Xiaomei Ductile Shear Zone on Hainan Island in a Nanoscale Perspective
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摘要: 为查明构造性质不明、空间位置不清楚的南海北缘琼南缝合带西段——九所－陵水断裂带，采集断裂带东段小妹韧性剪切带中花岗岩、石榴石石英片岩和花岗质片麻岩3类样品，探讨其纳米尺度特征.扫描电镜(scanning electron microscope，SEM)观测结果表明：这3类岩石存在多种纳米结构和构造，结合野外实践得到的该剪切带纳米颗粒的发育程度与其所受剪切作用有关.纳米颗粒的形成机制可能有：(1) 剪切力作用下层状硅酸盐热分解；(2) 颗粒塑性变形后发生脆性破裂，再经剪切作用研磨而成.与台湾太鲁阁深大断裂带中的韧性剪切带岩石样品进行SEM测试结果对比，发现其纳米颗粒特征和研究区具可比性，据此推测小妹韧性剪切带在区域构造归属上可能与九所－陵水深大断裂带密切相关.Abstract: The western Qiongnan suture zone, i.e., Jiusuo-Lingshui fault belt, at the northern margin of the South China Sea, is important in tectonics. To better understand the tectonic evidences and ductile shear belts, a study is done in this area. Three types of rocks are sampled from the Xiaomei ductile shear zone on Hainan island, including granite, garnet quartz schist and granitic gneiss. Their shear surfaces were observed and analyzed through scanning electron microscope (SEM) at nanoscale. The results show multiple nano-textures and nanostructures in all three kinds of rocks and a positive relationship between the development degree of nanoparticles and the stress of the rocks. The authors propose several possible formation machanism of nanoparticles on the shear surfaces in this area, which may be thermal decomposition of sheet silicates under shear stress, or brittle deformation of minerals (especially feldspar with mechanical twins) after ductile deformation, then followed by crushing and grinding of the particles to nanoscale under shear force. Samples from the Taroko ductile shear belt, a part of the Taroko fault zone in Taiwan show similar nano characteristics to the Xiaomei ductile shear zone, implying both may have been in the same tectonic setting of a deep fracture as the Jiusuo-Lingshui fault belt. This study can therefore provide important observational facts for determining the structural properties and spatial positions of the Jiusuo-Lingshui fault belt.
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图 1 陵水县小妹研究区区域地质图(改自1: 250 000海南省地质图)和空间位置
