P-T evolution of HP migmatitic paragneiss from the Beni Bousera metamorphic unit (internal Rif, Northern Morocco) constrained by phase equilibria modelling and melt reintegration

Abstract

The high-pressure (HP) migmatitic paragneiss of the Beni Bousera metamorphic unit (BBMU) from the Beni Bousera Massif underwent granulite-facies metamorphism. They mainly consist of garnet, biotite, kyanite, sillimanite, cordierite, spinel, quartz, plagioclase, K-feldspar, rutile, ilmenite and graphite. These minerals form a sequence of metamorphic assemblages corresponding to five metamorphic events (M1, M2, M3, M4, and M5). The first three events correspond to a latest Carboniferous-earliest Permian metamorphic evolution, while the last two are retrograde and Alpine in age. The p-T conditions of these metamorphic events were estimated using pseudosection calculations in the MnNCKFMASTHO system. Following prograde stage conditions of 9.3–10.9 kbar and 630–665°C (M1 stage), the p-T evolution culminates in melting of the paragneiss which occurred mainly at HP conditions, within the kyanite stability field, at 11.8–14.2 kbar and 820–845°C (M2 pressure-peak); anatexis then continued at high to medium pressure (HMP), within the sillimanite stability field, at 9.9–11 Kbar and 840–860°C (M3 thermal-peak), accompanied by the production of very small proportions of melt. The prograde p-T segment of the inferred clockwise path is characterized by a pressurization related to a post-Variscan collisional event triggered by the closure of the Paleotethys Ocean, and is followed by the exhumation of the granulitic unit to mid-crustal depth, with slight coeval heating. The retrograde Alpine evolution is coeval with the final exhumation of the migmatitic paragneiss and underlying peridotites during back-arc extension related to the westward retreat of the Alpine subduction. It occurred under sub-solidus conditions from 9.5–10.7 kbar and 818–848°C (M4) to 4.9–5.85 kbar and 710–785°C (M5) and corresponds to a near-isothermal and strongly decompressive metamorphic evolution (ΔP ≈ 5 Kbar).