Identification of α-PbO2-Type TiO2 in Jadeite Quartzite from Shuanghe, Dabie Mountains, China
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摘要: 大陆深俯冲深度对于了解大陆碰撞造山带中超高压变质岩的折返动力学具有重要意义.2005年笔者在中国大别山石马地区含柯石英榴辉岩绿辉石中发现了α-PbO2型TiO2晶体, 最近笔者用高分辨透射电子显微镜和能量散射X-射线谱仪测试技术在中国大别山双河地区超高压硬玉石英岩硬玉中鉴别出纳米级α-PbO2型结构的TiO2天然超高压相.α-PbO2型TiO2晶体的保存, 为超高压变质作用(6~7GPa, 730~870℃) 提供了新的证据, 同时指示陆壳物质的俯冲深度大于170~200km, 也指示了俯冲陆壳到地表的抬升, 虽然其速率还不能确定, 但可能是相当快速的.Abstract: The depth of the continental deep subduction is of vital importance for better understanding of the dynamics of exhumation process of ultrahigh pressure metamorphic rocks in the continental collision orogen belt. The crystal of α-PbO2-type TiO2 was found in omphacite from coesite-bearing eclogite at Shima in the Dabie Mountains, China. A natural ultrahigh-pressure phase of titanium oxide with α-PbO2-structure has been identified through high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectrometer (EDS) in jadeite quartzite at Shuanghe in the Dabie Mountains, China. The reservation of nano-scale lamellae of the α-PbO2-type polymorph of TiO2 provides a new evidence of ultra high-pressure, metamorphism at 6-7GPa, 730-870℃. It implies a burial of continental crustal rocks to depth between 170-200 kilometers or deeper, and also implies exhumation of subducted continental crust to the Earth′s surface was presumably rapid, though at an uncertain rate.
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图 1 大别山地区岩石构造单元图(董火根和郭振宇, 1996)
1. 各构造单元: Ⅰ—北大别岛弧杂岩, Ⅱ—中大别碰撞杂岩, Ⅲ—南大别活化盖层和扬子大陆基底, Ⅳ —古生界弧后盆地, Ⅴ—扬子大陆前陆逆掩带; 2. 超高压变质岩; 3. 镁铁-超镁铁质岩; 4. 中生代花岗岩基; 5. 晚中生代碱性花岗岩; 6. 主要断裂带
Fig. 1. Tectonic units in Dabie Mountains
图 2 金红石(Rut) 和α-PbO2型TiO2纳米相的HRTEM像(a), 相应的[111]R (Rut) /[110]α (α-PbO2型TiO2) 带轴的SAED图(b) 以及其指标化示意图(c)
图 2c中空心圆、实心圆和小实心圆分别表示金红石双晶、金红石基体(hklR) 和α-PbO2型TiO2结构(hklα) 的电子衍射斑点
Fig. 2. HRTEM images (a), SAED patterns (b) and the schematic indexing (c) of rutile and α-PbO2 type TiO2 nanometer phase, corresponding to zone axis [111]R (Rut) /[110]α (α-PbO2 type TiO2) respectively
图 5 金红石、金红石双晶和α-PbO2型TiO2纳米相5个不同带轴的SAED图及其指标化示意图
(a) [31 1]R/[310]α; (b) [100]R/[100]α; (c) [511]R/[510]α; (d) [311]R/[310]α; (e) [211]R/[210]α; 指标化示意图中各种符号(“o”、“·”和“·”) 所表示的内容与图 2c相同
Fig. 5. SAED patterns and schematic indexing of rutile, rutile twin andα-PbO2type TiO2nanometer phase obtained by tilting the crystal about[011]R/[001]α
图 6 由5张(图 5a-5e)SAED(一套弱的电子衍射斑点)及其夹角关系构筑成的α-PbO2型TiO2[001]α带轴的二维倒易点阵平面图
Fig. 6. A diagram of the two dimensional reciprocal lattice plane of α-PbO2 type TiO2[001]α zone axis determined from the five SAED patterns(a set of weak electr on diffraction spots) in Fig.5 and their inter ang led relationships
表 1 金红石和α-PbO2型TiO2的晶胞参数
Table 1. Crystal structures of rutile and α-PbO2-type TiO2
表 2 α-PbO2型TiO2的晶面间距
Table 2. Interplanar spacings for α-PbO2-type TiO2
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