Geochronology of the Ossa-Morena granitoids (Iberian Massif) in the framework of the Ibero-Armorican arc

Abstract

A comparison is made between the Ossa-Morena Zone (ZOM) in the Iberian Massif and the northernmost domains of the Armorican Massif, mainly based upon granitoids geochronology.In the ZOM evidence is given for a VAriscan granitic plutonism, mainly calc-alkalic, ranging from Middle/Upper Devonian up to Lower Permian. Several age groups can be distinguished in the ZOM and in the neighbouring Central Iberian Zone (ZCI): 280-290 Ma, 300-310 Ma, 310-330 Ma and 340-380 Ma. In opposition to the ZCI, the ZOM is characterized by the occurence of pre-Variscan granotoids in the 400-600 Ma range; they are tentatively gathered in four age groups: 420-470 Ma, 470-500 Ma, 500-550 Ma and 550-600 Ma. Both in the Portuguese and Spanish sectors of the ZOM pre-Variscan alkaline rocks occur in the age group ca, 485 Ma. Alkaline rocks dated ca, 260 Ma have been reported only in the Spanish sector.In the Armorican Massif, Cadomian (650-550) and Late-Cadomian (550-500 Ma) granitoids occur largely and (almost) exclusively in the northern Damnomean-Mancelian Zone (ZDM); they become virtually absent in the neighbouring Central Armorican Zone (ZCA), thus matching the situation in the Iberian Massif when one goes from the ZDM to the ZCI. Several age groups identical to ZOM can be distinguished in ZDN: 420-480 Ma, 500-550 Ma and 550-600 Ma. Granitoids in the 430-480 Ma range are scarce both in the ZDM and in the ZOM.The Variscan age groups 380-340 Ma, 330-310 Ma and 305-280 Ma can be distinguished both in the ZDM (and its northeastern adge) and in ZDM/Domnean domain (and its southern edge).The analogies recognized here between the Iberian ZOM and the Armorican ZDM (Domnonean domain in particular) support some geodynamic models recently proposed that visualize a Cadomian ZOM-ZDM active margin (cordillera or island arc) and an associated (late) Cadomian ZCI-ZCA basin. In Hercynian tomes the northward advancing of the postulated Cantabrian block would have caused a strong reactivation of the ZCI-ZCA belt. Obviously, much more geochronological data are necessary (mainly in the Portuguese sector of ZOM) to clarify the picture.