Earth's oldest preserved K-bentonites in the ca. 2.1 Ga Francevillian Basin, Gabon
Earth's oldest preserved K-bentonites in the ca. 2.1 Ga Francevillian Basin, Gabon
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地球上保存最古老的钾膨润土位于约。
DOI：
10.2475/04.2018.02
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发表时间：
2018
影响因子：
2.9
通讯作者：
A. Bekker
中科院分区：
文献类型：
--
作者：
O. Bankole;A. El Albani;A. Meunier;F. Pambo;J. Paquette;A. Bekker
膨润土是火山灰的蚀变产物，通常保存在基线以下的低能量沉积环境中。尽管火山凝灰岩在地球历史上随处可见，但膨润土的历史却早于大约 10 世纪。 1.5 Ga尚未描述。我们提供了加蓬未变质古元古代 Francevillian 盆地 FB 地层中钾膨润土层的矿物学、地球化学和年龄数据。 K-膨润土的粘土矿物主要由伊利石和大量高岭石以及痕量长序伊利石/蒙脱石 (R3) 混合层组成。高岭石含量以及共存的 1M 和 2M1 伊利石表明，在最低埋藏温度下，在很长一段时间内发生了成岩蒙脱石伊利石化作用。它们的化学特征表明源自钙碱性中间体到长英质岩浆，与俯冲环境中的大陆火山弧岩浆作用有关。锆石颗粒相对较小，呈圆形至亚圆形，并且产生的 207Pb/206Pb 日期范围较窄，加权平均值为 2971 ± 13 Ma，与底层结晶钙碱性太古宙基底花岗岩的年龄一致。这个年龄表明，在岩浆活动过程中，太古代基底花岗岩中的锆石并入了岩浆中。考虑到 FB 组膨润土源自沿西加蓬地块边缘发育的火山弧，并保存在 Francevillian 盆地的下部，我们推断该盆地反映了前陆盆地背景下的高速率但短暂的沉积作用。从古地理角度来看，这些钾膨润土可以作为古元古代加蓬和邻近克拉通的潜在相关标记。 2.1 Ga。根据目前的记录，这些是迄今为止报道的世界上最古老的膨润土床。
Bentonites are the alteration product of volcanic tephra typically preserved in low-energy, sedimentary environments below baseline. Although volcanic tuffs occur throughout the Earth's history, bentonites older than ca. 1.5 Ga have not been described. We present the mineralogy, geochemistry, and age data for K-bentonite beds within the FB Formation in the unmetamorphosed Paleoproterozoic Francevillian Basin, Gabon. The clay mineralogy of the K-bentonites consists predominantly of illite with substantial amounts of kaolinite and trace amounts of long-ordered illite/smectite (R3) mixed layer. The kaolinite content and co-existing 1M and 2M1 illites are indicative of diagenetic smectite illitization over a prolonged period of time with minimal burial temperature. Their chemical characteristics suggest derivation from calc-alkaline intermediate to felsic magma, related to continental volcanic arc magmatism in a subduction setting. The zircon grains are relatively small, rounded to sub-rounded, and yield 207Pb/206Pb dates that have a narrow range with a weighted mean of 2971 ± 13 Ma, consistent with the age of the underlying crystalline calc-alkaline Archean basement granitoids. This age indicates incorporation of zircons from the Archean basement granitoids into the magma during magmatic activity. Considering that the FB Formation bentonites were derived from a volcanic arc developed along the margin of the West Gabonian block and are preserved in the lower part of the Francevillian Basin, we infer that this basin reflects high-rate, but short-lived sedimentation in a pro-foreland basin setting. Paleogeographically, these K-bentonites could thus serve as a potential correlation marker for the Paleoproterozoic Gabonian and adjacent cratons at ca. 2.1 Ga. Based on the current records, these are the world's oldest bentonite beds so far reported.
影响因子：
5.3
作者：
Martin A
通讯作者：
Martin A