New constraints on the age of the Manaslu leucogranite: evidence for episodic tectonic denudation in the central Himalayas

The Manaslu leucogranite of central Nepal transects 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. ⁴⁰ Ar/ ³⁹ Ar 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. ⁴⁰ Ar/ ³⁹ Ar 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. This cooling event is attributed to tectonic denudation of the pluton by movement on structurally higher extensional faults, and it is suggested that gravitational collapse of the orogenic front occurred episodically over the late Oligocene-early Miocene interval in the Manaslu area. -Authors