Lu-Hf Garnet Geochronology of Eclogites from the Dabie Orogenic Belt, Eastern China.

The collision between the Sino-Korean and Yangtze cratons in the Triassic formed one of the most extensive ultrahigh-pressure metamorphic belts worldwide, and index metamorphic rocks are today exposed in the southern and eastern part of the Qinling-Tongbai-Dabie-Sulu collision zone. Coesite and microdiamond were recognized in eclogites and country rocks indicating a deep subduction of continental material at mantle depth. Various isotope systems (U-Pb, Sm-Nd, Ar-Ar, Rb-Sr) have been applied to investigate the metamorphic history of the belt. With these geochronometers the actual metamorphic peak is poorly constrained to 210-245 Ma. In this study we applied the Lu-Hf isotope system on eclogites and garnet-peridotite from the Dabie Mountains, the eastern part of the belt, which is suitable to date peak conditions of garnet bearing high pressure rocks. Sample locations were Bixiling, Shuanghe, Shima and Lidu. The Bixiling Complex is the largest coesite- bearing mafic-ultramafic body in the Dabie Mountains and occurs as a tectonic block within foliated gneisses. Additionally we will present data for 3 eclogite samples from the continental drilling program at Dabie-Sulu, a part of the belt which was offset 500km to the northeast by the Tan-Lu fault. One eclogite sample from Bixiling yields a garnet+omphacite isochron age of 221.4±1.7 Ma which is slightly higher than the Sm-Nd garnet ages of [1] (210-218 Ma) of eclogites at Bixiling. Systematic higher Lu/Hf compared to Sm/Nd ages might be achieved because (a) the higher closure temperature of Lu-Hf (compared to Sm-Nd) in garnet [2] and (b) because Lu is highly enriched in garnet cores. Therefore Lu-Hf preferentially dates garnet-cores while Sm-Nd more emphasizes garnet-rims which might be retrograde. Two Eclogite samples from Shuanghe and Shima yield garnet+clinopyroxene isochron ages of 223.2±6.0 Ma and 221.9±2.4 Ma respectively, and thus agree with the obtained age for the Bixiling eclogite sample. [1] V. Chavagnac and B. Jahn, Chemical Geology 133, 29-51, 1996. [2] E. Scherer, K.L. Cameron and J. Blichert-Toft, Geochimica et Cosmochimica Acta 64, 3413-3432, 2000.