PRE-METAMORPHIC AR-AR AGES FROM BIOTITE INCLUSIONS IN GARNET

Two populations of Ar-Ar ages in biotite inclusions occluded by garnet s from an Indian polymetamorphic gneiss have been measured using UV la ser ablation. One population yields a reproducible age of 455 My, iden tical to biotite ages in the adjacent groundmass, and is interpreted a s slow cooling following high-grade metamorphism during the Pan-Africa n event. The second population yields ages in the range 1800-2100 Ma a nd corresponds closely with zircon ages from the same rock. These resu lts imply that the Ar-Ar system in a small population of biotite inclu sions remained closed during high-grade metamorphism (750 degrees C an d 5 kb). The results of this first experiment accord with results for helium diffusion in garnet and suggest a mechanism for the long-term s torage of heavy noble,eases in the deep crust. Copyright (C) 1997 Else vier Science Ltd.