PETROTECTONIC CHARACTERIZATION OF THE KOKCHETAV MASSIF AND THE DABIE-SULU TERRANE – ULTRAHIGH-P METAMORPHISM IN THE P-T FORBIDDEN ZONE

More than 20 ultrahigh-pressure (UHP) terranes have been documented within major continental plate collision belts; they are markedly different from Pacific-type oceanic lithosphere subduction complexes. Each UHP terrane extends several hundred km or more and share common structural and lithological characteristics: (1) Scattered UHP index minerals (coesite and microdiamond) are preserved in zircon, garnet, and clinopyroxene in eclogites, garnet peridotites and supracrustal country rocks including gneiss, marble and metapelite. (2) Lithologies are mainly continental ± minor oceanic in bulk-rock compositions. (3) Exhumed UHP unit occurs as thin subhorizontal slabs, bounded by normal faults above, and reverse faults below, and are sandwiched in among high-P or low-grade metamorphic units. (4) Coeval island-arc volcanic-plutonic rocks do not occur. Both the Kokchetav Massif of northern Kazakhstan and the Dabie-Sulu terrane of east-central China are unique in the widespread occurrence of microdiamond in the former and coesite in the later. Available P-T estimates indicate that subduction zone UHP metamorphism of these two terranes occurred under P-T conditions allegedly unrealized in the Earth. Supracrustal rocks together with enclosing mafic-ultramafic rocks were recrystallized at mantle depths with very low geothermal gradients, in some cases less than 5oC/km. Only the deep underflow of old, cold continental lithosphere can produce such a prograde P-T path. Both mantle-derived and crustal-hosted garnet peridotites yield very high-P estimates (4.0-6.5 ± 0.2 GPa at 750-950 °C). Occurrence of majoritic garnet and high-P clinoenstatite in garnet peridotite is consistent with their deep upper mantle origin. Some garnet peridotites were emplaced in the crust prior to subduction, and have been subjected to in situ UHP metamorphism together with the subducted slab. The rare occurrence of diamond in Dabie-Sulu could be due to the lack or low content of a C-O-H fluid and high fO2 conditions. Such fluid with appropriate high fCO2 and low fO2 is essential for crystallization of crustal microdiamonds in gneissic rocks and dolomitic marbles from Kokchetav.