Hercynian metamorphism in nappe core complexes of the Alpine Betic-Rif belt, Western Mediterranean - a SHRIMP zircon study

Ion microprobe U Pb dating of a bimodal zircon population from a graphitic schist from the basal section of the Alpine Alpujarride nappe complex in th e Betic Cordilleras, southern Spain, has yielded different ages for zircons of contrasting morphology and composition. Approximately half the grains a re rounded and abraded, and have ages ranging from similar to2.2 Ga to simi lar to 350 Ma. These are detrital grains from the schist's sedimentary, par ent material, indicating,, its derivation from a composite basement and a P alaeozoic, possibly Carboniferous, age of deposition. The other half of the grains consist of euhedral cores if Hereynian age (305.3 +/- 3.2 Ma) surro unded by Late Alpine (19.7 +/- 2.2 Ma) metamorphic overgrowths (alpha = 0.0 5 errors). The Hercynian cores probably record the first, medium-high-grade amphibolite-facies metamorphism, relicts of which are seen in the schist c omplex, whereas the rims were formed during the final tectono-metamorphic s tage in the Alpine development of the orogenic belt. The bimodality, of the zircon population in age, morphology and internal structure may mainly ref lect the lithological bimodality of the rock (thin quartzitic layers or sch lieren alternating with graphitic micaceous folia) and the control that tha t exerted on fluid circulation in the metamorphic environment.