下地壳麻粒岩包体波速测定及其地质意义

Measurements of Elastic Wave Velocity of Lower Crustal Xenoliths and Their Geological Implications

下地壳岩石的波速测定可以提供有关下地壳物质组成及物理性质的重要信息。但是,由于下地壳难于直接采样,因此常常利用目前出露地表的古老麻粒岩地体样品来代表下地壳,但所获结果常常与实际情况不符。由新生代玄武岩携带到地表的下地壳包体则为我们提供有关下地壳的直接信息。本文对采自河北汉诺坝玄武岩中的下地壳包体进行了密度和高温高压波速测定。下地壳由不同类型的麻粒岩相岩石组成,其密度和波速有较大的差异,因此所获波速不能直接与地震测深得到的下地壳波速进行对比,而需要进行波速的温度、压力校正,并进行具体的分析。本文得到下地壳包体的原地波速6.13～6.90km/s,与该区地震资料得到的结果十分接近,并探讨了下地壳低速层的可能成因。

The measurements of the elastic wave velocity of lower crustal rocks may provide valuable information on the composition and physical properties of the lower crust. The samples from old granulite terrane cropped out on the earth surface are often taken as the representatives of the lower crust, because of the difficulty of directly sampling the lower crust. The obtained results, however, are often in compatible with the actual conditions, as the old granulite terrane had experienced long period of multi-stage deformation and metamorphic processes. In Contrast, the lower crustal xenoliths brought up by Cenozoic basalts onto the surface may provide us with direct information of the lower crust. The measurements of density and elastic wave velocity of lower crustal xenoliths from Hanuoba basalts, Hebei Province, China under high temperature and pressure conditions are presented in this paper. The study shows that the lower crust of the area consists of various types of granulite facies rocks. Their density and velocity are different significantly from each others. The measured velocity of lower crustal xenoliths, therefore, can not be directly compared with the velocity structures of the lower crust obtained from deep seismic sounding, without temperature and pressure corrections. An in-situ velocity of lower crust of 6. 13 - 6. 90 km/s is obtained from this study, which is comparable well to the seismic data from the studied area. In addition, the origin of the low velocity layer is discussed as well.