Arclogites and their role in continental evolution; part 2: Relationship to batholiths and volcanoes, density and foundering, remelting and long-term storage in the mantle
Arclogites and their role in continental evolution; part 2: Relationship to batholiths and volcanoes, density and foundering, remelting and long-term storage in the mantle
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弧长岩及其在大陆演化中的作用；
DOI：
10.1016/j.earscirev.2020.103476
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发表时间：
2021
影响因子：
12.1
通讯作者：
Balica, Constantin
中科院分区：
文献类型：
--
作者：
Ducea, Mihai N.;Chapman, Alan D.;Bowman, Emilie;Balica, Constantin
弧长岩是一种类似榴辉岩的岩石，由岩浆作用形成，是超基性残留物，并积聚在厚弧的根部（Ducea et al.， 2021年的合著论文）。据推测，在上板块地壳较厚的地区，它们在体积上是重要的组合，以补充与俯冲有关的岩浆岩。在地表和浅层地壳中，这些弧形成了层状火山，在太平洋周围的许多地方都发现了这些弧，这些弧暴露在灭绝的弧中，比如北美西部的科迪勒拉。弧长岩补充了这些俯冲带岩浆活动的浅层表现，因为它们代表了中间熔体萃取后留下的超镁铁质残留物。由于石榴石、角闪洞和铁钛氧化物的二氧化硅含量低，质量平衡计算表明，给定弧的二氧化硅含量与弧下的弧晶残留物的体积有关。研究表明，熔体提取发生在下地壳热区疏散的间断事件中，这有效地弥补了中上部地壳记录的岩浆爆发。这些循环事件导致岩石残余物的致密化并触发它们的沉没。我们发现，除了石榴石，单键钛氧化物在根的致密化中起着重要作用，甚至在无石榴石的电弧中也能引发塌陷。最近的模型表明，沉降可能是小规模的，横向剪切可能对消除亚弧残余起重要作用。因此，先前假设的大幅度隆升和大规模幔源岩浆活动可能不会伴随沉没事件。下降弧长岩体的部分熔融预计会产生少量的霞石（和/或白斑岩）规范岩浆，其成分与大陆内部发现的碱性地块相似，而不是玄武岩或中间熔体。在显生宙弧中，弧长岩的沉降速率约为20 ~ 40 km3/km/Myr。沉没弧长岩的命运可能包括：在660公里的不连续面处停止，与俯冲的海洋物质在沿核-地幔边界的“板块墓地”聚集和混合，以及/或通过地幔中的韧性流分解。无论如何，地幔中必须有一个储层，代表这些地壳下循环体随着时间的推移。我们认为最有可能的储层是EM1，这是海洋岛玄武岩中众所周知的和常见的再生物质之一，并且地幔体积的1-3%可能是由这种储层构成的。
Arclogites are eclogite-like rocks formed magmatically as ultramafic residues and cumulates in the roots of thick arcs (Ducea et al., 2021 companion paper). They are inferred to be volumetrically important assemblages to complement subduction-related magmatic rocks at depth, in areas where the upper plate crust is thick. At the surface and in the shallow crust, these arcs form stratovolcanoes found at many sites around the Pacific and batholiths, which are exposed in extinct arcs, such as the western North American Cordillera. Arclogites complement these shallower manifestations of magmatism in subduction zones in that they represent the ultramafic residues left behind following the extraction of intermediate melts. Mass balance calculations constrained by the low silica contents of garnet, amphibole, and iron‑titanium oxides dictate that the silica content of a given arc tracks with the volume of arclogitic residues beneath it. We show that melt extraction takes place in punctuated events of hot-zone evacuation of the lower crust which effectively make up the magmatic flare-ups documented in the mid- to upper-crust. These cyclic events lead to the densification of arclogitic residues and trigger their foundering. We show that in addition to garnet, Fesingle bondTi oxides play an important role in the densification of roots and can trigger foundering even in garnet-free arcs. Recent models show that foundering may be small scale and lateral shearing may have an important role in removing sub arc residues. Consequently, previously postulated large magnitude uplift and large-scale mantle-derived magmatism may not accompany foundering episodes. Partial melting of descending arclogitic bodies is expected to produce small amounts of nepheline (and/or leucite) normative magmas similar in composition to alkaline massifs found in continental interiors, not basaltic or intermediate melts. The foundering rates of arclogites are calculated to be around 20–40 km3/km/Myr in Phanerozoic arcs. The fate of foundered arclogite may include stalling at the 660 km discontinuity, accumulation and mixing with subducted oceanic material at “slab graveyards” along the core-mantle boundary, and/or disaggregation via ductile flow in the mantle. Regardless, there has to be a reservoir in the mantle representing these lower crustal recycled bodies over time. We show that the most likely such reservoir is EM1, one of the well-known and commonly identified recycled material in ocean island basalts, and that 1–3% of the volume of the mantle may be made of this reservoir.
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