18.104.22.168 Discussion on M2
The metamorphic grade within the Tethys Himalaya increases slowly and progressively from the upper structural unit (Zangla) towards the lower structural units (Chumik and Kenlung Serai units). The metamorphic grade reaches lower greenschist facies at the base of the TH along the Kurgiakh valley and lower amphibolite facies in the Chumik Marpo - Kamirup area. The transition between the TH and the HHCS is then characterized by a very rapid, although still progressive, increase in metamorphic grade from biotite zone to kyanite zone over a vertical distance of little more than one kilometre. This transition zone coincides with a major tectonic structure, the ZSZ. The footwall of the ZSZ is formed by leucogranitic intrusions and migmatitic rocks belonging to the HHCS.
The successive prograde assemblages from the ZSZ are typical of a medium-pressure Barrovian metamorphic field gradient. The successive apparition, with increasing metamorphic conditions, of biotite, garnet, staurolite and kyanite as well as the systematic absence of Chloritoid within our rocks indicates that they correspond to low-Al metapelites. Their estimated composition is shown by a star in AFM diagrams in figure 5.20. These AFM diagrams show the most important changes that metapelitic rocks experience with increasing metamorphism along the Kyanite geotherm. Our interpretation of the pressure and temperature stability conditions of the metapelite mineral assemblages characteristic of each metamorphic zone is based on a petrogenetic grid adapted to the observed compositions (Fig 5.20). Most of the reactions limiting these stability fields have a steep dP/dT slope, implying a strong temperature control on the characteristic mineral equilibria. Little information can however be obtained from this petrogenetic grid on the pressure variation between each zone.
A Barrovian field gradient is generally the consequence of orogenic (regional) metamorphism and corresponds to a prograde metamorphic path along a kyanite-type geotherm as shown in Figure 5.20. This figure however shows that such a prograde metamorphism along the kyanite geotherm implies an important pressure increase from biotite zone to kyanite zone and therefore, that a Barrovian metamorphic zonation should develop over a vertical distance of 10-20 kilometres. In the studied area, the Barrovian zones are, however, condensed in a 1-km. thick zone. We will here discuss two possible ways to produce such a condensed Barrovian zonation.
The first possible explanation is that the rocks at the base of the Tethys Himalaya reached greenschist facies conditions (biotite zone) along a particularly steep geotherm (HP / LT) such as to reach pressures of about 7-8 kbars. These rocks would subsequently have been heated by a deep-seated intrusion (i.e. leucogranites) producing a Barrow-type zonation over a short vertical distance through contact metamorphism.
The second hypothesis to explain the observed condensed Barrovian metamorphic zonation is that the rocks where initially metamorphosed along the kyanite geotherm (as would be expected in a collision orogen) and that these metamorphic zones were subsequently telescoped by extensional ductile simple shear deformation along the ZSZ during the exhumation of the HHCS.
The first hypothesis, although theoretically possible (Spear, 1993), does not seem to be appropriate in our case for the following reasons:
- In the studied area the footwall of the ZSZ is systematically in contact with large leucogranitic plutons which could be invoked as the necessary heat source. These leucogranitic bodies are however missing in northwestern Zanskar where the same telescoping of the metamorphic zones within the ZSZ was described by Herren (1987), which leaves us, for this region, without a heat source.
- In the adjacent regions of Lahul and Western Suru, deformations associated with the ZSZ are weak or non-existent and the Barrovian zones in these areas are much thicker (6,3 km between the garnet and the sillimanite isograd in the region of Suru).
- Structural observations indicate that the prograde metamorphic minerals grew before the intrusion of the leucogranites.
Moreover, this hypothesis leaves us with no explanation for the cause of migmatisation below the leucogranites, and thus the presence of the leucogranites themselves would be rather challenging to explain.
The second hypothesis seems much more likely because the telescoping of the Barrovian zones always coincides with ductile extensional shearing associated with the ZSZ. Moreover, the thermobarometric data presented below indicate a marked pressure difference between garnet zone and kyanite zone.