Department of Earth and Environmental Science, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto, 860-8555, Japan.
Geodynamics Research Center (GRC), Ehime University, 2-5 Bunkyo-cho, Matsuyama, 790-8577, Japan.
Sci Rep. 2020 Jul 15;10(1):11645. doi: 10.1038/s41598-020-68599-7.
Microdiamonds in metamorphic rocks are a signature of ultrahigh-pressure (UHP) metamorphism that occurs mostly at continental collision zones. Most UHP minerals, except coesite and microdiamond, have been partially or completely retrogressed during exhumation; therefore, the discovery of coesite and microdiamond is crucial to identify UHP metamorphism and to understand the tectonic history of metamorphic rocks. Microdiamonds typically occur as inclusions in minerals such as garnet. Here we report the discovery of microdiamond aggregates in the matrix of a metapelite from the Nishisonogi unit, Nagasaki Metamorphic Complex, western Kyushu, Japan. The Nishisonogi unit represents a Cretaceous subduction complex which has been considered as an epidote-blueschist subfacies metamorphic unit, and the metapelite is a member of a serpentinite mélange in the Nishisonogi unit. The temperature condition for the Nishisonogi unit is 450 °C, based on the Raman micro-spectroscopy of graphite. The coexistence of microdiamond and Mg-carbonates suggests the precipitation of microdiamond from C-O-H fluid under pressures higher than 2.8 GPa. This is the first report of metamorphic microdiamond from Japan, which reveals the hidden UHP history of the Nishisonogi unit. The tectonic evolution of Kyushu in the Japanese Archipelago should be reconsidered based on this finding.
变质岩中的微金刚石是超高压（UHP）变质作用的特征，这种变质作用主要发生在大陆碰撞带。大多数 UHP 矿物，除了柯石英和微金刚石外，在抬升过程中已经部分或完全退化；因此，柯石英和微金刚石的发现对于确定 UHP 变质作用和了解变质岩的构造历史至关重要。微金刚石通常作为石榴石等矿物的包裹体存在。在这里，我们报告了在来自日本九州西部长崎变质杂岩中的西之园单元的变质泥岩基质中发现微金刚石聚集体。西之园单元代表白垩纪俯冲杂岩，被认为是绿帘石-蓝片岩相变质单元，而变质泥岩是西之园单元中蛇纹岩混杂岩的一个成员。根据石墨的拉曼微光谱，西之园单元的温度条件为 450°C。微金刚石和 Mg-碳酸盐的共存表明，微金刚石是在高于 2.8 GPa 的压力下从 C-O-H 流体中沉淀出来的。这是日本首例变质微金刚石的报道，揭示了西之园单元隐藏的 UHP 历史。基于这一发现，应该重新考虑日本列岛九州的构造演化。
