Petrology and P-T evolution of high-grade garnet metabasites from the Tin Begane area (Laouni terrane, South Algeria)

Abstract

The south of the Tin Begane region (Laouni terrane, Central Hoggar, southern Algeria) has been subjected to high grade metamorphism and shows well-preserved relicts of garnet-bearing metabasites boudinaged within metapelites and migmatites. These rocks have preserved a number of spectacular mineral assemblages and reaction microstructures. Two successive metamorphic stages have been recognised, on the basis of metamorphic microstructures and mineral compositions: (i) the main primary paragenesis consists of garnet porphyroblasts, amphibole, clinopyroxene, titanite and quartz; (ii) the retrograde stage is evidenced by clinopyroxene-amphibole-plagioclase symplectites, which developed mainly at the expense of garnet, as well as peculiar clinopyroxene-ilmenite symplectites after titanite. Garnet porphyroblasts are almandine-grossular (Alm_46–58 Prp_7–14 Grs_28–42 Sps_1–5) and show a smooth zoning pattern from the core to the outer rim. The clinopyroxene is diopside but shows slight chemical variations depending on its microstructural position in the rock (inclusion in garnet: X_Fe = 0.39; Al_total = 0.06 apfu; Ca = 0.90 apfu; ameboid crystals in the matrix: X_Fe = 0.42–0.47; Al_total = 0.15–0.16 apfu; Ca = 0.90–0.93 apfu; clinopyroxene in symplectite: X_Fe = 0.37–0.52; Al_total = 0.06–0.11 apfu; Ca = 0.91–0.95 apfu). The amphibole is (ferro-)tchermackite to (ferro/magnesio)-hornblende, with some grains exhibiting a smooth chemical zoning profile from core to rim. Plagioclase, either from the main paragenesis or from the Cpx + Pl symplectites, is anorthite (X_An = 0.92–0.97). To reconstruct the pressure-temperature evolution of a selected sample, two P-T pseudosections were calculated in the NCKFMASHTO system. The first pseudosection was modelled using the bulk-rock composition of the sample, whereas the second was calculated for the overall composition of the late symplectites, estimated by balancing the reaction that produced them. The results indicate that the studied metabasites have experienced a clockwise P-T path, with a decompression from peak conditions estimated at 13–15 kbar and 750 ± 50 °C down to 7–4.5 kbar and 810 ± 50 °C. The clockwise P-T path can be interpreted in terms of crustal thickening followed by exhumation, most likely during the Pan-African orogenesis.