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首页> 外文期刊>Journal of Geophysical Research. Biogeosciences >Measured and calculated seismic velocities and densities for granulites from xenolith occurrences and adjacent exposed lower crustal sections: A comparative study from the North China craton
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Measured and calculated seismic velocities and densities for granulites from xenolith occurrences and adjacent exposed lower crustal sections: A comparative study from the North China craton

机译：测得并计算出的异质岩发生和邻近的下地壳下部碎屑颗粒的地震波速度和密度：来自华北克拉通的比较研究

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Granulites from the Neogene xenolith-bearing Hannuoba alkaline basalt and from the Manjinggou-Wayaokou exposed lower crustal section in the Archean Huai'an terrain, which occurs within and surrounds the Hannnoba basalt, provide a unique opportunity for a comparative study on petrophysical properties and composition of the lower crust represented by these two types of samples. P and S wave velocities and densities of 12 Hannuoba lower crustal xenoliths and one associated spinel Iherzolite xenolith as well as nine granulites and granulite-facies metasedimentary rocks from the Archean Huai'an terrain were measured in laboratory at pressures up to 600 MPa and temperatures up to 600 degrees C. Calculations of P and S wave velocities were also made for the same suite of samples based on modal mineralogy and single-crystal velocities whose variations with composition are considered by using microprobe analyses and velocities of end members. The measured and calculated V-p at room temperature and 600 MPa, where the microcrack effect is considered to be almost eliminated, agree within 4% for rocks from the Manjinggou-Wayaokou section and the adjacent Wutai-Jining upper crustal to upper lower crustal section. In contrast, the xenoliths show systematically lower measured V-p by up to 15% relative to calculated velocities, even if decompression-induced products of kelyphite and glass are taken into account. The lower measured velocities for xenoliths are attributed to grain boundary alteration and residual porosity. This implies that although granulite xenoliths provide direct information about lower crustal constitution and chemical composition, they are not faithful samples for studying in situ seismic properties of the lower crust in terms of measured velocities due to alterations during their entrainment to the surface, which changes their physical properties significantly. In this respect, granulites from high-grade terrains are better samples because they are not subjected to significant changes during their slow transport to the surface and because physical properties depend primarily on mineralogy in addition to pressure and temperature. On the other hand, calculated velocities for granulite xenoliths are consistent with velocities for granulites from terrains, suggesting that they can be also used to infer lower crust composition by correlating with results from seismic refraction studies. [References: 53]
机译：含新近纪异种岩的汉诺巴碱性玄武岩和太古淮淮地区裸露的下地壳剖面中的粒岩，出现在汉诺巴玄武岩内和周围，为比较岩石物性和成分提供了独特的机会这两种类型的样品代表的下地壳。在实验室中，在高达600 MPa的压力和最高温度下，测量了12个汉诺巴下地壳异质岩和一种伴生的尖晶石Iherzolite异质岩以及9种来自太安淮安地形的花岗石和花岗石相沉积沉积岩的P和S波速度和密度。到600摄氏度。还根据模态矿物学和单晶速度对同一组样品的P波和S波速度进行了计算，并通过使用微探针分析和末端成员的速度考虑了成分的变化。在室温和600 MPa下测量和计算的V-p（认为几乎消除了微裂纹效应），对于从曼景沟-药窑口段和相邻的五台-济宁上地壳到上下地壳段的岩石而言，其一致性在4％以内。相比之下，即使考虑了减压诱导的水晶石和玻璃的形成，异种石也显示出相对于计算出的速度有系统地降低了高达15％的V-p。异种材料测得的较低速度归因于晶界变化和残余孔隙率。这意味着尽管粒状异种岩提供了有关下地壳组成和化学成分的直接信息，但它们并不是研究下地壳的原位地震特性的真实样本，因为它们在夹带到地面时会发生变化，从而根据测得的速度进行测量。物理性能显着。在这方面，来自高级地形的花岗石是更好的样品，因为它们在缓慢运输到地面的过程中不会发生显着变化，并且物理性质除压力和温度外还主要取决于矿物学。另一方面，花岗石异种岩的计算速度与地形上的花岗石的速度一致，这表明它们还可以通过与地震折射研究的结果相关来推断下地壳成分。 [参考：53]

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