Infrared spectra in the region of 3 000 to 4 000 cm-1 have been measured by an FTIR spectrometer on minerals in UHP eclogites from the Bixiling and Shuanghe areas, the Dabie Mountains. It is shown that omphacite is the most important hydrous mineral (100-200 μg/g) in UHP eclogites. Garnet and quartz contain virtually no H2O or OH. Accessory mineral rutile contains a significant amount of OH (>1 000 μg/g). Coesite gives no indication of the presence of OH but apparent H2O bands. The results have considerable implications for existing forms and contents of hydroxyl in the continental deep subduction zone, the eastern Dabie Mountains.
Geological, geophysical and geochemical evidence for lower crustal delamination in the Qinling-Dabie oro-genic belt is presented and a chemical geodynamic model for lower crustal delamination is developed. The synthetic results suggest that eclogite from the Dabie-Sulu ultrahigh pressure metamorphic belt is the most likely candidate as the de-laminated material, and that a cumulative 37-82 km thick eclogitic lower crust is required to have been delaminated in order to explain the relative deficits in Eu, Sr, Cr, Ni, Co, V and Ti in the present total crust composition of the Qin-ling-Dabie orogenic belt. Delamination of the lower crust can well interpret many geological, geophysical and geochemical characteristics of the belt.
