科研之友
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摘要
Two Rongcheng eclogite-bearing peridotite bodies (Chijiadian and Macaokuang) occur as lenses within the country rock gneiss of the northern Sulu terrane. The Chijiadian ultramafic body consists of garnet lherzolite, whereas the Macaokuang body is mainly meta-dunite. Both ultramafics are characterized by high MgO contents, low fertile element concentrations and total REE contents, which suggests that they were derived from depleted, residual mantle. High FeO contents, an LREE-enriched pattern and trace-element contents indicate that the bulk-rock compositions of these ultramafic rocks were modified by metasomatism. Oxygen-isotope compositions of analysed garnet, olivine, clinopyroxene and orthopyroxene from these two ultramafic bodies are between +5.2 parts per thousand and +6.2 parts per thousand (delta(18)O), in the range of typical mantle values (+5.1 to +6.6 parts per thousand). The eclogite enclosed within the Chijiadian lherzolite shows an LREE-enriched pattern and was formed by melts derived from variable degrees (0.005-0.05) of partial melting of peridotite. It has higher delta(18)O values (+7.6 parts per thousand for garnet and +7.7 parts per thousand for omphacite) than those of lherzolite. Small O-isotope fractionations (Delta Cpx-Ol: 0.4 parts per thousand, Delta Cpx-Grt: 0.1 parts per thousand, Delta Grt-Ol: 0.3-0.4 parts per thousand) in both eclogite and ultramafic rocks suggest isotopic equilibrium at high temperature. The P-T estimates suggest that these rocks experienced subduction-zone ultrahigh-pressure (UHP) metamorphism at similar to 700-800 degrees C, 5 GPa, with a low geothermal gradient. Zircon from the Macaokuang eclogite contains inclusions of garnet and diopside. The 225 +/- 2 Ma U/Pb age obtained from these zircon may date either the prograde conditions just before peak metamorphism or the UHP metamorphic event, and therefore constrains the timing of subduction-related UHP metamorphism for the Rongcheng mafic-ultramafic bodies.
- 出版日期2007-2
