REE-assisted U–Pb zircon age (SHRIMP) of an anatectic granodiorite: Constraints on the evolution of the A Silva granodiorite, Iberian allochthonous complexes

Abstract

The A Silva granodiorite is a plutonic body intruded into the metasediments of the upper unit of the Órdenes Complex (Variscan belt, NW Spain). These metasediments represent the middle section of a magmatic arc located in northern Gondwana. The A Silva granodiorite has been classically considered a late Variscan granite. In this work, new field mapping, structural analysis, and SHRIMP U–Pb zircon dating indicate the granodiorite is significantly older. However, the data indicate a concordant age range between 540 and 460 Ma, and therefore CL images are not useful toward the interpretation of the geochronological results. This issue can be unravelled by using the hafnium and rare earth element composition of zircon in the assessment of the age. In this way, we determined that the age distribution was the result of lead loss, rather than a real age scatter or inheritance, and we could obtain a 206Pb/238U crystallization age of 510.28 (+1.57, −1.44)Ma using the TuffZirc algorithm. This age together with the well-preserved field relationships of the host rock permit us to interpret the A Silva granodiorite as multiple sheets intruded into a sequence of metatexitic host rocks after crustal thickening and subsequent decompression that developed coeval with partial melting during the latest stages of a regional extensional event. Taken together with the underlying Monte Castelo gabbro (499± 2 Ma), the whole plutonic complex reaches 8 km in thickness and forms an antiformal stack structure in a shear parallel (N–S) cross-section. This structure could be responsible for previously described, localized granulite facies metamorphism. The presence of a late Cambrian magmatic event has been widely reported in other areas of northern Gondwana and it is related to the opening of the Rheic Ocean.