Sm–Nd isotope geochemistry and tectonic setting of the metasedimentary rocks fromudthe basal allochthonous units of NW Iberia (Variscan suture, Galicia)

摘要

The basal units of the allochthonous complexes of NW Iberia are formed by thick metasedimentary rock sequencesudintruded by granitoids, ranging in composition from calc-alkaline (c. 493 Ma) to minor alkaline–udperalkaline massifs (c. 475–470 Ma), and mafic rocks. The granitoids were transformed into variably deformedudothogneisses and the associated mafic rocks were transformed into amphibolites, blueschists andudeclogites during eo-Variscan high-P metamorphism dated at c. 370 Ma. Two different superimposedudmetasedimentary rock sequences can be distinguished. The lower sequence (maximum depositional age atudc. 560 Ma) is mainly composed of metagreywackes, while the upper sequence (maximum depositional ageudat c. 500 Ma) consists of mica schists and other minor types. Major and trace element geochemistry of theudmetagreywackes of the lower sequence suggests that they were generated in relation to a peri-Gondwananudarc system built on the thinned continental margin, although some chemical transition to passive marginudgreywackes is also observed. This sedimentary sequence was probably deposited in an Ediacaran–Early Cambrianudback-arc setting or retro-arc setting, closer to the thinned platform of the continental margin. The geochemicaludfeatures of the sedimentary rocks of the upper sequence suggest some affinity with passive marginudsediments; they were probably deposited closer to the continental domain and to certain distance from theudmost active zones of the magmatic arc. The Nd model ages of 23 analysed samples are Paleoproterozoic andudrange between 1782 Ma and 2223 Ma (average value 1919 Ma). The Nd model ages are slightly younger inudthe upper sequence than in the lower sequence, but altogether they define a single population, and thereforeudthe two metasedimentary rock sequences can be clearly related. Sedimentation probably took place withinudthe same basin located in the continental platform of Gondwana, the main source areas of these sedimentaryudrocks did not change during the Late Neoproterozoic and Cambrian times. The Nd model ages are very oldudand they seem to be compatible with Paleoproterozoic or Archean source areas, with only minor participationudof younger sources probably represented by intrusive Cadomian–Pan-African granitoids.