Deserpentinization and high-pressure (eclogite-facies) metamorphic features in the Eoarchean ultramafic body from Isua, Greenland
Deserpentinization and high-pressure (eclogite-facies) metamorphic features in the Eoarchean ultramafic body from Isua, Greenland
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DOI：
10.1016/j.gsf.2021.101298
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发表时间：
2022-01
影响因子：
8.9
通讯作者：
J. Guotana;T. Morishita;I. Nishio;Akihiro Tamura;T. Mizukami;K. Tani;Y. Harigane;K. Szilas;D. Pearson
中科院分区：
文献类型：
--
作者：
J. Guotana;T. Morishita;I. Nishio;Akihiro Tamura;T. Mizukami;K. Tani;Y. Harigane;K. Szilas;D. Pearson
不连续的超镁铁岩体链构成了 3800–3700 Ma Isua 超地壳带 (ISB) 的一部分，该带位于格陵兰岛西南部的伊萨克片麻岩杂岩中。这些尸体是世界上最古老的超镁铁质岩石露头之一，因此是宝贵的地质记录。 ISB 西北翼透镜体 B 的超镁铁质岩石表现出蛇纹石化和去蛇纹石化几个阶段的特征，形成顺行和逆行矿物组合。富钛镁铁矿族矿物，如钛球粒陨石（Ti-Chn）和钛斜镁铁矿（Ti-Chu）经常作为副相出现在变质超镁铁岩中。富钛硅镁石矿物与变质橄榄石有关。主体橄榄石是高度镁橄榄石 (Fo96-98)，具有不同的 MnO 和 NiO 含量。变质橄榄石的稀土元素（REE）和高场强元素（HFSE）浓度高于典型地幔橄榄石。橄榄石的结构和化学特征表明变质起源是蛇纹石化超镁铁质原岩脱蛇纹石化的结果，而不是反映高度部分熔融的地幔残留物的原生组合。橄榄石、叶蛇纹石以及共生的 Ti-Chn 和 Ti-Chu 的密切关联表明，叶蛇纹石稳定场内的压力条件为 ∼1.3–2.6 GPa（<700 °C）。太太古代ISB内透镜B超镁铁体的整体岩石学和地球化学特征表明，这些是外来超镁铁岩，在相对较低的温度下记录了蛇纹石脱水，并在其复杂的变质历史中达到了榴辉岩相条件，类似于现代俯冲带通道中挖出的超高压超镁铁岩。
Discontinuous chains of ultramafic rock bodies form part of the 3800–3700 Ma Isua Supracrustal Belt (ISB), hosted in the Itsaq Gneiss Complex of southwestern Greenland. These bodies are among the world’s oldest outcrops of ultramafic rocks and hence an invaluable geologic record. Ultramafic rocks from Lens B in the northwestern limb of ISB show characteristics of several stages of serpentinization and deserpentinization forming prograde and retrograde mineral assemblages. Ti-rich humite-group minerals such as titanian chondrodite (Ti-Chn) and titanian clinohumite (Ti-Chu) often occur as accessory phases in the metamorphosed ultramafic rocks. The Ti-rich humite minerals are associated with metamorphic olivine. The host olivine is highly forsteritic (Fo96-98) with variable MnO and NiO contents. The concentrations of the rare-earth elements (REE) and high-field strength elements (HFSE) of the metamorphic olivine are higher than typical mantle olivine. The textural and chemical characteristics of the olivine indicate metamorphic origin as a result of deserpentinization of a serpentinized ultramafic protolith rather than primary assemblage reflecting mantle residues from high-degrees of partial melting. The close association of olivine, antigorite and intergrown Ti-Chn and Ti-Chu suggests pressure condition between ∼1.3–2.6 GPa within the antigorite stability field (<700 °C). The overall petrological and geochemical features of Lens B ultramafic body within the Eoarchean ISB indicate that these are allochthonous ultramafic rocks that recorded serpentine dehydration at relatively lower temperature and reached eclogite facies condition during their complex metamorphic history similar to exhumed UHP ultramafic rocks in modern subduction zone channels.