The Manaslu leucogranite of central Nepal transacts one segment of the South Tibetan detachment system, a major extensional feature that helped to moderate large topographic gradients in the Himalayan orogen in middle Tertiary time. 40Ar/39Ar ages for hornblendes from the northeastern contact aureole of the pluton indicate that intrusion occurred prior to 22-23 Ma, providing a minimum age for formation of the detachment system in the Manaslu region that is at least 2 m.y. older than previous estimates from other parts of the orogen. 40Ar/39Ar mica ages from the aureole, similar to previously published mica ages from the upper part of the leucogranite, indicate an episode of rapid cooling at ∼19-16 Ma. We attribute this cooling event to tectonic denudation of the pluton by movement on structurally higher exten-sional faults, and we suggest that gravitational collapse of the orogenic front occurred episodically over the late Oligocene-early Miocene interval in the Manaslu area.

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