Classificação sistemática de unidades magmáticas: exemplo das rochas alcalinas Cambro-Ordovícicas da Zona de Ossa-Morena

Abstract

The systematic classification of non-stratiform units provides a harmonized setting that can be used as a basis for mapping at different scales, correlating geological terrains, and conducting subsequent studies focused on petrogenesis or metagenesis. Here, a classification method based on 6 ranks is proposed for the alkaline rocks (s.l.) of the Ossa-Morena Zone (OMZ), emplaced during the Paleozoic rift and distinguished from other contemporary magmatic units (tholeiitic mafic rocks and calc-alkaline granitoids). The entire set of alkaline rocks in the OMZ is considered a Supersuite (rank 1), equivalent to a Supergroup in lithostratigraphic classification, mappable at a large scale (> 1:50,000). Three suites (equivalent to a Group) are defined, also mappable at a regional scale, distinguishing orthoderived units based on textura criteria: Plutonic Suite, Sub-volcanic Suite, and Mylonitic Suite (rank 2). Each suite can be divided into subsuites (rank 3) according to local groupings. In the OMZ Alkaline Supersuite, the Plutonic Suite comprises the Alter do Chão, Elvas, and Monesterio clusters, the Sub-volcanic Suite includes the São Romão and Feria clusters, and the Mylonite Suite includes a set of alkaline orthogneisses from the Tomar-Badajoz-Córdoba Shear Zone (from Assumar-Arronches to Las Minillas). Also at a regional scale, the suites/subsuites can be distinguished based on the lithotype (rank 4), in alkaline gabbros, alkaline granites, syenites, and albitites. The subsequent ranks are applied at a local scale, aiming to characterize each massif or individual lithological association: rank 5 (equivalent to Member) – the same intrusion non-continuous in outcrop (swarm) or two contiguous genetically associated units (composite body); and rank 6: the minimum mappable unit. This classification method was partially adapted from the classification systems recommended by the International Stratigraphic Guide (ISG) and the British Geological Surveys (Gillespie & Leslie, 2021).