Cl-rich amphiboles occur as corona surrounding garnet and omphacite grains, in symplectite after omphacite and in quartz-phengite vein in eclogites at Yangkou, in the Sulu ultrahigh-pressure (UHP) metamorphic terrane. The veins formed during the high-pressure eclogite-facies stage, the symplectite developed during the transitional eclogite- to amphibolite-facies stage, and the coronas were produced during the two stages. Pargasite and hastingsite are predominant in all occurrences and characterised by high Cl and K2O contents, ranging from 0.05 to 3.00 wt% and 1.00 to 4.00 wt%, respectively. The significant Cl contents in amphiboles suggest that fluids of high chlorinity attended the vein, corona and symplectite formation. All types of amphiboles show large variations of chlorine contents and are zoned with decreasing Cl contents from core to rim. Combining with the compositional data of apatite coexisting with amphibole, it can be determined that the fluids were evolved along a decreasing chlorine content from early to late stages, which is considered to be related to the water release by phengite-consuming reactions and the breakdown of nominally anhydrous minerals in country gneisses during the exhumation process of UHP terrane.

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