ON THE DEPTH OF THE FORMATION OF COESITEBEARING ECLOGITE OF DABIE UHPM ZONE,CENTRAL CHINA

The plastic deformation of garnet in coesite-bearing eclogite,quartz eclogite and garnet amphibolite of the ultrahigh-pressure metamorphic (UHPM) complex in Yingshan County in the Dabie Mt. is studied by the transmission electron microscope (TEM). It is shown than the garnets have undergone ductile deformation during eclogite and amphibolite metamorphic facies. Microstructures of the garnet vary greatly from coesite-bearing eclogite to quartz eclogite and garnet amphibolite.
The 3-D tectonic principal stresses and the hydrostatic pressure with a coupled tectonic stress(Ps) are reconstructed by differential stress and strain ratio (α) of garnet in minor coesite-bearing eclogite,and whereby the gravity and thickness of overlying rocks are determined. The formation depth of ≥ 32.09~32.11 km of the coesite-bearing eclogite in Yingshan County in the Dabie UHPM zone is obtained from P_G(P-P_S) where P is the pressure where the transformation of coesite phase took place,and P_S=(σ₁+σ₂+σ₃)/3. P is known to be 2.8 GPa by Qz-coes geobarometer. This result (ca 32km depth) is much smaller than 100km depth of eclogite in Dabie UHPM zone as achived by others by the method of weight/special weight (W/SW). Strong tectonism is thought to have made a great contribution to the formation of eclogite in the Dabie UHPM zome.