ISOTOPIC CONSTRAINTS ON THE FORMATION DEPTH OF ULTRAHIGH PRESSURE METAMORPHIC ROCKS IN THE DABIE REGION

The formation depth of metamorphic rocks in the Dabie ultra-high pressure metamorphic (UHPM) zone is a question attrating attention of geologists in the world. It has some bearing not only on our understanding of formation mechanism and evolution precesses of collision orogenic belt,but also on studies on earth's interior and geodynamic processes.
In this study, the isotopic data of metamorphic rocks in the Dabie UHPM zone are analyzed and discussed to give constraints on the formation depth of in the Dabie UHPM zone.
The ε_sr of eclogite in the Dabie UHPM zone varies from 18 to 42,and ε_Nd from -6.1 to -17,showing remarkable isotopic disequilibrium. The oxygen isotope studies indicate that the protoliths of these UHPM rocks have experienced oxygen isotope exchange with meteoric water(or sea water) depleted in ¹⁸O before metamorphism and remained unchanged in the processes of metamorphism. Except for one from the Bixiling,all samples of eclogite from Dabie UHPM zone have the ³He/⁴He ratios falling within the range between 0.79×10^-7 and 9.35×10^-7,indicating an important contribution of He from continental crust. All studies of Sr,Nd,O and He isotopes indicate that the UHPM rocks have retained the isotopic characteristics of the original surface rocks. No significant infulence of mantle derived materials has been found in them.
Some researchers attributed the above isotopic characterisitics, to rapid subduction and exhumation that no isotopic exchange between the two would take place,with no remains of mantle materials in these UHPM rocks. However,this assumption is not justified with pressent knowledge. It was estimated that the whole process of UHPM has spanned a time of at least 15Ma. During such a long period,it seems impossible that no isotopic exchange between mantle materials and original surface rock would occur at a depth of ≥ 100km where the temperature would rise to ≥ 700℃.Conversely,the isotopic characteristics will be well explained by assuming that the UHPM occurred in the crust instead in the upper mantle.