Contact metamorphism, PT conditions and model of the emplacement of the High Moulouya batholith (Moroccan Variscan Meseta). Insights from thermodynamic and gravity modelling

Abstract

In the Moroccan Eastern Meseta, the High Moulouya Batholith (HMB), which spans an area of approximately 2400 km2, is partially hidden by the Meso-Cenozoic cover. The HMB was emplaced in Cambro-Ordovician metasedimentary rocks that had undergone greenschist facies metamorphism (Chl+Mus+Qtz+Pl+Kfs). These hosting rocks were then affected by HT-BP superimposed contact metamorphism linked to HMB emplacement. Four metamorphic zones were identified, delineating four isograds corresponding to four main metamorphic reactions. These zones include the biotite zone, cordierite zone, andalusite zone, and sillimanite zone.  The biotite zone exhibits a mineral assemblage of Bt+Mus+Chl+FK+Qtz (±Pl ± Grt), marking the introduction of biotite through the discontinuous monovariant reaction: Chl+Qtz+FK+H2O=Bt+Mus . In the cordierite zone, the composition includes Bt+Mus+Crd+Chl+FK+Qtz (±Pl), and the reaction leading to Crd formation is: Chl+Bt+Mus+Qtz = Crd+FK+H2O. The successive development of aluminum silicates characterizes the andalusite and sillimanite zones, each associated with specific isograds corresponding to the following reactions: (Sill)+Qtz+H2O, Mus+Qtz=FK+And (Sill)+H2O, and/or And=Sill. The obtained results, using pseudosection modelling as well as petrogenetic grid calculations, allow to precise better the HMB emplacement PT conditions. Biotite appears at a temperature of 400°C, cordierite at 500 to 520°C and andalusite at 580 to 600°C. The stability field of the andalusite-sillimanite paragenesis allows constrains the pressure at 2.6 to 2.9 Kb, which is considered constant for the entire metamorphic aureole. Therefore, the granitoids of the HMB were emplaced at a depth of 8.5 to 9.5 km, developing a contact aureole for temperatures ranging from 400 to 625°C. Moreover, 2D gravity modelling revealed that the HMB has a laccolithic shape with a thickness ranging from 3 to 5 km. The presence of numerous feeders suggests that the emplacement of the HMB occurred through a dyking process.