Incipient boninitic arc crust built on denudated mantle: the Khantaishir ophiolite (western Mongolia)
Incipient boninitic arc crust built on denudated mantle: the Khantaishir ophiolite (western Mongolia)
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建立在剥蚀地幔之上的早期胶岩弧地壳：汗泰希尔蛇绿岩（蒙古西部）
DOI：
10.1007/s00410-017-1415-4
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发表时间：
2017
影响因子：
3.5
通讯作者：
Oyungerel Sambuu
中科院分区：
文献类型：
--
作者：
Omar Gianola;M. Schmidt;O. Jagoutz;Oyungerel Sambuu
大约570 Ma的老Khantaishir蛇绿岩是由长达4公里的方辉长岩地幔建造的，其中含有丰富的辉石岩和英安岩，其次是大约2公里的角闪辉长岩和辉长岩，以及大约2.5公里厚的火山单元，由被枕状熔岩和一些火山碎屑岩覆盖的岩墙+岩台杂岩组成。火山岩主要为玄武质安山岩和安山岩(或玻安岩)，平均SiO_2含量为58.2g±0.01wt%，xMGx=0.61wt%±0.03(xMGx=摩尔镁/(σ+FeOtot))，TiO2x=100.4wt%，CaO_2=37.5wt%±100.6wt%(误差值为2)。归一化微量元素配分模式显示出铅和锶的正异常，负的Nb异常，N-MORB和E-MORB之间的大离子亲石元素(LILE)浓度和明显亏损的HREE。这些特征表明，Khantaishir火山岩来自一种被中等板片成分改造的难熔地幔岩源，类似于沿Izu-Bonin-Mariana俯冲系统喷发的玻安岩以及Troodos和Betts Cove蛇绿岩。最引人注目的是，尽管几乎完全露头超过260平方公里，但没有任何先前存在的MORB地壳的残留物，这表明该蛇绿岩的岩浆套形成于完全剥蚀的地幔上，很可能是在俯冲开始时形成的。这块4-5公里厚的早期弧形地壳的结构类似于形成于大洋中脊的洋壳，但缺乏席状岩墙杂岩；没有观察到火山建筑物。然而，低的熔融压力和适中的H2O含量导致了高硅原始熔体、丰富的角闪辉长岩和快速富集体SiO_2。从观测到的原始熔体(56.6wt%SiO_2)开始的分离结晶模拟表明，25wt%的辉石+斜长石的分馏足以形成平均的Khantaishir火山壳。大部分分馏发生在地幔中，观察到的辉石岩透镜和方辉橄榄岩中和顶部的层解释了所需的累积体积。最后，俯冲开始时大量记录的高度贫化的玻安岩表明了俯冲开始和高度贫化地幔之间的因果关系。未来密集俯冲板块的下沉可能是由于密集的富饶地幔与浮力贫化地幔之间的不连续，而未来俯冲板块的浮力贫化地幔形成了新生地幔楔体。
The ~ 570 Ma old Khantaishir ophiolite is built by up to 4 km harzburgitic mantle with abundant pyroxenites and dunites followed by ~ 2 km of hornblende-gabbros and gabbronorites and by a ~ 2.5 km thick volcanic unit composed of a dyke + sill complex capped by pillow lavas and some volcanoclastics. The volcanics are mainly basaltic andesites and andesites (or boninites) with an average of 58.2 ± 1.0 wt% SiO2, XMg = 0.61 ± 0.03 (XMg = molar MgO/(MgO + FeOtot), TiO2 = 0.4 ± 0.1 wt% and CaO = 7.5 ± 0.6 wt% (errors as 2σ). Normalized trace element patterns show positive anomalies for Pb and Sr, a negative Nb-anomaly, large ion lithophile elements (LILE) concentrations between N- and E-MORB and distinctly depleted HREE. These characteristics indicate that the Khantaishir volcanics were derived from a refractory mantle source modified by a moderate slab-component, similar to boninites erupted along the Izu-Bonin-Mariana subduction system and to the Troodos and Betts Cove ophiolites. Most strikingly and despite almost complete outcrops over 260 km2, there is no remnant of any pre-existing MORB crust, suggesting that the magmatic suite of this ophiolite formed on completely denudated mantle, most likely upon subduction initiation. The architecture of this 4–5 km thick early arc crust resembles oceanic crust formed at mid ocean ridges, but lacks a sheeted dyke complex; volcanic edifices are not observed. Nevertheless, low melting pressures combined with moderate H2O-contents resulted in high-Si primitive melts, in abundant hornblende-gabbros and in a fast enrichment in bulk SiO2. Fractional crystallization modeling starting from the observed primitive melts (56.6 wt% SiO2) suggests that 25 wt% pyroxene + plagioclase fractionation is sufficient to form the average Khantaishir volcanic crust. Most of the fractionation happened in the mantle, the observed pyroxenite lenses and layers in and at the top of the harzburgites account for the required cumulate volumes. Finally, the multiply documented occurrence of highly depleted boninites during subduction initiation suggests a causal relationship of subduction initiation and highly depleted mantle. Possibly, a discontinuity between dense fertile and buoyant depleted mantle contributes to the sinking of the future dense subducting plate, while the buoyant depleted mantle of the future overriding plate forms the infant mantle wedge.
影响因子：
3.9
作者：
M. Regelous;K. Haase;S. Freund;M. Keith;C. Weinzierl;C. Beier;P. Brandl;T. Endres;H. Schmidt
通讯作者：
M. Regelous;K. Haase;S. Freund;M. Keith;C. Weinzierl;C. Beier;P. Brandl;T. Endres;H. Schmidt