Mechanisms and time scales of felsic magma segregation, ascent and emplacement in the Himalayas.
Résumé
We combine field, petrological, geochemical and experimental observations to evaluate the time scales of compaction-driven and shear-assisted melt extraction and ascent in the Himalayas. The results show that melt migration via compaction and channelling is inescapable and operates on timescales less than 1 m.y, and possibly as short as 0.1 m.y. Field and petrological data show that such a fast and efficient melt transfer result from a combination of favourable factors, including 1) low but constant melt viscosity (104.5 Pa s) during extraction and ascent, 2) grain size coarsening of the source rocks in response to prolonged heating prior to melting, and 3) high source fertility and thus high melt fraction, owing to elevated modal amounts of muscovite of leucogranite sources, all three factors dramatically increase source permeability. Calculations show that shear assisted melt extraction had a time interval recurrence in the 10-100 Kyr range, leading to sill thicknesses of 1-30 m. Yet, melts falling at the low end of the viscosity range when coupled to high shear velocities may lead to veins several hundred meters thick. The deepest structural levels (eg: central Zanskar range) show that in situ melts formed where pure shear compaction was greatest and where simple shear was also operative. Magma extracted from migmatite leucosomes was injected along planes of weakness parallel to the ductile shear fabric probably by some sort of hydraulic fracturing crack propagation mechanism. Large HHL (eg: c 5 km thick sills at Manaslu, Makalu, and northern Bhutan) may thus represent inflated laccoliths assembled via dykes that tapped a 100-300 m melt layer produced by compaction of GHS. Thermal simulations show that such melt layers may have incubation times of several m.y. Although transport time for magmas associated with the HHL is short, the time for assembly may take several m.y for the largest HHL, as geochronological data indicate (up to 5 m.y. for Manaslu, Shisha Pangma). Transport of leucogranite melt from mid-crust levels towards the surface was concomitant with active low-angle normal faulting along the South Tibetan detachment normal fault, a structure that effectively formed the lid to the extrusion of a partially molten layer of mid-crustal rocks (Channel Flow). Rapid cooling of the granites emplaced on top of GHS implies rapid extrusion and lateral flow of Greater Himalayan sequence rocks beneath the STD during the period ~20-17 Ma. Weakening of the crust by partial melting is thus likely to be pulsatory in time, and future thermomechanical models should incorporate such aspects to model tectonic evolution of hot orogens.