A DEVONIAN AGE FOR THE EL CASTILLO VOLCANIC ROCKS IN THE CENTRAL IBERIAN ZONE, SPAIN: IMPLICATIONS FOR CONTINENTAL RIFTING ALONG THE NORTHERN GONDWANAN MARGIN

Exposures of volcanic rocks (El Castillo) in the Central Iberian Zone near Salamanca, Spain, are a representative of Paleozoic volcanic activity along the northern Gondwanan passive margin. Alkaline basalts and mafic volcaniclastic rocks of this sequence are structurally preserved in the core of the Variscan Tamames Syncline. On the basis of a graptolite fossil record in immediately underlying clastic sedimentary rocks, the El Castillo volcanics have been regarded as Lower Silurian in age. In contrast, most Paleozoic volcanic units in western Iberia are rift-related mafic to felsic rocks emplaced during the Late Cambrian-Early Ordovician, and are attributed to the opening of the Rheic Ocean. New zircon U-Pb TIMS data from a mafic volcaniclastic rock from El Castillo yield a near-concordant, upper intercept age of 394.7 ± 1.4 Ma that is interpreted to reflect a Middle Devonian (Eifelian) age for the magmatism, and demonstrate that the El Castillo volcanic rocks lie unconformably above lower Silurian siliciclastics. The U-Pb age is coeval with a widespread extensional event in Iberia preserved in the form of a generalized paraconformity surface described in most of the Iberian Variscan realm. However, in the inner part of the Gondwanan platform, closer to the continent, the Cantabrian Zone developed a major, coeval increase in subsidence and the generation of an important sedimentary trough. From this perspective, the obtained age probably represents a discrete phase of incipient rifting along the southern flank of the Rheic Ocean. Paleogeographic reconstructions indicate that this rifting event was coeval with widespread orogeny and ridge subduction along the conjugate northern flank of the Rheic Ocean, the so called Acadian “orogeny”. Ridge subduction would have resulted in geodynamic coupling of the northern and southern flanks of the Rheic Ocean, and we speculate that the extension along the southern flank is a manifestation of slab pull along the northern flank. This suggested scenario challenges whether the coeval Devonian mafic rocks currently exposed in the Variscan suture of NW Iberia are remnants of a supra-subduction zone ophiolite, as currently interpreted, or result from crustal extension/thinning and incipient ocean basin development.