Janots, and 5 co-authors, reported in 2006 that REE mineralogy was characterized at the micrometer scale (using scanning electron microscopy) in the four tectono-metamorphic units of the Beni Mzala window, Sebtide Complex, Internal Rif, Morocco, which sample a HP-LT metamorphic gradient from subgreenschist to blueschist/ eclogite facies. These tectonic units are composed of garnet-free metapelites (and associated synmetamorphic veins) from the same homogeneous aluminium-rich and calcium-poor protolith. In that context, mineralogical differences between units are assumed to result from contrasted pressure and temperature histories.
Light REE-bearing Al-phosphates of the crandallite group with florencite-rich composition and variable goyazite content are the dominant LREE minerals in the lowest-grade units, Tizgarine (300 °C, 3 kbar as peak conditions) and Boquete de Anjera (350 °C, 10 kbar). In the latter unit, metamorphic florencite is associated with MREE-rich monazite. In the schist of the highest-grade units, Beni Mzala 2 (420–450 °C, 12 kbar) and Beni Mzala 1 (550 °C, 16 kbar), associations of allanite-rich epidote and synchisite (a LREE-fluorocarbonate) are found whereas florencite and monazite are no longer observed. At the wall of the quartz–kyanite veins, retrograde monazite-(Ce) with grain size of 20 to 50 μm is the only LREE-bearing mineral.
Th–Pb and U–Pb SIMS data on some of these grains yield ages of 21.3 ± 1.7 Ma and 20.9 ± 2.1 Ma, respectively, consistent with previous K–Ar ages obtained on retrograde clay–mica mixtures in rocks from the same locality. The identification of a stability field for monazite in high-pressure aluminous metapelites with an upper thermal-limit below 450 °C as well as the derivation of meaningful U–Pb and Th–Pb SIMS ages demonstrates the monazite potential for dating HP-LT metapelites. However, since, under these conditions, monazite growth occurs below its admitted closure-temperature, these ages must be interpreted on the basis of monazite textural relationships.
The present study, based on petrologic characterization at the micrometer-scale, allows us to picture the LREE–mineral succession in Al-rich metapelites submitted to a subduction-type metamorphic gradient. The first LREE-bearing minerals to appear in the metamorphic cycle are alumino-phosphates of florencite-type. Initial grains of florencite are possibly of authigenic origin but florencite is stable under low-grade metamorphic conditions up to ca. 350° C at least, for pressures around 10 kbar. We showed that monazite can crystallize during prograde metamorphism at temperatures as low as 350° C (i.e. under sub-greenschist facies). At higher grade (420–550° C, 12–15 kbar), monazite disappears and allanite-rich epidote becomes the dominant LREE-bearing mineral, even if the bulk calcium content of the rock is relatively low (CaO below 0.7 wt.%).
Crystals of retrograde monazite formed below 450° C under hydrothermal conditions were clearly identified. They yield U–Pb and Th–Pb Miocene ages in good agreement with previous K-Ar ages obtained on retrograde clay–mica mixtures sampled in the same unit. The identification of a stability field for monazite in high-pressure metapelites with an upper thermal limit below 450° C, the recognition of retrograde monazite as well as the derivation of meaningful U–Pb and Th–Pb ages clearly demonstrate the potential of monazite for dating HP–LT rocks and confirms that monazite is suitable for dating low-grade events (Rasmussen et al., 2001).
Actually, similar conclusions may also apply to thorite, ThSiO4. This accessory mineral is ubiquitous in the investigated metapelites and preliminary EMP data yield an upper chemical age limit that is rather consistent with a late Alpine crystallization. The low Pb content of the analyzed thorite crystals suggests that only little common lead is incorporated in that phase (and/or that Pb loss as occurred). In order to better constrain the monazite (and thorite) stability for dating low-grade events, further petrological and mineralogical data are required. This observational approach needs to be complemented by thermochemical data (Janots et al., 2004) in order to model accessory mineral stability in metamorphic rocks.
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