Preiswerkite and högbomite within garnets of Aktyuz eclogite, Northern Tien Shan, Kyrgyzstan

Abstract

We report the occurrence of preiswerkite and högbomite as inclusion phases within the garnets of eclogite from the Aktyuz area of Northern Tien Shan, Kyrgyzstan. Preiswerkite and högbomite occur both as a constituent of multiphase solid inclusions (MSI) and as single discrete grains in the mantle and rim of the garnets. However, they do not occur in the core of the garnet and in the matrix of the eclogite. Preiswerkite is associated with the minerals paragonite ± staurolite ± Mg-taramite ± Na-biotite ± hematite ± högbomite ± chlorite ± titanite ± phengite ± magnetite, and högbomite is associated with paragonite ± preiswerkite ± staurolite ± hematite ± chlorite ± Na-biotite ± magnetite in MSI. The average compositions of preiswerkite and högbomite are (Na_0.96K_0.02Ca_0.01)_0.99(Mg_1.52Fe²⁺_0.54^VIAl_0.93)_2.99(^IVAl_1.93Si_2.07)_4.00O₁₀(OH)₂ and (Mg_1.47Fe²⁺_3.02Zn_0.04Fe³⁺_1.45)_5.98(Fe³⁺_0.31Al_15.13Ti_0.56)₁₆O₃₀(OH)₂, respectively. Na-biotite, with an average composition of (Na_0.89K_0.07Ca_0.01)_0.97(Mg_1.66Fe²⁺_0.69^VIAl_0.63)_2.98(^IVAl_1.57Si_2.43)_4.00O₁₀(OH)₂, corresponding to the intermediate composition between preiswerkite and aspidolite (i.e., Na-phlogopite), is also observed. The compositions of the newly found preiswerkite and Na-biotite with similar X_Mg values (0.66-0.78) are arrayed along preiswerkite-aspidolite solid solution series. The mode of occurrence of inclusion phases in garnets may suggest that the activity of Na-Al-rich and Si-undersaturated aqueous fluids played a major role in the formation of preiswerkite during the prograde stage of high-pressure eclogitic metamorphism.