Contrasting high-pressure metabasites from the Santiago unit (Ordenes Complex, northwestern Iberian Massif, Spain)

The Santiago unit forms part of the Basal unit of the Ordenes Complex. It is part of the Gondwana continental paleomargin, partially subducted under Laurentia at the beginning of the collisional episode that gave rise to the European Variscan chain. It is composed mainly of pelitic-semipelitic schists, orthogneisses, and intercalations of metabasites. The metabasites intercalated in orthogneisses preserve evidence of their transformation into low-temperature (T) eclogites during an initial syn-D 1 high-pressure (P) episode (P min = 14.8 ± 0.3 kbar; T = 502 ± 10 °C). The metabasites intercalated in the schists conserve an internal schistosity included in albite porphyroblasts, corresponding to garnet amphibolites developed during the same high-P episode (P = 15.8 ± 0.3 kbar; T = 466 ± 14 °C). Petrographic evidence, the chemical zoning of the syn-D 1 garnets, and the thermometry indicate that the basic rocks included in the pelitic schists underwent metamorphic recrystallization prior to the peak pressure, whereas those included in the orthogneisses went directly from slightly hydrated igneous conditions to eclogitization. The subsequent extensional relaxing of the accretional prism produced the superposition of a hot mantle material layer over the Santiago unit. The development of an inverted metamorphic gradient is reflected in the metabasites included in the schists by the appearance of three metamorphic syn-D 2 zones: (1) tschermakite + oligoclase + chlorite; (2) tschermakite + andesine + biotite; and (3) ferrotschermakite + labradorite + garnet. The peak conditions reached by the top of the unit during D 2 are P = 9.3 ± 0.8 kbar and T = 528 ± 34 °C.