Elastic and Seismic Properties of Dabie-Sulu Ultrahigh Pressure Metamorphic Rocks |
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| Authors: | WANG Qian JI Shaocheng SUN Shengsi Matthew H SALISBURY Hartmut KERN |
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| Institution: | 1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;2. Département des Génies Civil, Géologique et des Mines, école Polytechnique de Montréal, Montréal H3C 3A7, Canada;3. State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;4. Geological Survey of Canada‐Atlantic, Bedford Institute of Oceanography, Dartmouth B2Y4A2, Canada;5. Institul fur Geowissenschaften, Universitat Kiel, 24098 Kiel, Germany |
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| Abstract: | Lamé modulus (λ) and shear modulus (μ) are among the most important, intrinsic, elastic constants of rocks. Using λ and μ could be much more advantageous than using P‐ and S‐wave velocities (Vp and Vs). Here we quantified these equivalent isotropic elastic moduli for 115 representative rocks from the ultrahigh pressure (UHP) metamorphic terrane of the Dabie‐Sulu erogenic belt (China) and their variations with pressure (P), temperature (T), density (p), Vp, Vs and minera logical composition. Both moduli increase nonlinearly and linearly with increasing pressure at low (<200–300 MPa) and high (>200–300 MPa) pressures, respectively. In the regime of high pressures, λ and μ decrease quasi‐linearly with increasing temperature with temperature derivatives dλ/dT and dμ/dT generally in the range of –10 × 10–3 to –1 × 10–3 GPa/°C. Dehydration of water‐bearing minerals such as serpentine in peridotites and chlorite in retrograde eclogites results in an abrupt drop in λ while μ remains almost unchanged. In λ‐ρ, μ‐ρ and λ‐μ plots, the main categories of UHP rocks can be characterized. Serpentinization leads to significant decreases in μ and λ as serpentine has extremely low values of λ, μ and ρ. Eclogites, common mafic rocks (mafic gneiss, metagabbro and amphibolite), and felsic rocks (orthogneiss and paragneiss) have high, moderate and low μ and λ values, respectively. For pyroxenes and olivines, λ increases but μ decreases with increasing Fe/Mg ratios. For plagioclase feldspars, both λ and μ exhibit a significant positive correlation with anorthite content. SiO2‐rich felsic rocks and quartzites are deviated remarkably from the general trend lines of the acid‐intermediate‐mafic rocks in Vs‐ρ, μ‐ρ, λ‐Vp, λ‐Vs and μ‐λ diagrams because quartz has extremely low λ (～8.1 GPa) and p (2.65 g/cm3) but moderate μ (44.4 GPa) values. Increasing the contents of garnet, rutile, ilmenite and magnetite results in a significant increase in the λ and μ values of the UHP metamorphic rocks. However, either λ or μ is insensitive to the compositional variations for pyralspite (pyrope‐almandine‐spessartine) solution series. The results provide potentially improved constraints on characterization of crustal composition based on the elastic properties of rocks and in situ seismic data from deep continental roots. |
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| Keywords: | seismic properties elastic constants ultrahigh pressure metamorphic rocks Dabie-Sulu deep continental roots |
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