LACK OF IN-SITU OCCURRENCE OF CRUSTAL DIAMONDS IN THE DABIE-SULU UHP TERRANES, EAST CENTRAL CHINA

Mineralogical evidence suggests that mantle-derived garnet peridotites from the Dabie-Sulu terrane yield the highest P/T ratios: 750–950°C and >=4.0–6.7 GPa. These P–T conditions deduced from matrix minerals and from low Al content of orthopyroxene coexisting with garnet lie within the forbidden zone for metamorphism. The very high-P estimates are supported by the presence of abundant fine-grained euhedral needles of Cpx, Rt, and Ap in eclogitic garnet from southern Sulu, suggesting high Na2O and octahedral silicon contents in precursor majoritic garnet recrystallized at depths > 200 km (Ye et al. 2000). Similarly, the occurrence of exsolution lamellae of Ilm ± Grt ± K-richterite in Cpx from garnet pyroxenite suggests that the pyroxenite was derived from precursor garnetite at depths > 300 km. Occurrence of high-P C2/C clinoenstatite from great depths has been confirmed by the occurrence of anti-phase domains. Some of the forbidden-zone garnet peridotites were emplaced in the crust prior to subduction, and been subjected to in situ UHP metamorphism together with the subducted slab. These independent P estimates lie within the diamond stability field and are appropriate for diamond formation. In the Dabie Mtn, more than 20 diamond crystals (150 to 700 µm) have been reported (Xu S. et al. 1992). Other coarse-grained diamond (0.7 x 0.5 mm) occurs in heavy mineral separates from a 2-ton eclogite sample from southern Sulu (Xu Z. 1998). However, our examination of diamond-bearing thin sections and many intensive search for mineral inclusions in garnet and zircon from both UHP eclogites and country rock gneisses failed positive identification. The original report of diamond in Dabie by Okay (1993) proved to be misidentification. The lack of diamond occurrence in Dabie-Sulu could be due to the lack or low content of C-O-H fluid with appropriate XCO2 and fO2 conditions. Such fluid is essential for precipitation of crustal microdiamonds in gneissic rocks and dolomitic marbles from both Kokchetav and Erzgebirge.