Rb-Sr and single - zircon grain 207Pb / 206Pb chronology of the Monesterio granodiorite and related migmatites. Evidence of a Late Cambrian melting event in the Ossa-Morena Zone, Iberian Massif

Abstract

The Monesterio granodiorite, a small granodioritic body placed in a migmatitic complex in the SW of the Olivenza-Monesterio antiform, is a key plutonic body to understanding the relationships among the magmatism, metamorphism, and deformation in the Ossa-Morena Zone, SW Iberian Massif. We dated the granodiorite with the single zircon stepwise-evaporation ²⁰⁷Pb/²⁰⁶Pb method, and the related migmatization event with the Rb-Sr method on leucosomes. Our results indicate that the Monesterio granodiorite crystallised at 510 ± 7 Ma and its protolith had a component with Upper Proterozoic zircons with a minimum age of 1696 Ma. Leucosomes give a Rb-Sr age of 511 ± 40 Ma (MSWD =1,7) with initial ⁸⁷Sr/⁸⁶Sr =0.70914 ± 0.00048. The lower initial ⁸⁷Sr/⁸⁶Sr of the granodiorite and its calc-alkaline chemistry precludes it from having derived from the same protolith as the migmatites. The existence of different magmatic bodies in the Ossa-Morena Zone with ages clustering around 500-510 Ma reveals the existence of a significant melting event during the Late Cambrian that involved protoliths with very different geochemical and isotopic signatures.