of detailed petrographic observation and geothermobarometric analyses
allows us to put narrow constraints on the metamorphic evolution of the
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.
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.
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.
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.
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.