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Deformation behavior and inferred seismic properties of tonalitic migmatites at the time of pre-melting, partial melting and post-solidification
  • Yongjiang Liu,
  • Yilun Shao,
  • Sandra Piazolo
Yongjiang Liu
Key Lab of Submarine Geoscience and Prospecting Techniques, MOE, College of Marine Geosciences, Ocean University of China

Corresponding Author:liuyongjiang@ouc.edu.cn

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Yilun Shao
College of Earth Sciences, Jilin University,
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Sandra Piazolo
School of Earth and Environment, University of Leeds
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As seismic data from the lower crust becomes more readily available, it is import ant to link seismic properties to the ongoing processes within lower crustal evolution. It includes high temperature, pre- and post-migmatisation solid state deformation as well as melt-present deformation condition. We selected two tonalitic migmatites with variable former melt content (one metatexite and one diatexite) from the lower crustal Daqingshan area, northern North China Craton (NCC) to assess the link between seismic properties and rock structure and rheology. Field observation along with microstructural features suggest that the character of hornblende and plagioclase within the residuum of the metatexite can be used to derive information regarding the pre-migmatisation deformation. Residuum’s plagioclase CPO (crystallographic preferred orientation) is consistent with high temperature dislocation creep as the main deformation mechanism; similarly, hornblende shows a strong CPO related to dislocation creep. During syn-melt (melt present) conditions, phenocrysts of plagioclase in the metatexite’s neosome and K-feldspar and peritectic hornblende in the diatexite’s neosome are present. The rheology of the rock was dominated by melt; hence is inferred to follow Newtonian flow. After melt crystallization deformation is minor but again dominated by dislocation creep. For seismic properties (seismic velocity, anisotropy, Vp/Vs ratio, etc.), in pre- and post-melt conditions, migmatites have normal values. While in syn-melt condition, seismic velocities have a greater decrease, Vp/Vs ratios have a great increase, seismic anisotropies are unusually high.