Pre-metamorphic Ar-Ar ages from biotite inclusions in garnet

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