Ion microprobe U–Pb dating of a bimodal zircon population from INTRODUCTION a graphitic schist from the basal section of the Alpine Alpujarride The Betic–Rif mountain belt forms the southwestern nappe complex in the Betic Cordilleras, southern Spain, has yielded termination of the Alpine orogenic belt in Western Eurdifferent ages for zircons of contrasting morphology and composition. ope and North Africa. Although located in one of the Approximately half the grains are rounded and abraded, and have very regions where the foundations of modern geology ages ranging from >2·2 Ga to >350 Ma. These are detrital were laid (e.g. Argand, 1924), fundamental aspects of its grains from the schist’s sedimentary parent material, indicating its tectogenesis are still disputed. For example, there is no derivation from a composite basement and a Palaeozoic, possibly consensus on the direction of the regional collisional Carboniferous, age of deposition. The other half of the grains consist convergence and the location of the subduction zone. of euhedral cores of Hercynian age (305·3 ± 3·2 Ma) surrounded Most researchers (e.g. Egeler & Simon, 1969; Platt & by Late Alpine (19·7 ± 2·2 Ma) metamorphic overgrowths Vissers, 1989; Platt & Whitehouse, 1999) have envisaged ( = 0·05 errors). The Hercynian cores probably record the first, a north–south convergence within the framework of medium–high-grade amphibolite-facies metamorphism, relicts of an Iberia–Africa collision. Recently, an alternative was which are seen in the schist complex, whereas the rims were formed suggested (Zeck, 1996, 1997, 1999): an east–west-directed during the final tectono-metamorphic stage in the Alpine development convergence under the influence of the opening of the of the orogenic belt. The bimodality of the zircon population in age, North Atlantic. In this model, the Alpine collisional morphology and internal structure may mainly reflect the lithological nappe pile was derived from Tethys-realm Betic–Ligurian bimodality of the rock (thin quartzitic layers or schlieren alternating lithosphere in a SW–NE-trending subduction zone system obliquely underthrusting eastward-drifting Atlantic-realm with graphitic micaceous folia) and the control that that exerted on Iberia. fluid circulation in the metamorphic environment. In addition, the timing of the subduction is poorly constrained. Comparison of the scarce thermochronological data with tomographic information (Blanco & Spakman, 1993) and the sea-floor spreading record for the North Atlantic (Srivastava et al., 1990) suggest