e-Terra Volume X – nº Y | 2010 VIII Congresso Nacional de Geologia Mação-Penhascoso laccolite granite: age and its implications (SW- Central-Iberia Zone) O lacólito granítico de Mação-Penhascoso: idade e as suas implicações (SW da Zona Centro-Ibérica) J. ROMÃO – manuel.romao@ineti.pt (Laboratório Nacional de Energia e Geologia, UGCG) G. DUNNING – gdunning@mun.ca (Memorial University, Departement of Earth Sciences, Canada) A. MARCOS – marcos@geol.uniovi.es (Oviedo University) R. DIAS - rdias@uevora.pt (Univ. de Évora, Depart. de Geociências, LIRIO e Centro de Geofísica de Évora ) A. RIBEIRO - aribeiro@fc.ul.pt (Universidade de Lisboa, Departamento de Geologia e CEGUL) ABSTRACT: The Mação-Penhascoso granite laccolith occurs in the SW border of the Centro-Iberian Zone near its boundary with Ossa Morena Zone. A U/Pb (ID-TIMS) age of “ca 483 Ma” was obtained from magmatic zircon of the main microgranite lithofacies. The Armorican Quartzite Formation (AQF) is bent by the intrusion and disconformably overlain by the Brejo Fundeiro Formation of Oretanian age. This suggests that the AQF, already compacted is of Tremadoc and Upper Cambrian age. We conclude that the AQF facies is diachronous between Upper Cambrian and Arenig from SW to NE across the Iberian Terranes; it was fed from a promontory of a stable and large Gondwana continent. KEYWORDS: Iberia, laccolith, stratigraphic and radiometric ages, diachronous. RESUMO: O lacólito granítico de Mação-Penhascoso ocorre no bordo SW da Zona Centro-Ibérica próximo da fronteira com a Zona de Ossa Morena. Uma idade U/Pb (ID-TIMS) de 483 Ma foi obtida em zircões magmáticos da sua litofácies microgranítica. A Formação do Quartzito Armoricano (FQA) é condicionada pela sua intrusão e discordantemente recoberta pela Formação do Brejo Fundeiro de idade Oretaniana. Isto sugere que a FQA, já compactada, é do Tremadociano ou do Câmbrico superior. Concluímos que as fácies da FQA são diacrónicas entre o Câmbrico superior e o Arenigiano de SW para NE no terreno Ibérico, tendo sido alimentadas a partir de um promontório do Continente Gondwana. PALAVRAS-CHAVE: Iberia, lacólito, idades estratigráficas e cronológicas, diacronia. 1. INTRODUCTION The Mação-Penhascoso granitic laccolith belongs to a group of granitoid rocks occurring in the SW border of the Central-Iberian Zone (CIZ) and these are confined to a sub-parallel alignment to the boundary between the CIZ and Ossa Morena Zone (OMZ) oriented NW-SE. It is a pre-orogenic tabular granitic body with subvolcanic textures, which is located south of the Amêndoa-Carvoeiro sinform D 3 (Romão, 2001). Its radiometric age has been intensively discussed since stratigraphic and structural observations seen in the field are not consistant with the previous Rb/Sr age determination which was 402±15 Ma (Abranches & Canilho, 1981/82). We intend to solve the mentioned contradiction by presenting a new U-Pb zircon (ID-TIMS) age for the Mação-Penhascoso granitic laccolith followed by a discussion of its implications for the lithostratigraphic sequence of Iberian Terrane (Dias et al., 2006). 2. GEOLOGICAL SETTING The Mação-Penhascoso granitic laccolith has a half-ring shape with two elongated edges in the NW-SE direction: the Penhascoso the SW side (1,5 - 2km wide) and the Mação village segment in the SE side (1 - 1,5 km wide). This laccolith, whose thickness is above 80m, underlies with the synclinal section of the metasediments that compose the Palaeozoic sedimentary succession. An intrusion in the lithologies of the units that belong to Grupo das Beiras (GB) and Grupo de Vale do Grou (GVG)