部分熔融对高喜马拉雅片麻岩电导率性质影响的实验研究

Abundant geological surveys in ancient mountain belts suggested that the Himalayan orogenic belts crust in the south of the Tibetan Plateau, which was grown from subduction and collision between the Indian and Eurasian continents, experienced crustal-scale metamorphism and partial melting in the past of ~20 Ma, then extruded plentiful various kinds of leucogranites. The locations of them correspond to the crustal high conductivity anomalies in the depth of 10-30 km, which were obtained from Magnetotelluric data recently. It’s regrettable that we cannot understand the compositions and structures of crustal high conductivity anomalies in the Himalayan orogenic belts until now because of missing the information about the electrical behaviors of partial melting. On the basis of the above summaries, using AC impedance spectroscopy, the in-situ experimental investigation on the electrical conductivity of Greater Himalayan gneisses during partial melting processes under the temperature and pressure which correspond to the 15-25 km depth of the Tibet Plateau is purposed for this application. Combining with the analysis results of compositions and structures of samples before and after experiments, the relationships between status parameters of experimental samples (i.e. temperature, pressure, melt fraction and water concentrations of the melts) and the effective electrical conductivity of partially melted two-phase bodies (i.e. the mixture of unmelted gneisses and leucogranitic melts) will be well-constrained after acquiring mixed samples’ and natural samples’ experimental data. Therefore, probing the origin of remarkable crustal electrical anomalies in the Himalayan orogenic belts, investigating melt fractions and distribution, studying the magmatic evolution of the Greater Himalayan Sequence can be assessed in the future.

大量的地质学证据表明由印度-欧亚大陆的俯冲和碰撞作用所形成的喜马拉雅造山带地壳在过去的~20 Ma内发生了一系列变质-深熔作用并产生大量不同类型的花岗岩，与现今大地电磁探测所得到的10~30 km深的地壳高导层数据相对应。但是，由于缺乏对部分熔融电导率性质的有效约束，喜马拉雅造山带地壳高导层的物质组成和结构构造到目前为止仍然知之甚少。基于此，本项目拟申请在青藏高原地下15-25 km深度的温度和压力范围内，利用交流阻抗谱法对部分熔融的高喜马拉雅片麻岩开展电导率的原位实验测量。通过获取混合样品和天然样品的实验数据，结合实验前后样品成分和结构的分析测试结果，系统性研究温度、压力、部分熔融程度和熔体含水量对部分熔融的二相体（即未熔片麻岩-花岗质熔体的均匀混合体）有效电导率的影响，从而为探索喜马拉雅造山带地壳高导层的成因机制，推断高喜马拉雅岩系部分熔融程度、熔体分布状况及岩浆演化过程带来帮助。

在项目执行期间，利用中国地震局地质研究所地震动力学国家重点实验室活塞-圆筒式压机，在高温高压条件下对含有7 vol.%淡色花岗质熔体的混合物进行了探索性的电导率实验测量工作。另外一项重要工作是用高导的石墨配比相对绝缘的造岩矿物，制备石墨-造岩矿物的二相混合体来模拟高导熔体和低导固体所组成的部分熔融二相混合体，在干燥、2~5 MPa、1mm/s滑动速率下开展长距离(1~5 m)的摩擦-导电性质实验测量。一共包括3个系列的实验样品：（1）不同比例石墨粉末与600目粒径石英颗粒混合物；（2）相同6 wt.%比例的石墨粉末与不同粒径石英颗粒混合物；（3）不同比例石墨粉末与500目粒径方解石颗粒混合物。最终得到了共计37组有效实验数据，揭示了石墨-造岩矿物混合物摩擦系数-电导率-滑动位移之间的关系。结合显微结构照片，拟合相关实验数据曲线，提出上述3个系列样品在经历摩擦滑动后所分别对应的结构演化模型与摩擦-导电机制。除此之外，得到以下结论：(1) 断层泥的碳质可能会使成熟断层主滑带的力学性质更加复杂，互相连通的石墨薄片产生的断层剪切带高导层不能成为成熟断层滑动减弱行为的可靠指标；(2) 野外地电阻率台站应平行于地震断层走向方向布设，以期能够更大程度地覆盖断层面与其内部的局部地震成核区；(3) 以熔体代表的高导相在二相混合体中的占比达到3.4 vol.%以上时能够解释喜马拉雅造山带的地壳高导层。本项目的研究成果能够为深入了解石墨对地震断层带摩擦滑动中所起到的作用、指导地震地电阻率前兆观测台站的布设和解释地下高导层的成因机制带来较大帮助。