Transformation from the Donghai Eclogite to Amphibolite in the Southern Sulu UHPM Terrene
YANG Tiannan,XU Zhiqin,CHEN Fangyuan Institute of Geology,Chines Academy of Geological Sciences,Beijing,100037
Abstract:The Donghai eclogite in the Southern Sulu ultra-high pressure metamorphic (UHPM) terrene experienced wide-range variations of pressure, temperature, stress and fluid actions, resulting in a lot of microtextures, which provide indications for the study of mineral reactions. An eclogite sample retaining a complete retrograde process was analyzed in this study. Its transforming procedure from the eclogite phase to the amphibolite phase was revealed through detailed observations of microtexture, mineral chemistry, chemical space, and mass-balance. The procedure is divided into two stages: the earlier one is symplectitization. In this stage, diopside+ albite symplectite replaced omphacite only, which resulted from reactions among end-members within omphacite grains. The product Fe2+ was combined with rutile to form ilmenite; some products, such as Ca and Mg, were expelled out of the omphacite system. The latter stage is fluid infiltration caused by fluid action, which resulted in a hydrous mineral assemblage: hornblende + epidote (or ziosite) (+ phengite) + albite, replacing the earlier assemblage: garnet + diopside + albite. Finally, more and more fluids intruded the partially retrograded eclogite and then the remnant garnet was replaced by ziosite + hematite (+ hornblende). As a result, the eclogite was transformed to epidote-bearing amphibolite.