Thermal Constraints on the Emplacement Rate of a Large Intrusive Complex: The Manaslu Leucogranite, Nepal Himalaya

The emplacement of the Manaslu leucogranite body (Nepal, Himalaya)has been modelled as the accretion of successive sills. Theleucogranite is characterized by isotopic heterogeneities suggestinglimited magma convection, and by a thin (<100 m) upper thermalaureole. These characteristics were used to constrain the maximummagma emplacement rate. Models were tested with sills injectedregularly over the whole duration of emplacement and with twoemplacement sequences separated by a repose period. Additionally,the hypothesis of a tectonic top contact, with unroofing limitingheat transfer during magma emplacement, was evaluated. In thislatter case, the upper limit for the emplacement rate was estimatedat 3·4 mm/year (or 1·5 Myr for 5 km of granite).Geological and thermobarometric data, however, argue againsta major role of fault activity in magma cooling during the leucograniteemplacement. The best model in agreement with available geochronologicaldata suggests an emplacement rate of 1 mm/year for a relativelyshallow level of emplacement (granite top at 10 km), uninterruptedby a long repose period. The thermal aureole temperature andthickness, and the isotopic heterogeneities within the leucogranite,can be explained by the accretion of 20–60 m thick sillsintruded every 20 000–60 000 years over a period of 5Myr. Under such conditions, the thermal effects of granite intrusionon the underlying rocks appear limited and cannot be invokedas a cause for the formation of migmatites. KEY WORDS: granite emplacement; heat transfer modelling; High Himalayan Leucogranite; Manaslu; thermal aureole