Coeval interaction between magmas of contrasting composition (Late Carboniferous-Early Permian Santa Eulália-Monforte massif, Ossa-Morena Zone): field relationships and geochronological constraints

Abstract

The Santa Eulália-Monforte massif is a post-kinematic late Carboniferous-early Permian (LC-EP) located in the Ossa-Morena Zone (Portugal). This paper examines field relationships between calc-alkaline pinkish granites and mafic-intermediate rocks from the external ring of this composite massif and presents new U-Pb zircon age determinations. The estimated 206Pb/238U ages of 297.4 ± 4.2 Ma for a sample of the calc-alkaline pinkish granite and of 303.1 ± 3.1 Ma for a sample of gabbro-diorite, point to a short time interval between the crystallization of both magmas, with the mafic and intermediate rocks being the oldest ones. At the outcrop scale, contacts of the mafic-intermediate rocks with the host pinkish granite are sharp and corroborate this age relationship, but do not justify why the host-enclave contacts often have curved and irregular shapes, indicative of liquid-liquid interaction. A full analysis of the distribution of U-Pb zircon ages and respective Th/U ratios seems to be possible to admit that the distinct compositionally magmas were roughly contemporaneous. The ages obtained are also important because they fit the LC-EP Iberian calc-alkaline suite formed within the time interval in which the Iberian-Armorican Arc was formed, developed and took place the subduction of the Paleotethyan oceanic plate under the Eurasian active margin. Taking this geodynamic setting as reference, the LC-EP Iberian calc-alkaline magmatism can be interpreted as most probably related to the Cimmerian cycle instead of the traditionally accepted model that ascribe a closer connection with the Variscan cycle.