P-T evolution of eclogites from the Agualada Unit (Ordenes Complex, northwest Iberian Massif, Spain) : Implications for crustal subduction

Abstract

Eklogite lenses in the Agualada Unit (western Ordenes Complex, Spain) contain the peak mineral assemblage garnet (prograde rim: Alm = 48 mol%, Ptp = 30 mol%), omphacite (Jd max = 36 mol%), quartz, rutile and rare zoisite, which equilibrated at T = 700°C and P > 12-14 kbar. Garnet shows discontinuous growth zoning, with a pyrope-poor intermediate zone (Alm = 51 mol%, Prp = 10 mol%) between a core zone where pyrope is slightly higher (Aim = 46 mol%, Prp = 16 mol%) and areas just inward from the rims where the maximum pyrope contents (Alm = 48 mol%, Prp = 30 mol%) are recorded. In atoll interiors, garnet contains inclusions of a first generation of omphacite (Jd max = 40 mol%). This omphacite is replaced in the matrix by a second generation (Jd max = 36 mol%) with higher Fe/Fe + Mg ratio. The compositions of garnet and omphacite suggest a complex syneclogitic tectonothermal evolution for the Agualada Unit, characterized by: (1) eclogite-facies metamorphism (T = 585°C P > 12-13 kbar), followed by (2) cooling during a slight decompression (T = 5OOºC, P > 1 l- 12 kbar), and (3) a final increase in P and T to form the garnet rim-matrix omphacite mineral assemblage. The Agualada Unit is part of a subduction complex which affected the Gondwana margin at the beginning of the Variscan cycle. The P-T evolution of the Agualada eclogites is closely related to the structural evolution of the accretionary complex and the whole erogenic wedge. The cooling event recorded by the Agualada eclogites may have resulted from the accretion of a new colder crustal slice under the unit, whereas the final progradation reflects the emplacement of the Agualada Unit directly under the mantle wedge. This evolution fits well with previously presented theoretical models, both for the tectonothermal evolution of accretionary complexes and for the dynamic evolution of erogenic wedges. P-T paths such as the one for the Agualada Unit eclogites, probably reflect a prolonged structural evolution. Although evidently rarely preserved, such paths are probably the rule rather than the exception during plate convergence.