Low-grade metamorphism around the down-dip limit of seismorgenic subduction zones : Example from an ancient accretionary complex in the Shimanto Belt, Japan
Low-grade metamorphism around the down-dip limit of seismorgenic subduction zones : Example from an ancient accretionary complex in the Shimanto Belt, Japan
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地震俯冲带下倾极限周围的低级变质作用：以日本四万十带的古代增生杂岩为例
DOI：
10.1016/j.tecto.2011.02.010
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发表时间：
2011
期刊：
影响因子：
2.9
通讯作者：
et al.
中科院分区：
文献类型：
--
作者：
Kameda;J.;Raimbourg;H.;Kogure;T.;et al.
利用综合透射电子显微镜-能量色散x射线能谱和x射线衍射分析方法，研究了日本九州岛东带古增生杂岩深部低变质过程中粘土矿物的反应。所分析的沉积（北川单元）包含一个早期的次水平板岩解理。伊利石结晶度数据和云母b尺寸表明，变质变形的条件为安次—副次级和中等压力。解理的形成与两个涉及粘土矿物的反应有关：(1)从原始沉积组构继承下来的1m基质云母重结晶，沿解理面形成厚的、无缺陷的2m1包；(2)由7Å berthierine生成绿泥石。根据成分数据及其相对丰度，粘土相间的平衡方程表明，基质云母的分解导致沿解理面形成共生云母和绿泥石，没有明显的元素外流。虽然数据没有显示层状硅酸盐的模态增加，但绿泥石和云母沿解理面生长可能对石料的流变特性有很大影响，并可能与~350°C时地震-地震转变的发生有关。
Reactions involving clay minerals during low-grade metamorphism at the depth of an ancient accretionary complex in the Shimanto Belt, Kyushu, Japan, were studied by integrated transmission electron microscopy–energy dispersive X-ray spectroscopy and X-ray diffraction analyses of the bulk rock and clay fraction. The analyzed metasediment (the Kitagawa unit) contain an incipient sub-horizontal slaty cleavage. Illite crystallinity data and mica b dimensions indicate that the conditions of metamorphic deformation were anchizone–epizone grade and intermediate pressure. Cleavage formation was linked to two reactions involving clay minerals: (1) the recrystallization of 1M-dominant matrix mica, inherited from the original sedimentary fabric, into thick, defect-free 2M1packets along cleavage planes; and (2) the formation of chlorite from 7Å berthierine. Balanced equations among the clay phases, based on compositional data and their relative abundance, suggest that the decomposition of matrix mica resulted in the formation of paragenetic mica and chlorite along the cleavage planes, without significant elemental outflux. Although a modal increase in phyllosilicates is not indicated by the data, the growth of chlorite and mica along cleavage planes may have a large influence on the rheological properties of a décollement and may be related to the occurrence of the seismic–aseismic transition at ~350°C.