Rios and Takasu (1999) consider that at the staurolite isograd, [X.sub.grs] increases with increasing temperature, which is difficult to determine taking into account that garnet tends to be resorbed back to higher [X.sub.grs] concentrations as staurolite grows.
However, this reaction, which promotes the disappearance of staurolite also coincides with the appearance of kyanite (at pressure greater than about 2.5 kbar), and if we were to encounter this reaction in the field, it could be used to define the kyanite isograd.
Taking into account that the Fe-Mg aluminosilicates are more Fe-rich at low-P than at middle-P, the mineral assemblages will probably loss the major KFMASH isograd producing reactions such as garnet-in and staurolite-in (Spear, 1993), which can explain the absence of garnet and staurolite.
Rios and coworkers (Rios, 1999, 2001, 2005; Rios and Takasu, 1999; Campos, 1999; Montenegro y Barragan, 1999; Garcia y Campos, 2000; Rios and Garcia, 2001; Castellanos, 2001; Mantilla et al, 2001, 2002, 2003; Gelvez and Marquez, 2002; Rios et al 2003a, 2003b, 2008a, 2008b; Cardona, 2003; Castellanos et al., 2004, 2008, 2010; Garcia et al, 2005; Gomez y Avila, 2006) have conducted in the last decade numerous studies on the geological evolution of the different metamorphic units outcropping in the Santander Massif, emphasizing on the metamorphic rocks of the Silgara Formation, modifying the classic scheme of metamorphic zones and isograds proposed by Ward et al.
The boundaries connecting the first appearance of a mineral and separating the zones are isograds (Tilley, 1924).