119 WESTERN PACIFIC EARTH SCIENCES Vol.2, No.2, P.119-148, 11 Figs., May, 2002 PETROTECTONIC CHARACTERIZATION OF THE KOKCHETAV MASSIF AND THE DABIE-SULU TERRANE- ULTRAHIGH-P METAMORPHISM IN THE SO-CALLED P-T FORBIDDEN ZONE JUHN G. LIOU 1 , RU-YUAN ZHANG 1 , IKUO KATAYAMA 2 , SHIGENORI MARUYAMA 2 AND W. GARY ERNST 1 1. Department of Geological and Environmental Sciences, Stanford University, Stanford, U. S. A 2. Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan ABSTRACT The Kokchetav Massif of northern Kazakhstan (metamorphosed at 540 - 530 Ma) and the Dabie-Sulu terrane of east central China (metamorphosed at 240-210 Ma) are characterized by their large geographical extent, widespread trace occurrences of UHP index minerals such as microdiamond and coesite, and P-T estimates within a field of extremely low geothermal gradient. The exhumed UHP/HP units occur as thin subhorizontal slabs, bounded by normal faults on the top, and reverse faults on the bottom. For the Kokchetav Massif, microdiamond and coesite are best preserved in supracrustal rocks (pelitic-psammitic gneiss, dolomitic marble and whiteschist); minor inclusions of coesite and its pseudomorphs occur in mafic eclogites. Rare Ti-clinohumite-bearing garnet peridotite appears to represent crustal-derived Fe-Ti ultramafics or slab/mantle metasomatic products. P-T estimates for diamond- grade eclogite and garnet peridotite based on the K 2 O-in-Cpx geobarometer yield maximum P-T conditions of 7 GPa, 1000 o C. On the other hand, UHP records for the Dabie-Sulu belt are mainly from mafic eclogite and ultramafic garnet peridotite within felsic gneissic units. Only traces of diamond were reported in eclogite and garnet peridotite, whereas gneissic rocks contain minor coesite inclusions in zircon. Hydrous phases and magnesite are common in various Dabie-Sulu UHP rocks. Both mantle-derived and crustal-hosted garnet peridotites yield very high-P estimates (4.0-6.5 ± 0.2 GPa at 750-950 o C). The occurrence of majoritic garnet and high-P clinoenstatite in garnet peridotite is consistent with their deep upper mantle origin.