Magmatism and early-Variscan continental subduction in the northern Gondwana margin recorded in zircons from the basal units of Galicia, NW Spain

In situ uranium-lead dating (LA-SF-ICPMS\udand SIMS) and Lu-Hf isotope analyses\ud(LA-MC-ICP-MS) of zircon from eclogite\udfacies rocks from the basal units of the\udVariscan Belt in Galicia constrain their magmatic\udand metamorphic evolution and give\udsome clues about the nature and origin of\udthe involved basement. The samples studied\udare two felsic gneisses, two eclogites, and one\udeclogitic gneiss of intermediate composition\ud(metatonalite). Oscillatory-zoned zircon cores\udfrom the felsic samples gave a main clustering\udof U-Pb ages at 493 ± 2 and 494 ± 2 Ma, and\udsome older ages that represent inherited cores.\udZircon grains from the intermediate and one\udof the mafi c rocks show no inherited cores\udand yielded ages of 494 ± 3 and 498 ± 6 Ma,\udrespectively, interpreted as time of protolith\udcrystallization. Variably developed homogeneous\udzircon rims in one felsic gneiss\udyielded an age of 372 ± 3 Ma, and very tiny\udzircons of one eclogite gave 350 ± 2 Ma, both\udof which we interpret as metamorphic ages.\udThe new age data demonstrate that the\udcalc-alkaline magmatic suite described in\udthe basal unit is ca. 20 Ma older than the alkaline\udto peralkaline plutonic suite of the same\udunit (dated at 472 ± 2 Ma; Rodríguez et al.,\ud2007), and thus probably represents a distinct\udgeologic event. Overgrowth rims are interpreted\udas metamorphic on the basis of their\udLu/Hf and Th/U ratios. The 372 ± 3 age is\udconsidered as dating the high-pressure (high-P)\udmetamorphism, and is essentially in agreement\udwith previous Ar-Ar and Rb-Sr data.\udThis high-P metamorphism marks the initial\udearly-Variscan subduction of the Gondwana\udmargin. The inherited zircon ages and Hf isotopic\udcomposition of zircons point to a considerable\udinput of crustal material with West African\udCraton provenance to the felsic magma