Deserpentinization and high-pressure (eclogite-facies) metamorphic features in the Eoarchean ultramafic body from Isua,Greenland

• 作者: Juan Miguel Guotana Tomoaki Morishita Ikuya Nishio Akihiro Tamura Tomoyuki Mizukami Kenichiro Tani Yumiko Harigane Kristoffer Szilas D.Graham Pearson
• 来源期刊: 地学前缘（英文版）
• 年，卷（期）: 2022,（1）
• 所属期刊栏目: Research Paper
• 页码范围: 297-309
• 页数: 13页
• 语种: 英文

摘要

Discontinuous chains of ultramafic rock bodies form part of the 3800-3700 Ma Isua Supracrustal Belt(ISB),hosted in the ltsaq 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 deserpen-tinization 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 meta-morphic origin as a result of deserpentinization of a serpentinized ultramafic protolith rather than pri-mary 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 ℃).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 condi-tion during their complex metamorphic history similar to exhumed UHP ultramafic rocks in modern sub-duction zone channels.