Tectonic setting, origin, and obduction of the Oman ophiolite
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DOI：
10.1130/0016-7606(1999)111
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发表时间：
1999
影响因子：
4.9
通讯作者：
M. Searle;Jon Cox
中科院分区：
文献类型：
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作者：
M. Searle;Jon Cox
阿曼山脉的塞迈尔蛇绿岩是世界上最大且保存最完好的洋壳和上地幔逆冲岩席（面积>10000平方千米，长约550千米，宽约150千米）；它在晚白垩世期间被推覆到阿拉伯大陆边缘。该蛇绿岩于96 - 94百万年前在一个东北倾的俯冲带上方的扩张中心形成，与蛇绿岩最上部的未成熟岛弧拉斑玄武岩（拉赛尔弧）的形成有关。在超过12千米的亏损方辉橄榄岩上地幔之下，同时发生的三叠纪（以及可能的侏罗纪）洋中脊玄武岩和碱性火山岩的俯冲作用，产生了在约850°C温度下形成的石榴石 + 单斜辉石角闪岩，其年龄为95 - 93百万年。俯冲不可能在洋中脊处开始，否则变质基底中角闪岩的原岩将与上方蛇绿岩火山岩具有相同的年龄和成分。在阿拉伯联合酋长国巴尼哈米德地区的阿曼山脉北部，约870米的麻粒岩相岩石（顽火辉石 + 尖晶石 ± 透辉石石英岩、石榴石 + 透辉石 + 硅灰石钙硅酸盐大理岩、含单斜辉石角闪岩）在与阿曼基底的石榴石 + 透辉石角闪岩相似的温度下形成，即800 - 850°C，但压力略高，可达9千巴。它们被解释为在仰冲后期阶段，由于逆冲顺序混乱，麻粒岩被推覆到地幔序列方辉橄榄岩之上而被抬升的更深层次的变质大陆边缘沉积岩。通过对东部地区阿斯西法榴辉岩的热 - 压力测定（至20 - 23千巴），已证明阿拉伯大陆地壳在仰冲的塞迈尔蛇绿岩之下俯冲到约78 - 90千米深度。在阿拉伯联合酋长国，俯冲的大陆地壳开始部分熔融，产生了不寻常的含黑云母 ± 白云母 ± 石榴石 ± 电气石 ± 堇青石 ± 红柱石花岗岩，这些花岗岩侵入到蛇绿岩的最上部地幔序列方辉橄榄岩和最下部地壳序列堆积辉长岩中。我们认为，在仰冲的最后阶段，阿拉伯板块的整个前缘（东北缘）在蛇绿岩之下发生俯冲，导致地壳岩石榴辉岩化。阿拉伯联合酋长国地区较高的温度和压力，可能是由于蛇绿岩剖面更厚或双倍厚度，导致变质基底出现蓝片岩、角闪岩和麻粒岩相条件，并且蛇绿岩下伏基底的地壳熔融产生了淡色花岗岩，这些花岗岩作为岩脉向上侵入到仰冲的蛇绿岩中。提出了一个蛇绿岩仰冲模型，该模型解释了阿曼山脉所报道的所有构造和变质条件。
The Semail ophiolite in the Oman Mountains is the world9s largest and best preserved thrust sheet of oceanic crust and upper mantle (>10 000 km 2 , ∼550 km long, ∼150 km wide); it was emplaced onto the Arabian continental margin during Late Cretaceous time. The ophiolite originated 96–94 Ma at a spreading center above a northeast-dipping subduction zone associated with initiation of immature island-arc tholeiitic lavas (Lasail arc) at the highest levels of the ophiolite. Simultaneous underthrusting of Triassic (and Jurassic[?]) mid-oceanic-ridge basalt and alkalic volcanic rocks beneath >12 km of upper mantle depleted harzburgites produced garnet + clinopyroxene amphibolites formed at temperatures of ∼850 °C, dated as 95–93 Ma. Subduction cannot have been initiated at a mid-oceanic ridge, otherwise the protolith of the amphibolites in the metamorphic sole would be the same age and composition as the ophiolite volcanic rocks above. In the northern part of the Oman Mountains in the Bani Hamid area, United Arab Emirates, ∼870 m of granulite facies rocks (enstatite + spinel ± diopside quartzites, garnet + diopside + wollastonite calc-silicate marbles, clinopyroxene-bearing amphibolites) were formed at temperatures similar to those of the garnet + diopside amphibolites of the Oman sole, 800–850 °C, but at slightly higher pressures, as much as 9 kbar. They are interpreted as deeper level metamorphosed continental margin sedimentary rocks exhumed by out-of-sequence thrusting placing granulites over mantle sequence harzburgites during the later stages of obduction. Subduction of the Arabian continental crust beneath the obducting Semail ophiolite to ∼78–90 km depth has been proven by thermobarometry of the As Sifah eclogites (to 20–23 kbar) in the eastern sector. In the United Arab Emirates the subducted continental crust began to partially melt, producing unusual biotite ± muscovite ± garnet ± tourmaline ± cordierite ± andalusite–bearing granites that intrude the uppermost mantle sequence harzburgites and lowermost crustal sequence cumulate gabbros of the ophiolite. We suggest that the entire leading (northeast) edge of the Arabian plate was subducted beneath the ophiolite during the final stages of obduction leading to eclogitization of the crustal rocks. Higher temperatures and pressures in the United Arab Emirates sector, possibly due to a thicker or double-thickness ophiolite section, led to blueschist, amphibolite, and granulite facies conditions in the metamorphic sole, and crustal melting in the subophiolite basement produced leucocratic granites that intruded up as dikes through the obducted ophiolite. A model for ophiolite obduction is presented, which accounts for all the structural and metamorphic conditions reported from the Oman Mountains.