[B-PT23_30PM1] 地球史解読：冥王代から現代まで

[BPT23-20] アカスタ片麻岩体に産する苦鉄質岩石類の微量元素多様性

*越田 渓子¹、石川 晃²、岩森 光³、小宮 剛² (1.東京大学大学院理学系研究科地球惑星科学専攻、2.東京大学大学院総合文化研究科、3.海洋研究開発機構・地球内部物質循環研究分野)

キーワード：太古代, 苦鉄質, マントル

The Hadean from birth of the Earth to 4.03 Ga is the earliest period of the history of the earth, and defined by no preservation of rock records in the earth. Eoarchean crustal records are also rare, so that the details of early Earth are not revealed yet.Acasta Gneiss Complex (AGC), located in the western part of the Slave Province, Canada, is one of the Early Archean terranes, and mainly consists of 3.6-4.0 Ga felsic and layered gneiss suites and mafic rocks. Minor mafic rocks are distributed all over the AGC and occur as rounded to elliptical enclaves and inclusions in the felsic and layered gneisses. These field occurrence of the mafic rocks suggest that they were formed before the formation of granitoid precursor of felsic gneisses and have potential to demonstrate the Early Archean mantle evolution. However, the AGC is subjected to numerous metamorphic and alteration events. The Acasta mafic rocks mainly consist of amphibolites with hornblende, plagioclase and quartz, suggesting that they underwent at least amphibolite facies metamorphism. No relict igneous minerals are preserved. At some localities, hornblendites with over 90 % modal abundance of hornblende occur as restites of anatexis. This study reveals the effects of alteration process by using whole-rock major and trace element compositions and constrained the Early Archean mantle characteristics from the least altered samples. The compositions of the amphibolites range from basalt to basaltic andesite (SiO2=48-57 wt. %, MgO=2.1?9.8 wt. %) and negative correlations can be seen between Al₂O₃ and MgO contents and Na₂O and MgO contents respectively. The hornblendites have higher MgO and lower Al₂O₃ and Na₂O contents than amphibolites, supporting the geological evidence that the hornblendites were derived from residue of anatexis. Amphibolites are divided into three groups based on their major elements and primitive mantle (PM)-normalized trace element patterns: Low-Al, Intermediate-Al and High-Al amphibolite respectively.The Low-Al amphibolites are plotted between the Intermediate-Al amphibolites and hornblendites on the Al2O₃ vs MgO diagram. They have relatively higher LREE contents than the Intermediate-Al amphibolites. They display negative Zr and Ti anomalies on the PM-normalized trace element patterns. Those characteristics are similar to those of hornblendites. On the other hand, PM-normalized trace element patterns of the High-Al amphiblites are highly scattered. The geochemical characteristics of the amphibolites suggest that the Low-Al amphibolites were formed as a residue with incomplete melt loss due to the partial melting of the Intermediate-Al amphibolites, whereas the High-Al amphiblites as the melts addition. The geological and geochemical evidence indicates that the compositions of almost mafic rocks at the AGC were affected by secondary partial melting, but some mafic rocks, the Intermediate-Al amphibolites, possibly preserve their primary characteristics.Except for Nb, the Intermediate-Al amphibolites have flat PM-normalized trace element patterns. Their negative Nb anomalies suggest that they were generated at the subduction setting, implying slab-dehydration process already occur in the Early Archean.