Hydrous peridotites with Ti-rich chromian spinel as a low-temperature forearc mantle facies: evidence from the Happo-O’ne metaperidotites (Japan)
Hydrous peridotites with Ti-rich chromian spinel as a low-temperature forearc mantle facies: evidence from the Happo-O’ne metaperidotites (Japan)
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DOI：
10.1007/s00410-009-0420-7
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发表时间：
2010-02
影响因子：
3.5
通讯作者：
M. Khedr;S. Arai
中科院分区：
文献类型：
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作者：
M. Khedr;S. Arai
日本中部希达边缘构造带东北部的八浦-奥恩橄榄岩杂岩，以高磷/特伦格变质作用为特征，被认为是古生代的蛇纹岩混杂岩。橄榄岩，是块状或叶理，已遭受水化和变质作用。它们的原岩主要为二辉橄榄岩至辉橄榄岩，具次级纯橄榄岩。我们发现了一个特征矿物组合，橄榄石+斜方辉石+透闪石+尖晶石+铬尖晶石，在低温（650 - 750°C）和高磷（16 - 20 kbar）下稳定，透闪石带的透闪石-铬橄榄岩。橄榄石是Fo 88-Fo 91，正辉石（Mg# = 0.91）由于低平衡温度而显示出低且均匀的Al 2 O3（高达0.25wt%）、Cr2 O3（高达0.25wt%）、CaO（高达0.36wt%）和TiO 2（高达0.06wt%）分布。铬尖晶石为自形体，主要包裹在斜方辉石中，具有高的TiO 2，平均含量为3.1wt%（最高可达5.7wt%），高的Cr# [=Cr/（Cr + Al）原子比]，平均值为0.95，但Fe 3 +[= Fe 3 +/（Cr + Al + Fe 3+）原子比，<0.3]。整体岩石化学表明，具有这种特殊尖晶石的Happo-O 'ne变橄榄岩的TiO 2含量较低（0.01- 0.02wt%），表明在变质作用期间没有从外部来源添加TiO 2;特殊尖晶石的高TiO 2是通过在冷却过程中形成低T、低Ti硅酸盐（<0.1wt% TiO 2）期间从含Ti高T辉石中释放Ti实现的。一些纯橄榄岩在水化作用下是完整的：它们的橄榄石是Fo 92，尖晶石显示出高Cr#，0.72。Happo-O 'ne变质橄榄岩（透闪石-尖晶石橄榄岩）位于地幔楔的角部，是由高含水橄榄岩相（尖晶石橄榄岩）过渡而来的一种含水低T、高P地幔橄榄岩相。它们在仁格变质带折返过程中经历了低温（<600°C）退变质作用，形成叶蛇纹石和透辉石。
The Happo-O’ne peridotite complex is situated in the northeastern part of the Hida Marginal Tectonic Zone, central Japan, characterized by the high-P/TRenge metamorphism, and is considered as a serpentinite mélange of Paleozoic age. Peridotitic rocks, being massive or foliated, have been subjected to hydration and metamorphism. Their protoliths are mostly lherzolites to harzburgites with subordinate dunites. We found a characteristic mineral assemblage, olivine + orthopyroxene + tremolite + chlorite + chromian spinel, being stable at low-T, from 650 to 750°C, and high-P, from 16 to 20 kbar, tremolite–chlorite peridotites of the tremolite zone. Olivines are Fo88–Fo91,and orthopyroxenes (Mg# = 0.91) show low and homogenous distributions of Al2O3(up to 0.25 wt%), Cr2O3(up to 0.25 wt%), CaO (up to 0.36 wt%) and TiO2(up to 0.06 wt%) due to the low equilibration temperature. Chromian spinels, which are euhedral and enclosed mainly in the orthopyroxenes, have high TiO2,3.1 wt% (up to 5.7 wt%) on average, and high Cr# [=Cr/(Cr + Al) atomic ratio], 0.95 on average but low Fe3+[=Fe3+/(Cr + Al + Fe3+) atomic ratio, <0.3]. The bulk-rock chemistry shows that the Happo-O’ne metaperidotites with this peculiar spinel are low in TiO2(0.01–0.02 wt%), indicating no addition of TiO2from the outside source during the metamorphism; the high TiO2of the peculiar spinel has been accomplished by Ti release from Ti-bearing high-Tpyroxenes during the formation of low-T, low-Ti silicates (<0.1 wt% TiO2) during cooling. Some dunites are intact from hydration: their olivine is Fo92and spinel shows high Cr#, 0.72. The Happo-O’ne metaperidotites (tremolite–chlorite peridotites), being in the corner of the mantle wedge, are representative of a hydrous low-T, high-Pmantle peridotite facies transitional from a higherTanhydrous peridotite facies (spinel peridotites) formed by in situ retrograde metamorphism influenced by fluids from the subducting slab. They have suffered from low-T(<600°C) retrogressive metamorphism to form antigorite and diopside during exhumation of the Renge metamorphic belt.