苏鲁-大别超高压变质岩的弹性力学性质与密度的关系

我们采集了31块来自中国大陆科学钻探(CCSD)主孔和预先导孔岩芯以及29块苏鲁-大别地区地表露头的典型超高压变质岩,在10～850 MPa的静水压力和室温下测量了60块标本的P波与S波速度,分别建立了纵横波速和围压之间的定量关系,并在此基础上计算了不同压力下各岩石标本的弹性参数即杨氏模量(E)、剪切模量(G)、体模量(K)和泊松比(v),着重探讨了超高压变质岩的弹性力学性质与密度之间的关系.这些最新成果将为超高压变质岩地区以及新老俯冲带地震波资料的解释提供关键的理论和实验基础.

Correlation between elastic properties and density for Sulu-Dabie UHP metamorphic rocks

Elastic properties of rocks provide important constraints for the chemical composition,structure and physical state of the Earth's interior.We have experimentally determined both P-and S-wave velocities of 60 typical UHP metamorphic rock samples among which 31 were collected from the CCSD(Chinese Continental Scientific Drilling)main and pre-pilot holes and 29 from the surface outcrops in the Sulu-Dabie orogenic belt(China)at ambient temperature and hydrostatic confining pressures up to 850 MPa.The velocity-pressure curves were well described by a four-parameter expression.The elastic constants(e.g.,Young's modulus E,shear modulus G,bulk modulus K and Poisson's ratio v)of each sample were computed from P-and S-wave velocities measured during depressurization.At each given pressure,the correlation between an elastic modulus(M)and density(p)can be well fitted by an empirical equation:M=ap~3 bp~2 cp,where a,b and c are adjustable coefficients.Except of monomineralic rocks such as quartzite,serpentinite,anorthosite,limestone and marble and serpentinized peridotite,all the other types of rocks have their Poisson's ratios falling along an upward convex trend line determined from the correlations between elastic moduli and density.Poisson's ratios increases as lithology changes from granite,schist,felsic gneiss,through diorite-syenite,intermediate gneiss and metasediment,to gabbro-diabase,amphibolite,and mafic gneiss,and then decrease as the rocks become ultramafic in composition(i.e.,pyroxenite and peridotite).Eclogite has a higher density but a lower Poisson' s ratio than peridotite.The increase in v with rock type,ranging from felsic gneisses to metagabbro,is correlated with systematic variations of quartz and feldspar contents in the rocks.An increase in quartz content induces a decrease in v while an increase in feldspar content causes an increase in v.An increase in the anorthite content of plagioclase also increases the Poisson's ratio of bulk rock.Serpentinization can significantly raise the Poisson' s ratio of peridotites because serpentinite possesses an extremely high value of v.However,the contribution of each constituent mineral to the bulk Poisson's ratio of a polymineralic rock can be analyzed for the moment only in a qualitative manner because mixture rules for Poisson's ratio are unavailable.The results provide a complete set of basic information for the interpretation of field seismic data from the UHP metamorphic terranes and modern and ancient subduction zones.