图中黑框为研究区位置，位于小妹水库堤坝下游河床中.AA′代表九所-陵水断裂带的可能实际露头(蓝色线)，BB′代表地球物理场资料给出的九所-陵水断裂带位置(绿色线).1.中二叠二长花岗岩；2.晚二叠二长花岗岩；3.中三叠二长花岗岩；4.早白垩二长花岗岩；5.晚白垩二长花岗岩；6.中二叠花岗岩；7.中三叠花岗岩；8.晚白垩花岗岩；9.晚二叠闪长花岗岩；10.晚白垩闪长花岗岩；11.中三叠石英二长岩；12.中侏罗石英二长岩；13.早白垩石英二长岩；14.中三叠石英正长岩；15.早白垩石英正长岩；16.中三叠钾长花岗岩；17.早白垩细晶斑岩；18.晚白垩花岗斑岩；19.长城系；20.第四系北海组；21.第四系烟墩组；22.第四系全新统；23.断层；24.糜棱岩带
Fig. 1. The regional geologic map and spatial location of Xiaomei research area, Lingshui County
图 2 样品采集位置剖面
a.花岗岩；b.石榴石石英片岩；c.花岗质片麻岩；蓝色实线为条带界限，蓝色虚线表示岩石间的接触关系
Fig. 2. The cross-section with sample collection spots
图 4 石榴石石英片岩SEM和能谱照片
a.纳米颗粒约50 nm，紧密镶嵌成条带状，条带之间有脊槽(粗箭头所示)，小箭头代表纳米颗粒突出聚集形成的滑脊，谱图 1和2显示信号强度大的Ti；b.纳米单体经过韧性变形，即异化团聚形成复体，大小达到600 nm以上，单体基本都融入到复体中，其直径大于100 nm，复体间呈松散堆积状；c.单体经过轻微异化，紧密镶嵌似葡萄状，层层压实，球度差，但圆度较佳，约80 nm，其上喷洒的金颗粒为13 nm.注：喷金均为100 s，金颗粒直径约小于10 nm.重量百分比、原子百分比和化合物百分比的单位均为“%”
Fig. 4. The SEM and EDS photos of garnet quartz schist
图 5 花岗质片麻岩SEM和能谱图像
a.花球一样的复体颗粒粘在边缘上，横向上具有优选排列，能谱3显示Fe含量高；b.似松果一样的复体，在破裂的洞中生长，从壁上的小松果，可以看出其黏性相当好，说明这是原地生长，而非搬运滚动；c.平直的纳米棒定向性非常明显，长度也较剥落后的更长，最先应该是平直的，摩擦作用增强后矿物剥落.样品喷金时间为100 s，金颗粒直径小于10 nm.重量百分比、原子百分比和化合物百分比的单位均为“%”
Fig. 5. The SEM and EDS photos of granitic gneiss
图 6 台湾太鲁阁地区岩石剪切表面SEM照片
a.副片麻岩表面SEM下观测到的纳米颗粒可能为粒化阶段产物；b.片麻岩表面SEM下观测到梅花状构造，对应纳米颗粒异化阶段
Fig. 6. The nano texture photos of rocks' shear surfaces from Taroko ductile shear zones under the SEM
表 1 样品野外和光学显微镜下特征
Table 1. Sample characteristics observed through field and the optical microscope
花岗岩 石榴石石英片岩 花岗质片麻岩 手标本现象 风化严重，新鲜面浅灰色，不显示条带 暗蓝色，致密坚硬；表面NNE向线理密集，侧面见层理；肉眼几乎识别不出矿物，抛光后可见颜色深浅不一的条带，远离表层的条带中石榴子石多 浅灰白色，花岗质条带，片麻状构造，NNE向排列 矿物种类 斜长石、石英为主，其次为黑云母、白云母 斜长石，石英，角闪石为主；长石大都绢云母化，角闪石多绿帘石化和绿泥石化，黑云母基本蚀变成绿泥石；且含较多石榴子石、锆石、榍石等副矿物 长石、石英、黑云母为主，部分长石绢云母化，少量黑云母绿泥石化 镜下矿物特征 颗粒粗大，长石多白云母化，一些为绢云母化，较多黑云母蚀变成绿泥石 颗粒极为细小，辉石的绿泥石化明显，而呈碎斑，直径0.2 mm，碎基0.05 mm；锆石和榍石也被绿泥石包围，细粒变晶结构；定向构造 颗粒粗大，结晶程度高；粒状变晶结构；云母等定向分布构成片麻理 变质变形 无定向性；镜下可见黑云母和斜长石受力弯曲，少量石英有波状消光 ac面上半定向-定向，片理发育，变质相主要为绿片岩相-低角闪岩相 ac面颗粒细小并呈定向排列，少量石英波状消光；ab面颗粒更细且石英波状消光更强烈普遍 
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表 2 3种岩石剪切表面纳米颗粒特征对比
Table 2. Comparison of nanoparticles from three kinds of rock surfaces
花岗岩 石榴石石英片岩 花岗质片麻岩 纳米颗粒产量 中间 最多 最少 纳米颗粒主要发育阶段 粒化、变异 粒化、变异、团聚、再生都有 团聚？ 颗粒聚集形态 紧密镶嵌、土堆状 层状、堆积状、条带状 堆积状为主 颗粒结构定向性 几乎不定向 主要显示定向性 几乎无定向 可能反映的受力情况 挤压和剪切 挤压和剪切 少量剪切
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