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 Alpujárride nappe complex in the Betic Cordilleras, southern Spain, has yielded different ages for zircons of contrasting morphology and composition. Approximately half the grains are rounded and abraded, and have ages ranging from ~ 2.2 Ga to ~ 350 Ma. These are deltrital grains from the schists sedimentary parent material, indicating its derivation from a composite basement and a Palaeozoic, possibly Carboniferous, age of deposition. The other half of the grains consist of euhedral cores of Hercynian age (305.3 ± 3.2 Ma) surrounded by Late Alpine (19.7 ± 2.2 Ma) metamorphic overgrowths (α = 0.05 errors). The Hercynian cores probably record the first, medium-high grade amphibolite-facies metamorphism, relicts of which are seen in the schist complex, whereas the rims were formed during the final tectono-metamorphic stage in the Alpine development of the orogenic belt. The bimodality of the zircon population in age, morphology and internal structure may mainly reflect the lithological bimodality of the rock (thin quartzitic layers or schlieren alternating with graphitic micaceous folia) and the control that that exerted on fluid circulation in the metamorphic environment.