Origin and episodic emplacement of the Manaslu intrusive complex, central Himalaya

The Manaslu granite is the most studied of the dozen or so plutons that mak e up the High Himalayan leucogranite belt. The inferred relationship of the Manaslu granite with important Himalayan tectonic structures has inspired repeated attempts to determine its crystallization age, but several geochem ical controls have hampered these geochronological investigations. These li mitations are circumvented by Th-Pb ion microprobe dating of monazite. We h ave determined 158 Th-Pb ages on monazites separated from 11 samples of the Manaslu leucogranite. Together with and earlier published datum, these res ults indicated that the Manaslu intrusive complex was constructed during pu lses of magmatism at 22.9 +/- 0.6 Ma (Larkya La phase) and 19.3 +/- 0.3 Ma (Bimtang phase). Monazite Th-Pb results for a single sample of the Dolpo-Mu gu granite, located 75 km NW of the Manaslu intrusive complex, yield an age of 17.6 +/- 0.3 Ma and suggest a protracted (similar to 0.5 my) magmatic h istory that appears unrelated to the two phases of Manaslu magmatism. Age c onstraints on the emplacement of the Manaslu intrusive complex require that extensional structures cut by the Manaslu intrusive complex be 23-19 Ma or older. Geochemical constraints are consistent with the Larkya La phase of magmatism resulting from muscovite dehydration melting. The available timin g and geochemical constraints are consistent with the Bimtang phase being p roduced from a higher-temperature pulse of dehydration melting. Thermo-kine matic modeling indicates that (1) two phases of leucogranite magmatism sepa rated by similar to 4 my can be produced by shear heating along the Himalay an decollement assuming a shear stress of MPa, and (2) scenarios involving only decompression to achieve the same result appear to require extreme con ditions not permitted by available geological constraints.