Relative thermobarometry

In order to compare the P-T results with those obtained along another transect about 50 km westward in the Milke Danda ridge (Groppo et al., 2009), peak and prograde metamorphic conditions for the studied samples were calculated using the same approach, software and database used by Groppo et al., 2009. P-T conditions were estimated using the “Average PT” module in the THERMOCALC v. 3.25 software - (Powell and Holland, 1994) and the Holland and Powell (1998 – revised in 2002) thermodynamic dataset. Activity-composition relationships were calculated using AX. The “Average PT” method estimates P-T conditions using an independent set of reactions representing all the equilibria between the end-members of the equilibrium assemblage. This method has a number of advantages compared with conventional thermobarometry, including its consistency with other mineral equilibria methods (such as P-T pseudosections: e.g. Groppo et al., 2009; Imayama et al., 2010), and the possibility of realistic assignments of uncertainties. In addition, this method is able to find a result only if the given mineral assemblage defines a sufficient number of reactions between end-members that themselves intersect in the P-T range under investigation (i.e. “Average PT” is particularly suitable for low-variant assemblages).

The “Average PT” mode was used for all the samples except for samples 09-71a, 09-71b (LHS) and 09-68, 09-69, 09-70 (IMS orthogneiss) which do not define enough reactions for Average PT to work. Samples 09-71a and 09-71b represent different levels from the same outcrop: therefore, the “average T” mode was used for sample 09-71b - whose equilibrium assemblage defines very well constrained temperature conditions - and then the “average P” mode was used for sample 09-71a in order to obtain pressure conditions at the equilibration temperature resulting from sample 09-71b. For the anatectic samples 09-11, 09-13a and 09-14 possible complications arise from the presence of melt in equilibrium with the peak mineral assemblage. The melt solution model cannot be considered in the Average PT calculation. According to Clemens and Watkins (2001), the H2O-undersaturated character of granitoid magmas derived from de-hydration melting of micas at high metamorphic grades, suggests that pure aqueous fluid is not usually present, in excess, at these conditions. This implies that αH2O is mostly < 1 during the de-hydration melting processes. Therefore, the presence of melt in the equilibrium assemblages of samples 09-11, 09-13a and 09-14 was simulated by considering that a free fluid phase is lacking and that αH2O < 1. A αH2O = 0.7 was used for the three samples. Although this approach certainly simplifies the complexity of the problem, the obtained P-T results discussed in the following are consistent with petrogenetic grids, therefore we are confident that the relative P-T variations between samples (if not their absolute values) are quite realistic.