The Lesser Kabylia metamorphic basement: Unraveling pre-Alpine history through petrological and geochronological studies (Texenna, Algeria)

The Lesser Kabylia massif, situated within the internal zone of the Alpine Algerian Tell in the Maghrebides, contains a metamorphic basement of uncertain Paleozoic age. Despite the negligible tectono-metamorphic Alpine overprint, the pre-Alpine history of the basement is poorly known. Therefore, to fill the gap, we carried a petrological and LA-ICP-MS zircon U-Pb dating study in various rock types of this basement.

The metamorphic basement of Lesser Kabylia is divided into two units: (1) a lower-crustal unit characterized by high-grade metamorphism and (2) an upper-crustal unit composed of low-grade to non-metamorphic rocks, including strata from Cambrian-Ordovician to Silurian-Devonian ages. This structural arrangement is comparable to other internal zones of the Maghrebide belt and Betic cordilleras. However, the Lesser Kabylia metamorphic basement exhibits a more complex structure. It is further subdivided into the Kerekera nappe thrust over the Beni Ferguen nappe.

Near the Texenna village, where the basement is part of the Kerkera nappe, the high-grade metamorphic lower-crustal unit is dominated by felsic migmatites enclosing lenses of mafic to ultramafic granulites. The felsic migmatites are commonly composed of Grt–Pl–Ksp–Qtz–Bt, locally also with sillimanite and spinel. The mafic granulites are composed of Opx–Cpx–Amp–Pl–Qtz–Ilm, locally with spinel and/or biotite. Pseudosection modeling using Perple_X software has been done on a felsic migmatite containing garnet and sillimanite, and for a mafic granulite, yielding peak P-T conditions of 8–6 kbar and ∼725 °C, followed by cooling with slight decompression. Zircon U–Pb dating by LA–ICP–MS revealed predominant Permian dates of 266–295 Ma, interpreted as the age of the high-grade metamorphism. It is not clear whether the Carboniferous dates in the range of 300–320 Ma have geological meaning. One granitic leucosome sample reveals a prominent Permian zircon population, potentially indicating coeval migmatization of the lower crust with the emplacement of the nearby granites (e.g., the Collo granite).

Our findings suggest that the basement of the Lesser Kabylia was affected by Variscan medium-pressure, high-temperature metamorphism, which may have resulted from the closure of the Paleo-Tethys Ocean or its intracontinental propagator near the edge of Gondwana and at the southern part of the European Variscan belt, sealed during Pangea formation.