Evidence of fluid activity in high-grade metamorphic rocks – constraints from chlorine concentration of biotite and mode of occurrence of zircon

Abstract

The Bt-Grt-Sil gneiss from Balchenfjella, East Antarctica characteristically contains rounded, coarse-grained Zrn in the rim of Grt porphyroblasts and in the matrix locally. Chlorine-rich Bt is also included in the outer side of Grt than Zrn. The presence of Cl-rich Bt, and the abrupt increase of Zrn inclusions in the Grt rim implies the infiltration of Cl-rich fluid and the addition of Zr, respectively. Because Bt inclusion predates Zrn inclusion, theses two might represent different events. The P-T condition for the entrapment of Cl-rich Bt in Grt is estimated to be ca. 800oC, 9kbar. The leucocratic vein consisting of Kfs, myrmekite, Bt-Qtz intergrowth and etc. is present in the matrix, and similar pseudo-inclusion exists in the garnet. From the petrographical and chemical constraints, pseudo-inclusion would be three-dimensionally connected with the matrix.