Anticlockwise metamorphic evolution of the Tin Hallen area (Ahnet Terrane, NW Hoggar, Algeria): Evidence for granulitic and blueschist-facies metamorphism in Fe-Ti metagabbros

Abstract

The Tin Hallen area (Ahnet terrane, NW Hoggar, Algeria) represents one of the most well-preserved examples of blueschist-facies metamorphism in West Gondwana, associated with FeTi garnet-bearing metagabbros. The textural analysis reveals four distinct stages. The first stage (M1) is magmatic, characterized by an orthopyroxene, plagioclase ± clinopyroxene, and ilmenite assemblage, primarily observed in areas distant from shear zones. The second stage (M2) involves the development of granulite-facies coronae and symplectites composed of garnet, clinopyroxene2, quartz, rutile, and brown amphibole. The third stage (M3) is defined by the appearance of green amphibole, sphene and epidote. Finally, the fourth stage (M4) is marked by the formation of glaucophane-bearing schists, indicating an advanced retrograde evolution within shear zones. Through the integration of thermodynamic modeling (P-T-M_H2O pseudosections), petrology, mineral chemistry, and reaction textures, a counterclockwise P-T path has been established for the area. The Tin Hallen metagabbros display a prominent granulitic imprint surrounding magmatic relicts in H₂O-undersaturated conditions. This evolution reflects a pressure increase at nearly constant temperature, progressing from c. 6 kbar and c. 900 °C (M1) to 9–10.5 kbar and 850–900 °C (M2), likely linked to the thickening of the magmatic arc root, as suggested by other international studies. The granulitization is followed by further pressure increases and temperature decreases, leading to the mylonitic garnet amphibolite stage (M3) at ~14 kbar and 740 °C, along with a significant rise in M_H2O. Moreover, the presence of an original blueschist-facies stage (M4) with high-pressure, low-temperature (HP-LT) conditions (c. 8 kbar and 470 °C) is characteristic of cold subduction gradients (10–15 °C/km).