Formation of Amphibole and Clinozoisite-Epidote in Eclogite owing to Fluid Infiltration during Exhumation in a Subduction Channel

Abstract

Two eclogites, one from the southern Dabie Shan, China, and the other from the western Erzgebirge, Germany, were studied because of amphibole and clinozoisite^epidote formation either as typical porphyroblasts or as oriented fine-grained matrix minerals. A nearly isobaric prograde path was derived for the eclogite from the Erzgebirge, mainly on the basis of garnet zonation and the reconstruction of P^T pseudosections in the system SiO2^TiO2^ Al2O3^MgO^MnO^FeO^CaO^Na2O^K2O^H2O^O2. Peak P^T conditions were 2·8 GPa and 6708C. The retrograde path shows significant cooling during pressure release. In contrast, the Dabie Shan eclogite contains sparse Mg-rich garnet cores pointing to a granulite stage. Subsequently, peak-pressure conditions of 3·5^4·0 GPa were reached at 6208C, followed by a retrograde path that was initially nearly isothermal and then similar to that of the Erzgebirge eclogite. Most of the amphibole and clinozoisite^ epidote grew during early retrogression in the pressure interval 1·8^2·3 GPa (Erzgebirge) or 2·4^2·7 GPa (Dabie Shan) owing to infiltration of hydrous fluids amounting to 1wt % and 0·5 wt % H2O, respectively. As a result of this hydration omphacite was partially decomposed, but the remaining omphacite demonstrates that infiltration of H2O essentially ceased at 1·8 GPa (Erzgebirge) or 2·4 GPa (Dabie Shan). The fluid infiltration and hence the growth of amphibole and clinozoisite^epidote is proposed to have occurred in a deep-seated subduction channel during ascent of eclogitic material. Subsequently, the eclogites were captured between colliding continental plates and further exhumed in an exhumation channel.