Abstract The rheological characters of omphacites in Dabie ultra‐high‐pressure eclogite have been studied in terms of fabric, dislocation and micro‐structures. 1. The eclogite has undergone high‐temperature deformation, thus forming omphacite lattice preferred orientation. In addition to creep dislocation, the omphacite ductile deformation may have other mechanisms, such as diffusion creep and grain boundary migration. 2. The main‐phase deformation of eclogite is coaxial, but asymmetry strain also exists due to strain partitioning in the Dabie orogenic belt. 3. The twin measured by the universal stage is (100), indicating that omphacite high‐T deformation was superimposed by low‐T deformation. 4. Subgrain structure is common in omphacite, but the deformation features of the omphacites in the Shuanghe area and Bixiling area are different, the latter being dominated by dynamic recrystallization. 5. The Flinn plots show that the strain of omphacite belongs to the constriction ellipsoid and stretching strain, which is similar to the result of the omphacite fabric analysis.
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 Hannuoba 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°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.
Velocity data from five published seismic refraction profiles traversing the Dabie Sulu area are summarized. The data are combined with seismic velocity measurements of regional eclogites and associated middle and lower crustal rocks at pressures and temperatures up to 600 MPa and 600 ℃ to study the seismic velocity structure and composition of the region. The Dabie Sulu UHP metamorphic area shows a four layered crustal structure. Its crustal thickness is thinner than the average of Eastern China and global continental crust. Both the upper and middle crusts in the Dabie Sulu are dominated by felsic gneisses with eclogite 10%. Eclogite may increase in proportion with depth, making up 24 44% and 41 68% of the upper lower and lowermost crusts, respectively, in addition to felsic and mafic granulites. The amphibolite facies metamorphism and typical middle crustal velocity of the dominant felsic gneisses suggest a middle crustal exposure level for the majority of the Dabie Sulu.
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