Estudo geocronológico das unidades metassedimentares do soco Pré-Mesozoico a Oeste da Zona de Cisalhamento Porto-Tomar

Abstract

To the west of the Porto-Tomar Shear Zone, several pre-Mesozoic metasedimentary outcrops belonging to the basement of the Lusitanian Basin are poorly exposed. The traditional view suggests that these units belong to Ossa Morena Zone (OMZ), while more recent models propose the existence of the Finisterra Terrane (TF), considered exotic relative to the Iberian Massif (Moreira et al., 2019). To better constrain these geodynamic models, we used U-Pb geochronology in zircon of the most representative lithotypes to define maximum depositional ages (MDA) and determine the provenance of sedimentary sources, revealing possible tectonostratigraphic affinities. Samples were collected in the Tomar - Abrantes and Coimbra - Albergaria-a-Velha sectors, with previously published data re-evaluated for the Porto - Espinho sector (Almeida, 2013). U-Pb ages ranging from 505 to 3268 Ma, with an absence of Mesoproterozoic and upper Paleoproterozoic ages. The 505 Ma age was interpreted as lead loss, with the age of 570±2 Ma interpreted as MDA. The distribution of detrital ages and the MDA indicate an affinity with the Série Negra or Cambrian units of OMZ with characteristic peaks from Cadomian/Pan-African events, with Stenian-Tonian and Eburnean ages. This data supports previously U-Pb ages of metapelitic lithotypes in this sector. In the Coimbra - Albergaria-a-Velha sector, the quartz-phyllite CMB-2A from the Arada Unit (Moreira et al., 2019) presents an MDA of 550 Ma. However, two younger ages (374 and 418 Ma) suggest a possibly even more recent MDA. The age distribution (374-3258 Ma), particularly the abundance of Mesoproterozoic and Silurian ages, suggests an affinity with the South Portuguese Zone (SPZ) or the Meguma Terrane. In the Porto - Espinho sector, reinterpretation of geochronological data from the Lourosa (UL) and Espinho (EU) units shows similarities to a typical SPZ/Meguma Terrane distribution with records from the Mesoproterozoic and Devonian/Silurian periods, more prominent in the UL. The age distribution of the EU aligns better with the Upper Devonian units of SPZ. Thus, the TF may represent a sector interdigitating ZOM and SPZ units, highly deformed with the ZCPT activity.