Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XXII, No.4


Two Metamorphic Events of the Nepal Himalayas Prior and Posterior to India-Eurasia Collision

Arita, Kazunori;Gautam, Pitambar;Ganzawa, Yoshihiro

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K-Ar ages were newly determined on biotite from a pelitic gneiss and on hornblende from a calcareous gneiss both from the Higher Himalayas in Nepal. The corresponding ages, 56.7±2.8 Ma and 61.5±3.1 Ma, respectively, are quite old compared with their fission-track ages. We made a synthesis of the various isotopic ages from the Nepal Himalayas available so far, and attempted an analysis on the age versus closure temperature basis. The findings suggest that the Higher Himalayas have undergone two major reheating episodes, during the Himalayan orogeny, which are reflected in two different cooling trajectories roughly corresponding to late Cretaceous to Eocene and Eocene-Oligocene to the present times. A possible two-stage model involving two metamorphic events, which occurred prior and posterior to the India-Eurasia collision, is proposed and the subsequent cooling histories are discussed. Implication of the (re-) magnetization events revealed by paleomagnetic data on the Nepal Lesser Himalayas to the model is also considered.