The combination of detailed petrographic observation and geothermobarometric analyses allows us to put narrow constraints on the metamorphic evolution of the studied area.
The transition between the Tethys Himalaya and the High Himalayan Crystalline Sequence is characterized by a progressive though very rapid increase in metamorphic grade which coincides with the Zanskar Shear Zone. This metamorphic zonation is the result of two subsequent tectonometamorphic events. The first event corresponds to an episode of crustal thickening associated with the southwestward thrusting of the Nyimaling-Tsarap Nappe and the burial of the HHCS below the TH, resulting in a regional metamorphism of Barrovian type. Petrographic observation and thermobarometric data indicate that the various Barrovian metamorphic zones equilibrated at significantly different peak pressures and temperatures along the kyanite geotherm.
The upper structural levels are represented by the weakly metamorphosed sedimentary series of the TH. The maximal pressures and temperatures at which these upper structural levels equilibrated (excepting the special case of the Kenlung Serai unit) can be estimated at 3 - 4 kbar and 300 - 400° C on the basis of «illite crystallinity» and mineral assemblages. Such pressures and temperatures are indicative of an overburden of 10 to 15 kilometres.
The lowermost structural level is represented by the migmatitic zone which recorded peak metamorphic pressures and temperatures of 10 - 12 kbar and 750°- 800°C corresponding to an overburden of 35 to 45 kilometres.
In-between these two extremes, all the metamorphic zone have recorded intermediate peak metamorphic P/T conditions.
The subsequent closing together of the Metamorphic zones is the result of extensional movements along the ZSZ associated with the exhumation of the HHCS. Both petrographic observation and thermobarometric estimates indicate that the exhumation of the HHCS must have been a rapid process, such as to allow for nearly isothermal decompression. The migmatitic zone is marked by a rapid pressure drop from 10-12 kbar to 3-4 Kbar. The kyanite zone records the growth of several retrograde mineral phases also indicative of isothermal decompression.
A complete P-T path (Fig 5.28) can be established for the kyanite zone of Zanskar in associating our data which constrain the retrograde metamorphic history (M3) of this zone with the data obtained by Vance and Mahar, 1998 for the prograde part of the path (M2). Peak pressure and temperature conditions that were calculated for the kyanite zone of SE Zanskar are nearly identical to those obtained by Vance and Mahar, 1998 for the same zone in NW Zanskar. This P-T path shows, in good acceptance with theoretical modelling (Spear, 1993), that crustal thickening (M2) is marked by a steep (nearly isothermal) prograde path, followed by an episode of nearly isobaric thermal relaxation. Thermal relaxation is however limited in Zanskar because crustal thickening is very quickly followed by rapid tectonic exhumation (M3), marked by an isothermal decompression path. A rapid exhumation and cooling of the HHCS might explain why, contrarily to the MCT zone (Epard et al. 1995; Vannay and Grasemann, 1998), peak metamorphic condition where very well preserved in Zanskar.
One of the major side effects of the tectonic exhumation of the HHCS was to trigger vapour-absent melting by crossing the muscovite-breakdown reaction, thus producing melts of leucogranitic composition.
|Discussion on M3||Chapter 6 "Leucogranites"|
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