Effect of partial melting on Vp and Vs in crustal enclaves from Mazarrón (SE Spain)

The combination of compressional and shear wave velocity is an important tool in discriminating rock types and identifying residing melts within the continental crust. Here we report the laboratory measurements for Vp and Vs obtained in varying conditions up to those exceeding the beginning of melting (950 degrees C at 500 MPa confining pressure) on two residual garnet-biotite-sillimanite-cordierite-spinel metapelitic enclaves recovered from the Neogene dacites of Mazarron (SE Spain). The enclaves preserve widespread interstitial rhyolitic glass as evidence of primary melt extraction. At 500 MPa, the experimental Vp ranges from 721 divided by 7.46 km s(-1) at room temperature to 5.44 km/s at 950 degrees C The mean Vs is 4.5 km/s at room temperature with shear-wave splitting of 0.25 0.3 km/s, measured along three mutually orthogonal directions. When temperature increases, the Vs evidences a reversible slope change at about 650 degrees C, and the shear-wave splitting reduces to zero (isotropic material) at 850-950 degrees C, where the sample Vs is similar to 3.0 km/s. The Vp anisotropy is 7-10% up to 700 degrees C increasing to similar to 20% at 950 degrees C, while the Vs anisotropy continuously decreases with temperature from 5% to 7% at room temperature to zero at 950 degrees C. No mineral reactions are observed up to 650-700 degrees C; however, microstructure equilibrates due to the relaxation of the primary glass at the glass transition temperature. Between 850 degrees C and 950 degrees C, a new melting reaction is observed producing biotite + spinel + ilmenite + plagioclase + melt. At melting, the rock becomes isotropic for Vs, and Poisson's ratio increases to 0.30. Our seismic data show that the seismic signature of high grade metapelitic rocks is similar to that of mafic materials. The evolution of Vp and Vs at melting conditions is compatible with the geophysical data of the area, supporting the hypothesis of the current existence of anatectic melts at intermediate depths of the Alboran domain.