Recycling of ultramafic lower crustal cumulates via delamination or foundering is often invoked as a mechanism to return mafic material to the mantle during continental crust formation. These recycled pieces of the lower crust are rarely sampled but are preserved in several locations including the Kohistan and Talkeetna arc sections, Sierra Nevada and Colorado Plateau pyroxenite xenoliths and, as discussed here for the first time, the exhumed Higashi-Akaishi (HA) ultramafic body in Japan. The HA is located in the Besshi region of the Sanbagawa metamorphic belt in southwestern Japan and is dominantly composed of dunite with lesser garnet pyroxenite and harzburgite lenses. Although the petrogenetic history of the HA body is still debated, our new bulk major and trace element compositions, radiogenic isotope data, as well as petrologic and field observations, are consistent with a lower crustal cumulate origin for the HA dunite and pyroxenite, with a later slab-derived fluid overprint. Clinopyroxene and olivine in the foliated HA dunite have compositions consistent with ultramafic cumulates with high Mg#s (Mg# clinopyroxene = 0·94, Mg# olivine = 0·88), high NiO in olivine (∼0·26 wt %) and low-Al clinopyroxene. In addition, the bulk major element chemistry of the HA dunite and garnet pyroxenite follow systematic behavior in Mg# vs SiO₂ wt %, similar to those observed in other lower crustal cumulate lithologies and corresponding intrusive lithologies, pointing to different liquid lines of descent for the corresponding melts. Our new thermobarometric estimates (peak pressure–temperature at 2·6 GPa, 713ºC) are consistent with a hot slab surface subduction path, rather than the lower crustal temperatures recorded in arc sections (Kohistan & Talkeetna: 1 GPa, 800ºC). A pervasive slab-fluid influence is also indicated in the HA lithologies by LREE and Ce enrichments and strong Nb and Zr depletions. The trace elements and the pressure–temperature estimates, as well as the thermodynamic modeling results necessitate removal of the HA body from the lower crust and incorporation into cooler portions of a mantle wedge. At lower crustal conditions, the bulk density of the HA lithologies is greater than the background mantle, indicating the feasibility of lower crustal foundering into a mantle wedge where the HA was incorporated in the subduction channel to reach its peak conditions. Hydration of the HA body while in the subduction channel likely provided the change in density necessary to facilitate its rapid exhumation to the surface. Thus, the HA cumulate likely represents a piece of the subduction system that is rarely preserved, as well as a key component in the compositional evolution of the continental crust.

中文翻译：

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东赤石超镁铁质岩体的成岩作用：对地壳下沉和地幔楔形过程的影响

超镁铁质下地壳的再循环通过分层或沉没积累起来，通常被用作一种机制，以在大陆壳形成过程中将铁镁质物质返回到地幔中。这些回收的下地壳碎片很少进行采样，但保存在多个地点，包括科希斯坦和塔吉特纳弧形断面，内华达山脉和科罗拉多高原的辉石红柱石，以及首次在这里讨论的发掘出的东-赤石超镁铁矿。日本的身体。HA位于日本西南部Sanbagawa变质带的Besshi地区，主要由榴辉岩和较少的石榴石辉石辉石和Harzburgite晶状体组成。尽管关于HA体的成岩史仍存在争议，但我们新的主要组成和微量元素组成，放射性同位素数据，以及岩石学和野外观测，与HA榴辉岩和辉石的较低地壳累积成因相一致，而板状流体的叠印作用较晚。叶状HA榴辉石中的斜辉石和橄榄石的成分与超镁铁质粉体一致，Mg＃s较高（Mg＃斜辉石= 0·94，Mg＃橄榄石= 0·88），橄榄石中NiO含量较高（〜0·26 wt％），低铝斜py。此外，HA榴辉石和石榴石辉绿岩的本体主要元素化学性质遵循Mg＃与SiO的系统行为 橄榄石中的NiO含量较高（〜0·26 wt％），而铝的斜ino含量低。此外，HA榴辉石和石榴石辉绿岩的本体主要元素化学性质遵循Mg＃与SiO的系统行为 橄榄石中的NiO含量较高（〜0·26 wt％），而铝的斜ino含量低。此外，HA榴辉石和石榴石辉绿岩的本体主要元素化学性质遵循Mg＃与SiO的系统行为₂重量％，类似于在其他下地壳中累积的岩性和相应的侵入性岩性，指出相应熔体的下降液体线不同。我们新的热压法估算值（峰值压力-温度在2·6 GPa，713ºC）与热平板表面的俯冲路径一致，而不是与弧段中记录的较低地壳温度一致（Kohistan＆Talkeetna：1 GPa，800ºC）。在HA岩性中，LREE和Ce富集以及Nb和Zr的强烈耗尽也表明了普遍的板状流体影响。痕量元素和压力-温度估算值以及热力学模型结果必须将HA体从下地壳中移除，并纳入地幔楔的较凉部分。在较低的地壳条件下 HA岩性的堆积密度大于背景地幔，表明较低的地壳沉入地幔楔的可行性，在该地幔楔中，HA被整合到俯冲通道中以达到其峰值条件。HA体在俯冲通道中的水合作用可能会提供必要的密度变化，以促进其快速发掘到地面。因此，HA累积可能代表了一个俯冲系统中很少被保留的部分，并且是大陆壳成分演化的关键组成部分。HA体在俯冲通道中的水合作用可能会提供必要的密度变化，以促进其快速发掘到地面。因此，HA累积可能代表了一个俯冲系统中很少被保留的部分，并且是大陆壳成分演化的关键组成部分。HA体在俯冲通道中的水合作用可能会提供必要的密度变化，以促进其快速发掘到地面。因此，HA累积可能代表了一个俯冲系统中很少被保留的部分，并且是大陆壳成分演化的关键组成部分。