The Cordierite-Orthoamphibole Rocks of the Variscan Dome of Gavarnie-GèDre-Héas: The Gedrite of Gèdre (Hautes Pyrénées, France)

Abstract

Abstract The low- P /high- T Variscan metamorphic dome of Gavarnie-Gedre-Heas, Central Pyrenees, harbors a small (1 m 3 ) but special cordierite and orthoamphibole-bearing enclave embedded within an anatectic granite. Compared to other cordierite-orthoamphibole occurrences described in the literature, the enclave shows singular characteristics: (1) its mineral composition is unique with cordierite (Crd) + gedrite (Ged) + biotite ss (Bt) + staurolite (St) + garnet (Grt) + spinel (Spl) + corundum (Crn) + ilmenite (Ilm) and sulfides (Sulf), but lacking quartz (Qtz), plagioclase (Pl), and aluminosilicate minerals; and (2) its chemical composition, with low SiO 2 , high Al 2 O 3 , K 2 O, and TiO 2 and high Fe 2 O 3T /MgO. The Crd-Oam-bearing rocks also occur as layers in stromatic migmatites that show a mineral composition (Bt + Crd + Ged + Pl + Qtz + Ilm + Grt ± St) and a chemical composition more akin to other described occurrences. The cordierite-orthoamphibole enclave records a succession of metamorphic reactions: (1) local Crd-Oam growth at the expense of Bt with release of K 2 O; (2) development of coronas of Crd ± Ilm ± Grt around St; (3) isochemical breakdown of St to Crd + Spl + Crn. In the Ged-bearing layers in interlayered migmatites, the formation of the Crd-Oam assemblage involves the consumption of Bt, Pl, and Qtz. Mass balance calculations show that in the enclave most reactions took place in an open system with addition of Si and release of K 2 O. Pseudosections indicate a clockwise P-T path, with high-temperature conditions (700–730 °C) at decreasing pressure (from 5.6 to 3 kbar). This fits the P-T path of other metamorphic domes of the Variscan Central Pyrenees.