西オーストラリア, ピルバラ地塊, マーブルバー地域における太古代初期(3.5Gyr)の珪質泥岩と砂岩の地球化学  [in Japanese] Geochemistry of early Archean (3.5Gyr) siliceous mudstone and sandstone from the Marble Bar area in the Pilbala craton, Western Australia  [in Japanese]

西オーストラリア, ピルバラ地塊, マーブルバー地域のSalgash亜層群(3.5Gyr)の最上部に分布する珪質泥岩/砂岩の全岩化学組成(主要, 微量, 希土類元素)を検討した.本検討試料は, 他の太古代初期の緑色岩帯中の頁岩に比較して, Ni, Cr含有量が異常に低く, 他の微量元素組成の特徴も, 花崗岩やTTGなどの珪長質な岩石に由来することを示す.コンドライト規格化希土類元素パターンも, 花崗岩やTTGのパターンに類似しているが, 半数程度の試料に長石の局所的な濃集による正のEu異常が認められる.一部の珪質泥岩は負のEu異常を示し, 太古代初期にすでに分化した花崗岩地殻が地球表層に露出していたことを示唆している.これらの珪質泥岩/砂岩は, 主に花崗岩からなる大陸地殻を後背地として, 大陸縁辺域において海溝充填型タービダイトとして形成されたと考えられる.

Major, minor and rare earth element abundances are reported for early Archean (3.5Gyr) siliceous mudstone and sandstone from the Marble Bar area in the Pilbara craton, Western Australia. Major and minor elemental patterns normalized to NASC (North American Shale Composite) indicate that these clastic rocks were mostly derived from felsic rocks such as granite and tonalite-trondhjemite-granodiorite (TTG). The Ni and Cr concentrations of siliceous mudstone are anomalously lower than those of other Archean shales in greenstone belts, suggesting that komatiitic components did not contribute substantially to the mudstone. The chondrite-normalized REE patterns of siliceous mudstone and sandstone are similar to those of granite and TTG, except for positive Eu anomalies. The positive Eu anomalies are probably due to local accumulation of feldspar during sedimentation. Negative Eu anomalies in two siliceous mudstone samples imply that a differentiated and fractionated granitic rock with negative Eu anomalies had already been exposed in the early Archean. In general, the siliceous mudstones have a greater contribution of granite than do the sandstones. These geochemical lines of evidence suggest that the siliceous mudstones and sandstons were deposited near a continental margin with a provenance composed mostly of granite.