New U-Pb ages for Early Ordovician magmatism in Central Spain

Felsic orthogneisses occur widely in the Ollo de Sapo Domain of the Central Iberian Zone, most of which were previously considered to represent a Prec ambrian basement to the Palaeozoic sequences of this domain. However, new U -Pb dating across the Berzosa-Riaza shear zone (Sierra de Guadarrama, Centr al Spain) indicates an Early Ordovician age for the most representative typ es of orthogneisses in the eastern part of the Ollo de Sapo Domain. Dated r ocks include the volcaniclastic Cardoso gneiss (480 +/- 2 Ma); from the low -medium-grade hanging wall; the Riaza gneiss (468(-8)(+16) Ma. mylonitic gr anite) in the Berzosa-Riaza shear zone; three types of 'leucogneiss' (Buitr ago gneiss: 488(-8)(+10) Ma, megacrystic granite; 482(-11)(+14) Ma, aplitic vein; and 482(-8)(+9) Ma, gneissic leucogranite); and the La Morcuera gran itic augen gneiss (477 +/- 4 Ma) in the high-grade footwall. The new age of the Cardoso gneiss brackets to the Mid-Late Arenig, the so-called Early Or dovician Sardic unconformity, characteristic of the Central Iberian Zone. T hese new ages suggest that the broadly coeval volcanism and plutonism were closely associated with the Sardic events, and that these orthogneisses wer e part of a felsic magmatic belt which extended along the Ollo de Sapo Doma in of the Central Iberian Zone. This magmatic belt is interpreted to have b een active during the Early Ordovician break-up of the peri-Gondwanan margi n of the Iapetus Ocean.