Sm-Nd age determination of eclogites from Sredna Gora Mountains – HP metamorphic event or final retrogression

Abstract

The eclogites from Sredna Gora Mts. occur as small isolated bodies enclosed in amphibolites, two-mica schists and migmatized gneisses. They have tholeiitic composition and characteristics of MOR basalts. Three equilibrium mineral associations are distinguished in eclogites: (1) high-pressure (HP) – Grt+CPx(Omp)+Zo+Rt; (2) middle-pressure-high temperature (MP/HT) – Opx+CPx+Pl and middle pressure – middle temperature (MP/MT) presented by Amp+Pl+Qzt+Ep+Ilm+Ttn. The P-T conditions of HP stage reached values of 1.6–2.02 GPa and 650–690 °С. After the HP stage and CPx-Pl symplectite formation, the eclogites underwent a short-lived granulite facies overprint under temperatures similar to those of the HP event. The P-T conditions of the retrograde amphibolite facies overprint are in the range 620–650 °C and 0.6–0.8 GPa. The obtained age of 319±28 Ma by Sm-Nd garnet+whole rock method is interpreted as age of the final retrograde metamorphism under P-T conditions of high temperature amphibolite facies.