NEW CONSTRAINTS ON THE AGE OF THE MANASLU LEUKOGRANITE - EVIDENCE FOREPISODIC 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 hel ped to moderate large topographic gradients in the Himalayan orogen in middle Tertiary time. Ar-40/Ar-39 ages for hornblendes from the north eastern contact aureole of the pluton indicate that intrusion occurred prior to 22-23 Ma, providing a minimum age for formation of the detac hment system in the Manaslu region that is at least 2 my. older than p revious estimates from other parts of the orogen. Ar-40/Ar-39 mica age s from the aureole, similar to previously published mica ages from the upper part of the leucogranite, indicate an episode of rapid cooling at approximately 19-16 Ma. We attribute this cooling event to tectonic denudation of the pluton by movement on structurally higher extension al faults, and we suggest that gravitational collapse of the orogenic front occurred episodically over the late Oligocene-early Miocene inte rval in the Manaslu area.