Experimental Constraints in Favor of Partial Melting of the Middle Crust for the low Resistivity Zone under the Tibetan Plateau
Résumé
Over the past 70-50 Ma the Indo-Asian collision induced 1400 km of north-south crustal shortening creating the Himalaya-Tibetan orogenic system. Establishing the three-dimensional geometry of thermomechanical structures of the lithosphere is a key parameter to understand the evolution of the Himalaya-Tibetan orogen. Magnetotelluric data can image the electrical resistivity under the surface. This parameter is influenced by the presence of an interconnected network of fluids in a solid matrix. Magnetotelluric data under the southern margin of the Tibetan Plateau show widespread low resistivity zones (10 to 3 ohm.m) interpreted as possible partial melting of the middle crust. Existing experimental studies on electrical resistivity of partially molten crustal rocks have been conducted on granulite samples at 0.1 MPa confining pressure under dry conditions. These studies establish the resistivity of the partially molten crustal rocks at a temperature similar to 1100°C, which is well above the expected temperature prevailing in the middle crust of orogenic belts. In order to better reproduce natural conditions and interpret the resistivity values in terms of petrological processes, we have conducted experiments on metapelitic rocks undergoing muscovite dehydration melting. The starting material is a paragneiss presenting alternating muscovite/biotite-bearing layers with quartzo-feldspathic layers and containing on average 2 wt% water. An impedance spectrometer was coupled to an internally heated Paterson-type deformation apparatus in order to measure the in situ resistivity variations in static conditions and during deformation in torsion (simple shear, strain rates from 1x10-5 s-1 to 1x10-4 s-1) at 300 MPa confining pressure and temperatures between 500°C and 850°C. In the temperature range 500 - 850°C, resistivity decreases from 4600 ohm.m to 8 ohm.m, and indicates the onset of fluid-absent partial melting at temperature similar to 700°C. Our results show that the resistivity measured during partial melting at 800 - 850°C under static conditions is very low (10 - 8 ohm.m), as melt percentage reaches 30%. Both temperature and melt fraction ranges are consistent with conditions of melt extraction during migmatization and with temperatures of leucogranitic pluton emplacements. We conclude that the resistivity measured in natural metapelitic rocks, undergoing partial melting under P-T conditions relevant to the formation of migmatites in continent-continent collisional belts, corresponds to the anomaly detected under the Tibetan Plateau (10 ohm.m at 20 - 25 km depth). The migration of these partial melts most likely lead to the formation of leucogranitic plutons in the middle crust (with expected resistivity similar to 3 - 4 ohm.m) of the Greater Himalaya Sequence triggering channel flow and associated ductile extrusion.