High-pressure metamorphism in metabasites from the Betic Cordilleras (S.E. Spain) and its evolution during the Alpine orogeny

The Nevado-Filábride complex is the lowest tectonic unit of the Betic Zone sensu stricto (ss) of the Betic Cordilleras (S.E. Spain). The upper series of this complex consists of a metamorphosed sequence intruded by basic and ultrabasic igneous rocks. High-pressure metamorphism in the eclogite and blueschist facies is recorded in the metabasites, but this was partially obliterated by further successive metamorphic stages in the almandine-amphibolite and greenschist facies.Coronitic and granoblastic eclogites appear side by side in the large stocks of basic rocks. The coronitic eclogites originate from coarse-to medium-grained olivine gabbros, and the granoblastic eclogites from fine-grained basic rocks (dolerites and porphyritic basaltic rocks). Higher chemical mobility and rate of diffusion, as well as the availability of fluids during the eclogite facies metamorphism, are responsible for the greater degree of recrystallization found in the granoblastic eclogites. The availability of fluids during this metamorphic stage was controlled by the difference in the hydration of the protolith and by variable proximity to surrounding water-rich metasediments.The minerals in the eclogites are chemically homogeneous, suggesting that they are almost completely equilibrated, even in the coronitic eclogites. The estimated equilibrium P-T conditions were found to be the same (approximately 550° C at 12 kbar pressure) in both coronitic and granoblastic eclogites, and it has, therefore, been deduced that the coronitic eclogites do not represent the first and lower-grade step of a prograde metamorphism in which the granoblastic eclogites are the higher-grade step.No relationship was found between shearing and eclogite crystallization. Nevertheless, a first fabric/foliation developed in the later blueschist facies stage, and syntectonic growth of the minerals was detected in glaucophane-bearing rocks.The further metamorphic evolution of the metabasites from high-to intermediate-pressure conditions is documented by the formation of minerals belonging to albiteepidote and almandine-amphibolite facies assemblages. The application of the amphibole zonation model, in order to deduce the P-T path, does not give realistic values.High-pressure metamorphism is related to an early subduction event in the Betic Cordilleras, with a later more-or-less isothermal uplift to shallower levels.