Elastic and Seismic Properties of Dabie-Sulu Ultrahigh Pressure Metamorphic Rocks

摘要

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 orogenic belt(China) and their variations with pressure(P),temperature(T),density(ρ),Vp,Vs and mineralogical composition.Both moduli increase nonlinearly and linearly with increasing pressure at low(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/oC.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 ρ(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.