Constraint on the eclogite age of the Sanbagawa metamorphic rocks in central Shikoku, Japan
Constraint on the eclogite age of the Sanbagawa metamorphic rocks in central Shikoku, Japan
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日本四国中部三波川变质岩榴辉岩年龄的制约
DOI：
10.1080/00206814.2019.1581997
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发表时间：
2019
影响因子：
2.6
通讯作者：
Daichi Kato
中科院分区：
文献类型：
--
作者：
Shogo Aoki;Kazumasa Aoki;Yuta Tsuchiya;Daichi Kato
为了限定三巴川榴辉岩原岩从增生到俯冲相关变质作用的时间，我们对日本国中部别石地区色巴榴辉岩相区泥质片岩进行了锆石U-Pb定年和稀土元素组成分析。锆石碎屑火成岩岩心年龄约为2000 ~ 100 Ma，变质岩边缘年龄约为90 Ma。结果表明，该原岩的增生年龄约为100 ~ 90 Ma，明显低于前人报道的该区其他榴辉岩相区约130 Ma的增生年龄。变质边缘域ree降低，未见Eu异常，表明其形成于约90 Ma榴辉岩相变质期。结合前人对三八川变质岩构造的研究，认为别石地区至少存在两个不同增生时代的榴辉岩相单元；约130毫安单位（Besshi单位）和约100-90毫安单位（assemi -gawa单位），它们在结构上相互接触。很可能较老的单元被俯冲到超过50公里的深度，并停滞不前，直到较年轻的单元被俯冲到相同的深度。可能，这两个单元在地幔深度并置，并在约90 Ma之后同时开始向地表挖掘。并置和掘出过程可能与俯冲带构造侵蚀、热板块俯冲角变浅、地幔回流和掘出路线流体入渗等因素有关。
To constrain the timing from the accretion to the subduction-related metamorphism of the protolith in the Sanbagawa eclogites, we performed zircon U–Pb datings and REE composition analyses on pelitic schist of the Seba eclogite-facies region in the Besshi area in central Shikoku, Japan. The detrital igneous cores of the zircons show ages from ca. 2000 to 100 Ma, and the metamorphic rims show ca. 90 Ma. These results show that the protolith was accreted at ca. 100–90 Ma, which is significantly younger than the previously reported accretion age of ca. 130 Ma of other eclogite-facies regions in this area. And, the metamorphic rim domains show HREE decrease without Eu anomalies, suggesting that they were formed at ca. 90 Ma eclogite-facies metamorphism. Our results combined with previous reports for the tectonics of the Sanbagawa metamorphic rocks suggest that there are at least two eclogite-facies units with different accretion ages in the Besshi area; ca. 130 Ma unit (Besshi unit) and ca. 100–90 Ma unit (Asemi-gawa unit), which structurally contact with each other. It is likely that the older unit was subducted into a depth of over 50 km and stagnated until the younger unit was subducted to the same depth. Probably, both units were juxtaposed at a mantle depth and began to exhume to the surface at the same timing after ca. 90 Ma. The juxtaposition and exhumation process might have relation to multi-factors such as tectonic erosion along the subduction zone, shallowing subduction angle of the hotter slab, backflow in the mantle and fluid infiltration along exhumation route.