Geochemical Characteristics Of Iron Isotope And Rutile Of UHP Eclogites During Exmhumation In The Dabieshan

The Dabie-Sulu orogenic belt is located in eastern China, and was built by the collision between the Yangtze Block and North China Block in the Triassic. The Tancheng-Lujiang fault has separated this orogenic belt into the Dabie orogen in the west and the Sulu orogen in the east. The occurrence of UHP accessory minerals such as coesite and micro-diamond, and the studies of cooling histories of UHP mctamorphic rocks provide evidence for subduction of continental crust up to mantle depths （>100 km）, followed by a rapid exhumation process. The samples of this paper are fresh eclogites and retrograde eclogites collected from Shuanghe and Bixiling, which both located in the Central Dabie mid-T/UHP eclogite-facies zone. Based on trace elemental data of rutile and iron isotopic composition and ferric content of bulk rock and mineral seperates, the purpose of this paper is to trace geochemical behavior of trace element and iron isotope during exhumation.The first research subject is to analyse detailed structure, trace element isotope content, oxygen isotopic composition, and raman spectrum of big grain rutile from Shuanghe. Compared to the rutile from retrograde eclogite, the rutile from quartz vein has more V and U with less Nb, Ta, Cr and W. The rutile from retrograde elcogite and quartz vein have similar oxygen isotopic composition, Zr/Hf ratio, and contents of Zr, Hf and W. The rutile from quartz vein has lower Nb/Ta ratio than the rutile from retrograde eclogite, which indicates that the residue phase controlling Nb-Ta behavior during dehydration might be rutile. As a product of retrograde metamorphism, titanite exists around rutile from retrograde eclogite, and back-diffusion of Nb from titanite to rutile could be observed based on the data of HMP. Rutile sub-grains and kink bands indicate that there were four stages after fomation of big grain rutile, as follow:i) Formation of rutile sub-grains; ii) Formation of ductile kink bands; iii) Formation of brittle kink bands; iv) Formation of fracture.The iron stable isotope compositions (δ⁵⁶Fe) and iron valence states of ultrahigh-pressure （UHP） eclogites from Bixiling in the Dabie orogen belt, China, were measured to trace changes of geochemical conditions during vertical transportation of earth materials, for example, oxygen fugacity. The bulk Fe^3-/∑Fe ratios of retrograde eclogites, determined by Mossbauer spectroscopy, are consistently higher than those of fresh eclogites, suggesting oxidation during retrograde metamorphism and fluid infiltration. The studied eclogites （five samples） display limited MORB-like （-0.10‰） δ⁵⁶Fe values, which are indistinguishable from their protoliths, i.e., gabbro cumulates formed through differentiation of mantle-derived basaltic magma. This suggests that Fc isotope fractionation during continental subduction is limited. Garnet separates display limited δ⁵⁶Fe variation ranging from-0.08 ± 0.07‰ to 0.02 ± 0.07%o, whereas coexisting omphacite displays a large variation of δ⁵⁶Fe values from 0.15 ± 0.07‰ to 0.47= 0.07‰. Omphacite also has highly variable Fe^3-/∑Fe ratios from 0.367 ± 0.025 to 0.598 ± 0.024, indicating modification after peak metamorphism. Omphacite from retrograde eclogites has elevated Fe^3-/∑Fe ratios （0.54-0.60） compared to that from fresh eclogites （-0.37）, whereas garnet displays a narrow range of ferric iron content with Fe³⁺/∑Fe ratios from 0.039= 0.013 to 0.065±0.022. The homogenous δ⁵⁶Fe values and Fe^3-/EFe ratios of garnet suggest that it survived the retrograde metamorphism and preserved it Fe isotopic features and ferric contents of peak metamorphism. Because of similar diffusion rates of Fe & Mg in garnet and omphacite and constant △26Mgomphacne-gamet values （1.14 ± 0.04‰）, equilibrium iron isotope fractionation between garnet and omphacite was probably achieved during peak metamorphism. Elevated Fe³⁺/Fe ratios of omphacite from retrograde eclogites and variant △56Feomphacite-garnet values of the studied eclogites （0.13 ±0.10‰ to 0.48±0.10‰） indicate that oxidized geofuild infiltration resulted in elevation of δ⁵⁶Fe values of omphacite during retrograde metamorphism. The calculated iron isotopic fractionation between Fe^3- and Fe2- in omphacite should be ～0.98%o at 600-650℃, which can be summarized as δ⁵⁶Fe³⁺ omphacite-δ⁵⁶Fe²⁺ omphacite= 75-84×10⁴/T² （T in K）.