The metamorphic evolution of the high pressure mafic granulites of the Bacariza Formation (Cabo Ortegal Complex, Hercynian belt, NW Spain) ☆

Abstract

The high pressure mafic granulites of the Bacariza Formation outcrop in the two uppermost structural units of the Cabo Ortegal Complex (La Capelada unit and Cedeira unit) were separated by a Variscan thrust. In both cases, they appear as heterogeneous metabasites in normal contact between ultramafic rocks and other more homogeneous and less differentiated metabasic rocks, also affected by catazonal metamorphism. The main difference between the mafic granulites in the two units is the degree of deformation, which is higher in the underlying Cedeira unit. Petrologie and mineralogical data indicate that the high-pressure (HP) granulites (Gt-Cpx ± Amp-Pl ± Qtz ± Scp-Rt ± Ilm-Czo) are already retrograde (M2 Stage), post-dating an earlier eclogite facies metamorphism (Ml Stage) characterised by the mineral associations: Gt-Cpx ± Amp ± Ky ± Qtz-Rt and Gt-Cpx ± Amp ± Qtz ± Zo-Rt. The main structure relatted to the exhumation processes is the development of a general mylonitic foliation that, although initiated in granulite facies conditions, was mainly equilibrated in amphibolite facies (M3 Stage). This foliation was affected by isoclinal folds, which led to the formation of the Variscan thrusts responsible for the present stacking position. Thrust conditions were transitional between amphibolite and greenschist facies (M4 Stage). Thermobarometric data point to different P—T exhumation paths in the two units. Estimated P—T conditions were higher in La Capelada unit during Ml (P ≥ 13 kbar; 860 °C) and M2 (15 kbar; 800 °C) than in the Cedeira unit (Ml: P ≥ 11 kbar, 770 °C; M2: 12 kbar; 750 °C). Temperatures for the M3 stage were comparable (720 °C) in both units but rocks from the Cedeira unit show a much bigger drop in pressure. This resulted in an isothermal decompression type path for the Cedeira unit, while both P and T decreased more steadily in La Capelada rocks. These were always located at deeper level than the Cedeira rocks before the Variscan stacking. The difference in the two paths is related to different exhumation rates; higher in rocks from the Cedeira unit than in those from La Capelada. Exhumation processes coeval with underthrusting, and a different location of the rocks with respect to the main shear zone responsible for the exhumation would account for the distinct paths.