Thermal Evolution of Leucogranites in Extensional Faults : Implications for Miocene Denudation Rates in the Himalayas.
Résumé
The crustally derived High Himalayan leucogranites (HHL) are characterised by strong isotopic heterogeneity and occurrence of magmatic muscovite. Such attributes indicate that the HHL were non-convecting magma bodies and crystallised at pressure-equivalent depths of at least 10 km. We have performed 1D thermal modelling in order to simulate the process of incremental growth of a laccolith whose roof is tectonically removed during intrusion, in a context of crustal exhumation dueto channel flow. The objective is to define under what conditions HHL laccoliths emplaced close to active normal faults may be built without convecting while crystallising muscovite. The results indicate that for a HHL thickness in the range 5-10 km, denudation rates cannot be higher than 4 mm/yr, and are more likely below 3 mm/yr. At such denudation rates, the intrusion process needs to start at depths of ca. 22 km, except when the final laccolith thickness is 10 km, in which case the depth of first emplaced sills cannot exceed 18 km. Thick HHL laccoliths (>7 km) may require a minimum denudation rate, on the order of 1 mm/yr, to prevent wholesale convection and allow muscovite crystallisation. Yet, emplacement of such thick HHL laccoliths during normal faulting implies that the top part of the leucogranite nearly reaches the surface while its base is still fed by active intrusions. Overall, such relatively low denudation rates suggest that, when HHL were intruded, the overlying crustal column was not undergoing vigorous erosion. Within the framework of a crustal channel flow, this suggests that the zone of focused erosion during Miocene was located further south of the HHL belt. Our results show also that, to explain the steep cooling patterns documented in many HHL, denudation must have been active after HHL solidification, especially when they are intruded close to their source region. However, to preserve the HHL from exhumation and erosion until the present time, the average denudation rate after emplacement cannot have exceeded 0.5 mm/yr.