U-Pb chronometry of polymetamorphic high-pressure granulites: An example from the allochthonous terranes of the NW Iberian Variscan belt

Abstract

Secondary ion mass spectrometry (SIMS) U-Pb geochronology of zircons from high-pressure granulite units within the allochthonous complexes of the northwestern Iberian Variscan belt illustrates the complexity of dating high-pressure events using the U-Pb chronometer. Zircons from four rocks belonging to the high-pressure (P), high-temperature (T) units were dated by SIMS. A gabbro body with preserved igneous texture within the Órdenes Complex high-P granulites yielded a crystallization age of ca. 515 Ma. A high-P mafic granulite within the same unit contained zircons that had U-Pb ages clustered around 387 Ma, and it preserved no record of events between the crystallization of precursor gabbros and the granulite event (sensu lato). A mafic high-P granulite from an equivalent structural unit in the Cabo Ortegal Complex contained abundant zircons that recorded crystallization of igneous protolith at 490–520 Ma and crystallization of new zircon from a melt phase starting at ca. 404 Ma, and zircons with 206Pb/238U ages between ca. 480 and 430 Ma that are interpreted to reflect partial Pb loss during the granulite-facies event. Bright luminescent cores in zircons from a leucosome pod within the outcrop area of the latter granulite yielded a mean age of 397 Ma, whereas dark U-rich rims in zircons from this leuco-some yielded a mean age of 390 Ma. These data, in conjunction with previous isotope dilution–thermal ionization mass spectrometry (ID-TIMS; zircon, monazite, titanite) U-Pb ages and 40Ar/39Ar dating of fabrics from rocks within the same units, point to a Silurian age for the high-P metamorphism. Timing of the peak pressure cannot be further constrained with available data because, in this case, we infer that the U-Pb system in zircon recorded only a segment of the retrograde path. We suggest that the high-P metamorphic event is related to the accretion and understacking of these units to the margin of Baltica or Laurentia following the closure of the Iapetus or Tornquist Oceans and concomitant opening of the Rheic Ocean.