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 make 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 geochemical controls have hampered these geochronological investigations. These limitations are circumvented by Th–Pb ion microprobe dating of monazite. We have determined 158 Th–Pb ages on monazites separated from 11 samples of the Manaslu leucogranite. Together with an earlier published datum, these results indicate that the Manaslu intrusive complex was constructed during pulses 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-Mugu granite, located 75 km NW of the Manaslu intrusive complex, yield an age of 17.6 ± 0.3 Ma and suggest a protracted (∼0.5 my) magmatic history that appears unrelated to the two phases of Manaslu magmatism. Age constraints 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 timing and geochemical constraints are consistent with the Bimtang phase being produced from a higher-temperature pulse of dehydration melting. Thermo-kinematic modeling indicates that (1) two phases of leucogranite magmatism separated by ∼4 my can be produced by shear heating along the Himalayan décollement assuming a shear stress of 45 MPa, and (2) scenarios involving only decompression to achieve the same result appear to require extreme conditions not permitted by available geological constraints.