Petrogenesis of the Higashi-Akaishi Ultramafic Body: Implications for Lower Crustal Foundering and Mantle Wedge Processes
Petrogenesis of the Higashi-Akaishi Ultramafic Body: Implications for Lower Crustal Foundering and Mantle Wedge Processes
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DOI：
10.1093/petrology/egaa089
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发表时间：
2021-01
影响因子：
3.9
通讯作者：
M. Guild;C. Till;T. Mizukami;S. Wallis
中科院分区：
文献类型：
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作者：
M. Guild;C. Till;T. Mizukami;S. Wallis
大陆地壳形成过程中，超镁铁质下地壳堆积物通过拆沉作用或沉降作用的再循环常被认为是镁铁质物质返回地幔的一种机制。这些回收的下地壳的碎片很少取样，但保存在几个地点，包括科希斯坦和塔尔凯特纳弧部分，塞拉利昂内华达州和科罗拉多高原辉石岩捕虏体，并在这里讨论的第一次，在日本挖出的东赤石（HA）超镁铁岩体。HA位于日本西南部三八川变质带的别志地区，主要由纯橄榄岩和少量石榴辉石岩和方辉橄榄岩透镜体组成。虽然HA体的成岩历史仍有争议，我们的新的散装主要和微量元素组成，放射性同位素数据，以及岩石学和野外观察，是一致的HA纯橄榄岩和辉石岩的下地壳堆积物的起源，后来板衍生流体叠印。叶片状HA纯橄榄岩中的单斜辉石和橄榄石具有与超镁铁堆晶岩一致的成分，具有高Mg#s（Mg#单斜辉石= 0.94，Mg#橄榄石= 0.88）、橄榄石中的高NiO（NiO = 0.26wt%）和低Al单斜辉石。此外，HA纯橄榄岩和石榴辉石岩的主体元素化学遵循Mg#与SiO2 wt %的系统行为，类似于在其他下地壳堆积岩岩性和相应的侵入岩岩性中观察到的，指向相应熔体的不同液体下降线。我们新的温压估算（峰值压力-温度为2.6 GPa，713ºC）与热板表面俯冲路径一致，而不是弧段记录的较低地壳温度（Kohistan和塔尔凯特纳：1 GPa，800ºC）。普遍板流体的影响也表明在HA岩性LREE和Ce的富集和强烈的Nb和Zr的亏损。微量元素和压力-温度的估计，以及热力学模拟的结果，有必要删除HA机构从下地壳和纳入较冷的部分地幔楔。在下地壳条件下，HA岩性的体积密度大于背景地幔，这表明下地壳沉降成地幔楔的可行性，其中HA被纳入俯冲通道，以达到其峰值条件。HA主体在俯冲通道中的水合作用可能提供了必要的密度变化，以促进其快速折返到表面。因此，HA堆积可能代表了一块俯冲系统，很少被保存下来，以及在大陆地壳的组成演化的关键组成部分。
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.