Reaction Space: Transition from Eclogite to Amphibolite Facies in An Ultra-high Pressure Metamorphic Terrane from Zhucheng, Shandong, E. China

摘要

Eclogites and the related rocks are widely distributed over an ultra-high pressure metamorphic terrane in SE Shandong Province, eastern China. The petrography and mineral chemistry of the rocks preserved the transitional hltory from eclogite to amphibolite facies. The reaction space proposed by-Thompson (1982) was applied to investigate the metamorphic processes. In a closed system (N'CM'A"SO, the reaction space was conducted based on the two basis reaction vectors: a) 2di-J-Ik + 2mm =py and fl) py-|-2qz-|-Ik -|- 2-pl =2ab+ 2TW. Advancement of retrograde altaration of eclogite along the vector di+ Ik + // + qz=ab leads to the formation of symplectic intergrowth of clinopy-roxene plus nlbite around omphacitc. Abundance of quartz in eclogites limits the volume and shape of the reaction polytopc, hence the retrograde metamorphic production, e. g. symplectites, are more easily developed in quartz-basaltic eclogitos other than olivine-basaltic ones. If the system (N'CTvf'A 'STI") is open to water, formation of the hornblende is attributed to the hydration of eclogites during retrograde mctamorphism. Its reaction space is defined by the four independent net transfer reactions: A) ab= ed +4qz; B) 2ab+ 2vw =py+2qz+ Ik + 2jil; 0) ab=di+ Ik + fl +qz; D) 4tr -|-5qz+ 'Aed = 3py-|-1 ldi+ 3/iL +4H2O. The transfer of garnet and clinopyroxene into amphibole is mainly controlled by the reaction vector D . The other basis reaction vectors (A, B, G) are responsible for changes of mineral compositions and modes. Furthermore, least-squares approximation was used to estimate the mineral abundances.