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Isothermal decompression of garnet metabasites from Laouni terrane in the LATEA, Central Hoggar, Algeria

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Abstract

We study the first occurrence of clinopyroxene-free garnet bearing metabasites encased in migmatitic gneisses and metapelites, in the area of Tin Begane belonging to the Laouni terrane in the Pan-African Trans-Saharan belt (Hoggar, Algeria). They present two successive parageneses after a primary assemblage characterized by the presence of high titanium amphiboles equilibrated with coarse-grained garnet, plagioclase, biotite, quartz, and rutile. The two retrograde parageneses show decompression textures with the developing of coronas- and worm-like symplectites consisting of orthopyroxene + plagioclase ± amphibole for the secondary paragenesis and plagioclase + amphibole for the late stage paragenesis. These garnet metabasites do not show high pressure paragenesese compared to those previously studied from the Laouni terrane and which are known to present at least one high pressure paragenesis involving primary pyroxenes. We investigate the exhumation of these metabasites by combining detailed petrographic and thermobarometric studies in order to constrain the P-T conditions for each metamorphic stage. The results suggest an isothermal decompression from peak granulitic conditions (850 ± 50 ∘ C and 0.85 ± 0.15 GPa) down to transitional granulitic conditions (830 ±50 ∘ C and 0.5 ±0.1 GPa) followed by a cooling episode to the amphibolite-greenschist facies transition at 480 ± 80 ∘ C and 0.4– 0.5 GPa. These conditions are consistent with a tectonic exhumation process most likely provoked by a lithospheric thinning accompanied by a magmatic episode and partial melting of the lower continental crust.

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Acknowledgements

We would like to thank Yasuhito Osanai and KoichiroWatanabe from Kyushu University, Japan for providing logistics and technical support. T. Adachi is gratefully acknowledged for his time conducting EPMA analyses and for his critical review which helped to improve the manuscript. We warmly thank Renaud Caby for his guidance through the numerous insightful comments and discussions regarding the evolution of the LATEA and the Tuareg shield. We thank Abdullah M. Al-Amri and Domenico Doronzo as well as all the editorial team for their comments and support throughout the submission process. We are also grateful to the reviewers Jean-Paul Ligeois and Francesca Micheletti for their constructive and helpful comments. We wish to express our gratitude to OPNA for logistic support during fieldwork.

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  1. Laboratoire de Géodynamique, Géologie de l’Ingénieur et de Planétologie, Faculté des Sciences de la Terre, de Géographie et de l’Aménagement du Territoire,, USTHB., BP 32 El Alia, Bab Ezzouar, 16111, Alger, Algérie

    Brahim Ayad, El-Hocine Fettous & Aziouz Ouabadi

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  1. Brahim Ayad
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Ayad, B., Fettous, EH. & Ouabadi, A. Isothermal decompression of garnet metabasites from Laouni terrane in the LATEA, Central Hoggar, Algeria. Arab J Geosci 9, 238 (2016). https://doi.org/10.1007/s12517-015-2198-4

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