Vol. 43 Supp. SCIENCE IN CHINA (Series D) December 2000 Eclogitization of glaucophanites by fluid infiltration GAO Jun ( ) 1 , R. Klemd 2 & LIU Shengwei ( ) 1 1. Institute of Geology, Chinese Academy of Geological Sciences, Beijing100037, China; 2. Institute of Mineralogy, Wurzburg University, Wurzburg 97074, Germany Correspondence should be addressed to Gao Jun (email: gaoj@cags.cn.net) Received August 31, 2000 Abstract The characteristic textures of eclogitic veins infilling the interstitial spaces among zoisite-glaucophanite pillows, the fractures crosscutting massive zoisite-glaucophanites and the relic conduits within the eclogite boudins demonstrate that the eclogitization of glaucophanites was accompanied by the presence of a free fluid phase in the Western Tianshan high-pressure metamorphic belt. Non-planar populations of fluid inclusions, parallel to C-axis, have been preserved within the omphacites in eclogitic veins. The microthermometric results indicate that fluids are low-salinity (1 4 wt%NaCl) aqueous fluids. The chemical compositions of major, trace and rare earth elements of eclogitic veins and related host rocks indicate that the eclogitization of glaucophanites required the addition of external Si and Na elements, and was accompanied by the loss of large-ion lithophile elements, rare earth elements and water. The record of eclogitization of glaucophanites along fractures and zones of high permeability by fluid infiltration has been preserved in the Wetsern Tianshan high-pressure metamorphic belt. Keywords: fluids, glaucophanites, eclogitization , Western Tianshan. Intimate interlayering of blueschists (glaucophanites) and low-temperature eclogites on a metre to centimetre or even millimetre scale has occurred world-wide in many high-pressure low-temperature metamorphic belts [1] . It is still a controversial scientific topic of metamorphic geology about how to interpret the genetic relations between these two kinds of rocks representing two distinct mineral parageneses. The previous results about this topic are summrized as: (1) eclogites formed under conditions with relatively high PT and low activity of H 2 O [2 4] ; (2) differences in the bulk-rock compositions of protoliths controlled the rock types having formed [5 11] ; (3) the infiltration of fluids on different scales during the retrograde process produced the heterogeneity of retrogression from eclogite to blueschist [12] ; (4) the parageneses of eclogites formed under a relatively high activity of H 2 O, and probably coexisted with free-phase fluids [13 15] . We have found a lot of textures indicating a transition from glaucophanites to eclogitic rocks in the Western Tianshan high-pressure metamorphic belt recently. In this paper, we will discuss the reason causing the eclogitization of glaucophanites by the studies on the petrography, rock chemistry and fluid inclusions of eclogitic veins and related host rocks. 1 Outline of the high-pressure metamorphic belt The Western Tianshan high-pressure low-temperature metamorphic belt, which extends west