Tectonic and metamorphic evolution of the Temsamane units, External Rif (northern Morocco): implications for the evolution of the Rif and the Betic-Rif arc

Journal of the Geological Society, Jul 2007 by Negro, F, Agard, P, Goff�, B, Saddiqi, O

First P-T estimates in the Temsamane units

The P- T conditions have been derived from stability fields of observed metamorphic assemblages using the PTAX program, a development of the GEO-CALC program (Berman et al. 1987). The internally consistent thermodynamic database of Berman (1988) is completed by thermodynamic properties for Mg-carpholite (Vidal et al. 1992) and chloritoid (B. E. Patrick & R. G. Berman, unpubl. data). Activity models, based on simple ideal site solution, for Mg-carpholite, chloritoid and chlorite are taken from Vidal et al. (1992) and Theye et al. (1992), and from Massonne (1995) for phengite. As carpholite has not been found in these units, a theoretical carpholite composition, in equilibrium with chloritoid and chlorite, has been estimated based on partitioning coefficients. Values of /^[car/chl] = (Mg/Fe)^sub car^/(Mg/Fe)^sub Chl^ = 1.1 and K^sub D^/[car/cld] = 6.5 have been used for carpholite in equilibrium with chlorite and chloritoid, respectively, in agreement with values observed in the Betic Cordilleras or in Crete (Theye et al. 1992; Aza��n & Goffe 1997). A synthetic grid is presented in Figure 6a, corresponding to the mean compositions observed in the Ras Afraou, Tres For�as and Khebaba units.

Multi-equilibrium calculations have been performed for the Khebaba unit based on chlorite-chloritoid equilibria (Vidal et al. 1999). These estimates were made using the TWQ 2.02 program associated with the Berman (1988) thermodynamic database and completed by thermodynamic data and solid solution models for chlorite (Vidal et al. 2001, 2005) and chloritoid (Vidal et al. 1994, 1999). Mineral analyses of chlorite were selected following the criteria defined by Vidal et al. (2001 , 2005) to discard any analysis that cannot be expressed as a linear combination of (Fe1Mg)-amesite, sudoite, clinochlore and daphnite, and the possible Fe^sup 3 ^ content in chlorite. Calculations were performed in the FMASH system using chlorite-chloritoid-quartz-water equilibria (Fig. 6b), and the following end-members: clinochlore, daphnite, (Fe.Mg)-amesite and sudoite for chlorites; and (Fe,Mg)-chloritoid. The plotted results (Fig. 6a) correspond to calculation with up to three independent equilibria (Vidal & Parra 2000). Monte Carlo simulation was performed to calculate the error bars on P- T estimates (Fig. 6a) and discard the out-of-equilibrium mineral assemblages (see details given by Vidal & Parra 2000).

In the Ras Afraou, Tres For�as and Khebaba units, temperature has been estimated based on observed compositions and following the reaction (Fig. 6a)

chlorite pyrophyllite = chloritoid quartz water.

Temperatures have also been estimated using the chlorite-quartz and chlorite-chloritoid thermometers (Vidal et al. 1999, 2001).

In the Ras Afraou and Tres Forcas units, pressure has been estimated based on the Si content of phengite (Si^sub max^ = 3.4) and on the composition of chlorite through the reaction (Fig. 6a)

muscovite 4- chlorite albite water = paragonite celadonite.