Cordierite metapelites: tracers of the late orogenic PanAfrican evolution of the Laouni terrane. LATEA, Tuareg shield, Algeria

In orogens, interactions between heat transfer and tectonics lead to rheological instabilities that initiate and localize the deep seated rock exhumation. The presence of Precambrian metamorphic and magmatic rocks within the LATEA-Iskel collision zone, South of Laouni, offers a good opportunity to understand these interactions. The Hoggar, belonging to the Tuareg shield, show a succession of juvenile Neoproterozoic and Achaean and/or Paleoproterozoic terranes which have been differently remobilized during the PanAfrican orogeny. The Laouni terrane, in the LATEA superterrane, is constituted of Proterozoic formations affected by two metamorphisms, one of high temperature with a discussed age, Paleoproterozic [1], [2], [3] and PanAfricain [4], [5], [6] and the other of high pressure with a PanAfrican age [2]. The magmatism, at the Neoproterozoic, is calc-alcalin highly potassic syn-orogenic and alcalin post-orogenic. The structure of Laouni is described to be constituted by a PanAfrican thrust sheet set followed by a transcurrent tectonics [1]. Recent and ongoing works highlight a North-South extension exhumating the lower continental crust and the high pressure rocks and which is compatible with an East- West shortening. We propose to study, in the Tin Began region s most meridional structural trend containing exhumed PanAfricane eclogite lenses, the different parageneses developed in the migmatitic metapelites, their temperature and pressure conditions and their relation with the fabrics of deformation. The main foliation is marked by a paragenesis composed of prismatic and needle like sillimanites, bands and ribbons of cordierite, ribbons and lenses of quartz, primary elongated garnets and bands of primary biotites sometimes alternating with plagioclase-quartz leucosome beds. It is associated to two ductile and brittle shear networks and it is involved in N-S to NW-SE folding oblique to the NW-SE trend. This foliation carries N-S mineral lineation of a high temperature (sillimanite-cordierite) which evolves into an elongation lineation in the same direction. It is sutured by biotites and muscovites which suggest a stretching continuum of N-S direction. The stability conditions of the two principal foliation parageneses and of the associated structures are estimated for a temperature of 800 ± 50 °C and a pressure evolving from 8 to 5 Kbar. This translates an exhumation continuum from the crustal depths by an isothermal decompression at a high temperature. The eventual conditions of this evolution are of 500 ± 50 °C and 4-5 Kbar. The presence of numerous plutons of a mixt origine (mantle and crust) and coeval mafic and ultramafic rock complexes of a mantelic origin, suggest that these high temperature conditions ( 750 °C) are provoked by a heat advection of a deep origin leading to a partial melting of the lower continental crust and which contributed to its exhumation by diapirism linked to the decreasing viscosity.