5.3.5 Thermobarometry

On the basis of the petrographic observations and the petrogenetic grid (Fig 5.20), we could establish pretty precisely the temperature stability conditions of the characteristic mineral assemblages of each zone. To quantify these temperatures more accurately and to obtain information on the pressure conditions of each zone, we resorted to thermobarometric analyses.

13 samples were selected for thermobarometry, on the basis of mineral and textural characteristics suggesting that peak metamorphic equilibria have been preserved. 6 of these samples are metabasites and 7 are metapelites.

Of the 6 metabasic samples, two (G1, G2) come from the garnet zone and two from the staurolite zone (S2, S3). These samples contain the stable assemblage: Hbl + Grt + Pl + Qtz ± Ep. The two last samples are from the migmatitic zone (M1, M2) and are characterized by the assemblage: Cpx + Hbl + Grt + Pl + Qtz (+ Ttn).

Of the 7 metapelitic samples one comes from the garnet zone (G3), with the assemblage Grt + Bt + Ms + Pl + Qtz + Chl, one from the staurolite zone (S1), with the assemblage St + Grt + Bt + Ms + Pl + Qtz + Chl, three from the kyanite zone (K1, K2, K3) with the assemblage Ky + St + Grt + Bt + Ms + Pl + Qtz and two from the migmatitic zone (M3, M4) with the assemblage Sill + Grt + Bt + Kfs + Pl + Qtz ± Ms.

5.3.5.1 Analytical procedure

Chemical analyses were made at the University of Lausanne using a Cameca SX 50 electron microprobe operated with an acceleration voltage of 15 kv and a beam current of 30 nA for garnet, 15 nA for biotite, muscovite and hornblende, and 10 nA for plagioclase. An average of 10 analyses were made on rims of relevant minerals. Special care was taken to analyse the outermost rim of relevant adjacent phases such as to reduce the disequilibrium effects. Average rim compositions are presented in Tables A1 to A5. Analytical procedure and uncertainty propagation follow the approach of Hodges & McKenna (1987). The thermobarometry calculations were performed with a program written by K.V. Hodges, using the solution models and calibrations summarized in table 5.3.

A different procedure was used for samples M1 and M2 which contain the assemblage garnet + plagioclase + hornblende + clinopyroxene + quartz + titanite. The thermobarometry calculations for these two samples were performed with the thermodynamic data base and computer program TWQ 2.02 (Berman, 1991; Berman et al., 1995; and references therein).

5.3.5.2 Results:

The calculated pressures, temperatures and depth of equilibration for each sample are summarized in table 5.2. When complete assemblages were present in the metapelites for solution of the GASP and GMAP barometers, both solutions are shown. The GAPH barometer yields two solutions.

The results of rim thermobarometry are represented in P-T diagrams (Fig 5.21 and Fig 5.22) and are grouped by metamorphic zone. The calculated values are shown with their error ellipses. The ellipses represent 95% confidence precision of the thermobarometric calculation based on the propagation of analytical uncertainties, using the Monte-Carlo approach. The coherence between P-T results obtained with different sets of thermobarometers, either for the same metapelitic samples or for metapelitic and metabasic samples from the same zone indicates well equilibrated assemblages. All samples fit, within uncertainties, the stability conditions deduced from the petrogenetic grid for the garnet to kyanite zone assemblages. This indicates that the P-T results represent equilibration conditions close to the peak of metamorphism.

The pressure and temperature profiles through the Zanskar Shear Zone and the migmatitic zone are plotted in two schematic logs in Figure 5.23 as a function of distance from the top of the ZSZ. This figure shows the rapid downward increase of both temperature and pressure.

In agreement with the field metamorphic zonation and the minerals assemblages, the P-T results confirm that a coherent and continuous Barrovian-type field gradient is preserved within the ZSZ. This gradient is characterized from the garnet zone to the kyanite zone by a temperature and pressure range of T ~ 550° to 700°C and P ~ 590 to 910 MPa.

With a «normal» lithostatic gradient of 27 MPa / km., the difference in equilibration depth between the kyanite and the garnet zone can be constrained to 12±3 km. These thermobarometric data confirm that the metamorphic zones equilibrated along the kyanite geotherm at significantly different depths and that the observed telescoping of these metamorphic zones within the ZSZ is the result of extensional tectonics along this structure.

The two metabasic samples from the migmatitic zone (M1, M2) preserved peak metamorphic temperature and pressures also indicating equilibration along the kyanite geotherm. The calculated temperatures (820° and 870°C) are a little higher than the temperatures predicted by our textural and petrographic observations for the migmatitic zone. Although temperatures above 800°C could be compatible with the mineral assemblages of the metabasic rocks (orthopyroxene does not form at HP, even at T well above 800°C), such high temperature are however incompatible with the metapelitic assemblages, because the break-down of biotite, which has to start above 800°C, was not observed in the migmatitic zone. The P/T conditions obtained through thermobarometric calculations are probably overestimated because the chemical composition of the minerals is somewhat out of range for the applied calibration. Pognante (1992) obtained pressures between 9.5 and 12 kbar and temperatures between 750° and 770° C for similar Grt - Cpx - Hbl - Pl - Qtz bearing metabasites from the same area (Temasa Valley).

The thermobarometric results confirm that during prograde metamorphism, the migmatitic zone reached P-T conditions close to but not necessarily sufficient to allow for the vapour-absent melting of muscovite.

The two pelitic samples from the migmatitic zone (M3, M4) yield temperatures and pressures much lower than expected (T = 580°C and P = 3 - 4,5 kbar), These inconsistencies are most likely the result of late-stage re-equilibration of the phases used for thermobarometry at conditions different from maximum pressures and temperatures and thus reflect final equilibration during retrograde evolution. As mentioned earlier, in the paragraph dealing with the migmatitic zone, vapour-absent melting must have occurred through isothermal decompression of muscovite-rich rocks. The low P/T values obtained for M3 and M4 are an additional indication that isothermal decompression must have occurred at the top of the HHCS as the retrograde path joining M1 and M3 is indeed very steep (Fig 5.22).

We will see in the next paragraph, that isothermal decompression of the HHCS is also marked in the ZSZ by the growth of new mineral assemblages.