The formation depth of coesitebearing eclogite, Dabie UHPM zone, China, modified by tectonooriginal additional hydrostatic pressure

The knowledge that hydrostatic pressure is equal to the gravity value of the overlying rocks in studying a dynamic state of certain underground site is argued. It is suggested that the stress field Τ in the crust is a combination or superposition of total hydrostatic pressureP with differential stress σ, and the total hydrostatic pressureP at any point in the crust comprises two parts: one is spherical stress tensor P_G, caused by the gravity, and the other is spherical stress tensor P_S caused by tectonic stress; thereforeP could not be attributed to the gravity of overlying rocks only. The results obtained by a finite-element simulation indicate that the tectono-original additional hydrostatic pressures P_S decrease gradually from the compressive zone (P _C ^S ) to the shear zone (P _SH ^s ) and to the tensile zone (P _T ^s ), i.e. P _C ^S > P _SH ^s > P _T ^s . in the same depth. On the basis of the above-mentioned research, the method of measurement and calculation of metallogenetic depth corrected by P_S is established, i.e. first tectonic additional hydrostatic pressure P_S is removed from general hydrostatic pressureP to get P_G, gravity additional pressure, then accordingly the depth data are measured and calculated. The plastic deformation of garnet in ccesite-bearing eclogite, quartz eclogite and garnet amphibolite of the ultrahighpressure metamorphic (UHPM) complex in the Yingshan County in Dabie Mt. is studied by the transmission electron microscope (TEM). It is shown that the garnets have ductile deformation in the metamorphic condition of eclogite and amphibolite facies. Microstructures of the garnet vary greatly among coesite-bearing eclogite, quartz eclogite and garnet amphibolite. The tectonic principal stress in three-dimensional space and the tectonic additional hydrostatic pressure are reconstructed by differential stress and strain ratio (α) of garnet in minor coesite-bearing eclogite, then the gravity and thickness of overlying rocks are determined. The formation depth of ⩾ 32.09–32.106 km of the coesite-bearing eclogite in Yingshan County in the Dabie UHPM zone is obtained from difference P_G which ms fromP minus P_S P_S = (σ₁ + σ₂ + σ₃)/3. The value ofP is 2.8 GPa resulting from Qz-coes geobarmeter. This result is at all different from other researchers’ conclusion that the formation depth of eclcgite in Dabie UHPM zone is ⩾ km by the method of weight/ special weight (W/SW). The strong tectionic action makes a great contribution to the formation of eclogite in the Dabie UHPM zone at so shallow depth of about 32 km, therefore it is thought that attention should be paid to the tectono-original additional hydrostatic pressure in the study on UHPM zone.